REV. C

Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.

AD603*

FEATURES
"Linear in dB" Gain Control
Pin Programmable Gain Ranges

11 dB to +31 dB with 90 MHz Bandwidth
+9 dB to +51 dB with 9 MHz Bandwidth

Any Intermediate Range, e.g., 1 dB to +41 dB with

30 MHz Bandwidth

Bandwidth Independent of Variable Gain
1.3 nV/

Hz Input Noise Spectral Density

0.5 dB Typical Gain Accuracy

MIL-STD-883 Compliant and DESC Versions Available

APPLICATIONS
RF/IF AGC Amplifier
Video Gain Control
A/D Range Extension
Signal Measurement

Low Noise, 90 MHz

Variable-Gain Amplifier

PRODUCT DESCRIPTION

The AD603 is a low noise, voltage-controlled amplifier for use
in RF and IF AGC systems. It provides accurate, pin selectable
gains of 11 dB to +31 dB with a bandwidth of 90 MHz or
+9 dB to +51 dB with a bandwidth of 9 MHz. Any intermediate
gain range may be arranged using one external resistor. The
input referred noise spectral density is only 1.3 nV/

Hz and power

consumption is 125 mW at the recommended

5 V supplies.

The decibel gain is "linear in dB," accurately calibrated, and
stable over temperature and supply. The gain is controlled at a
high impedance (50 M

), low bias (200 nA) differential input;

the scaling is 25 mV/dB, requiring a gain-control voltage of only

FUNCTIONAL BLOCK DIAGRAM

SCALING

REFERENCE

GAIN

CONTROL

INTERFACE

AD603

PRECISION PASSIVE
INPUT ATTENUATOR

FIXED GAIN

AMPLIFIER

*NORMAL VALUES

R = 2R LADDER NETWORK

VPOS

VNEG

GPOS

GNEG

VINP

COMM

0dB

6.02dB 12.04dB 18.06dB 24.08dB 30.1dB 36.12dB 42.14dB

20 *

694 *

6.44k *

OUT

FDBK

1 V to span the central 40 dB of the gain range. An over- and
under-range of 1 dB is provided whatever the selected range. The
gain-control response time is less than 1

s for a 40 dB change.

The differential gain-control interface allows the use of either
differential or single-ended positive or negative control voltages.
Several of these amplifiers may be cascaded and their gain-con-
trol gains offset to optimize the system S/N ratio.

The AD603 can drive a load impedance as low as 100

with

low distortion. For a 500

load in shunt with 5 pF, the total

harmonic distortion for a

1 V sinusoidal output at 10 MHz is

typically 60 dBc. The peak specified output is

2.5 V mini-

mum into a 500

load, or

1 V into a 100

load.

The AD603 uses a proprietary circuit topology--the X-AMPTM.
The X-AMP comprises a variable attenuator of 0 dB to
42.14 dB followed by a fixed-gain amplifier. Because of the
attenuator, the amplifier never has to cope with large inputs and
can use negative feedback to define its (fixed) gain and dynamic
performance. The attenuator has an input resistance of 100

laser trimmed to

3%, and comprises a seven-stage R-2R ladder

network, resulting in an attenuation between tap points of
6.021 dB. A proprietary interpolation technique provides a
continuous gain-control function which is linear in dB.

The AD603A is specified for operation from 40

C to +85

and is available in both 8-lead SOIC (R) and 8-lead ceramic
DIP (Q). The AD603S is specified for operation from 55

C to

+125

C and is available in an 8-lead ceramic DIP (Q). The

AD603 is also available under DESC SMD 5962-94572.

*Patented.
X-AMP is a trademark of Analog Devices, Inc.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700

World Wide Web Site: http://www.analog.com

Fax: 781/326-8703

© Analog Devices, Inc., 2000

AD603SPECIFICATIONS

REV. C

Model

AD603

Parameter

Conditions

Min

Typ

Max

Unit

INPUT CHARACTERISTICS

Input Resistance

Pins 3 to 4

100

103

Input Capacitance

Input Noise Spectral Density

Input Short Circuited

1.3

nV/

Noise Figure

f = 10 MHz, Gain = max, R

= 10

8.8

1 dB Compression Point

f = 10 MHz, Gain = max, R

= 10

dBm

Peak Input Voltage

1.4

OUTPUT CHARACTERISTICS

3 dB Bandwidth

OUT

= 100 mV rms

MHz

Slew Rate

500

275

Peak Output

500

2.5

3.0

Output Impedance

10 MHz

Output Short-Circuit Current

Group Delay Change vs. Gain

f = 3 MHz; Full Gain Range

Group Delay Change vs. Frequency

= 0 V; f = 1 MHz to 10 MHz

Differential Gain

0.2

Differential Phase

0.2

Degree

Total Harmonic Distortion

f = 10 MHz, V

OUT

= 1 V rms

dBc

3rd Order Intercept

f = 40 MHz, Gain = max, R

= 50

dBm

ACCURACY

Gain Accuracy

500 mV

+500 mV

0.5

MIN

to T

MAX

1.5

Output Offset Voltage

= 0 V

MIN

to T

MAX

Output Offset Variation vs. V

500 mV

+500 mV

MIN

to T

MAX

GAIN CONTROL INTERFACE

Gain Scaling Factor

39.4

40.6

dB/V

MIN

to T

MAX

dB/V

GNEG, GPOS Voltage Range

1.2

+2.0

Input Bias Current

200

Input Offset Current

Differential Input Resistance

Pins 1 to 2

Response Rate

Full 40 dB Gain Change

dB/

POWER SUPPLY

Specified Operating Range

4.75

6.3

Quiescent Current

12.5

MIN

to T

MAX

NOTES

Typical open or short-circuited input; noise is lower when system is set to maximum gain and input is short-circuited. This figure includes the effects of both voltage

and current noise sources.

Using resistive loads of 500

or greater, or with the addition of a 1 k

pull-down resistor when driving lower loads.

The dc gain of the main amplifier in the AD603 is

35.7; thus, an input offset of 100

V becomes a 3.57 mV output offset.

GNEG and GPOS, gain control, voltage range is guaranteed to be within the range of V

+ 4.2 V to +V

3.4 V over the full temperature range of 40

C to +85

Specifications shown in boldface are tested on all production units at final electrical test. Results from those tests are used to calculate outgoing quality levels. All min
and max specifications are guaranteed, although only those shown in boldface are tested on all production units.

Specifications subject to change without notice.

(@ T

= +25 C, V

= 5 V, 500 mV

+500 mV, GNEG = 0 V, 10 dB to +30 dB Gain

Range, R

= 500

, and C

= 5 pF, unless otherwise noted.)

AD603

REV. C

ABSOLUTE MAXIMUM RATINGS

Supply Voltage

. . . . . . . . . . . . . . . . . . . . . . . . . . . .

7.5 V

Internal Voltage VINP (Pin 3) . . . . . . . . . . .

2 V Continuous

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

for 10 ms

GPOS, GNEG (Pins 1, 2) . . . . . . . . . . . . . . . . . . . . . . .

Internal Power Dissipation

. . . . . . . . . . . . . . . . . . . . 400 mW

Operating Temperature Range

AD603A . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

C to +85

AD603S . . . . . . . . . . . . . . . . . . . . . . . . . . 55

C to +125

Storage Temperature Range . . . . . . . . . . . . 65

C to +150

Lead Temperature Range (Soldering 60 sec) . . . . . . . . +300

NOTES

Stresses above those listed under Absolute Maximum Ratings may cause perma-

nent damage to the device. This is a stress rating only; functional operation of the
device at these or any other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute maximum rating
conditions for extended periods may affect device reliability.

Thermal Characteristics:

8-Lead SOIC Package:

= 155

C/W,

= 33

C/W

8-Lead Ceramic Package:

= 140

C/W,

= 15

C/W

PIN FUNCTION DESCRIPTIONS

Pin

Mnemonic

Description

Pin 1

GPOS

Gain-Control Input "HI"
(Positive Voltage Increases Gain)

Pin 2

GNEG

Gain-Control Input "LO"
(Negative Voltage Increases Gain)

Pin 3

VINP

Amplifier Input

Pin 4

COMM

Amplifier Ground

Pin 5

FDBK

Connection to Feedback Network

Pin 6

VNEG

Negative Supply Input

Pin 7

VOUT

Amplifier Output

Pin 8

VPOS

Positive Supply Input

CONNECTION DIAGRAMS

8-Lead Plastic SOIC (R) Package

8-Lead Ceramic DIP (Q) Package

TOP VIEW

(Not to Scale)

GPOS

GNEG

VINP

VPOS

VOUT

VNEG

FDBK

COMM

AD603

CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection. Although
the AD603 features proprietary ESD protection circuitry, permanent damage may occur on devices
subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recom-
mended to avoid performance degradation or loss of functionality.

WARNING!

ESD SENSITIVE DEVICE

ORDERING GUIDE

Temperature

Package

Part Number

Range

Description

Option

AD603AR

C to +85

8-Lead SOIC

SO-8

AD603AQ

C to +85

8-Lead Ceramic DIP

Q-8

AD603SQ/883B

C to +125

8-Lead Ceramic DIP

Q-8

AD603-EB

Evaluation Board

AD603ACHIPS

C to +85

Die

AD603AR-REEL

C to +85

13" Reel

SO-8

AD603AR-REEL7

C to +85

7" Reel

SO-8

*Refer to AD603 Military data sheet. Also available as 5962-9457203MPA.

REV. C

AD603

THEORY OF OPERATION

The AD603 comprises a fixed-gain amplifier, preceded by a
broadband passive attenuator of 0 dB to 42.14 dB, having a
gain-control scaling factor of 40 dB per volt. The fixed gain is
laser-trimmed in two ranges, to either 31.07 dB (

35.8) or

50 dB (

358), or may be set to any range in between using one

external resistor between Pins 5 and 7. Somewhat higher gain
can be obtained by connecting the resistor from Pin 5 to com-
mon, but the increase in output offset voltage limits the
maximum gain to about 60 dB. For any given range, the band-
width is independent of the voltage-controlled gain. This system
provides an under- and overrange of 1.07 dB in all cases;
for example, the overall gain is 11.07 dB to 31.07 dB in the
maximum-bandwidth mode (Pin 5 and Pin 7 strapped).

This X-AMP structure has many advantages over former methods
of gain-control based on nonlinear elements. Most importantly,
the fixed-gain amplifier can use negative feedback to increase its
accuracy. Since large inputs are first attenuated, the amplifier
input is always small. For example, to deliver a

1 V output in

the 1 dB/+41 dB mode (that is, using a fixed amplifier gain of
41.07 dB) its input is only 8.84 mV; thus the distortion can be
very low. Equally important, the small-signal gain and phase
response, and thus the pulse response, are essentially indepen-
dent of gain.

Figure 1 is a simplified schematic. The input attenuator is a
seven-section R-2R ladder network, using untrimmed resistors
of nominally R = 62.5

, which results in a characteristic resis-

tance of 125

20%. A shunt resistor is included at the input

and laser trimmed to establish a more exact input resistance of
100

3%, which ensures accurate operation (gain and HP

corner frequency) when used in conjunction with external resistors
or capacitors.

The nominal maximum signal at input VINP is 1 V rms (

1.4 V

peak) when using the recommended

5 V supplies, although

operation to

2 V peak is permissible with some increase in HF

distortion and feedthrough. Pin 4 (SIGNAL COMMON) must
be connected directly to the input ground; significant impedance in
this connection will reduce the gain accuracy.

The signal applied at the input of the ladder network is attenu-
ated by 6.02 dB by each section; thus, the attenuation to each of
the taps is progressively 0 dB, 6.02 dB, 12.04 dB, 18.06 dB,
24.08 dB, 30.1 dB, 36.12 dB and 42.14 dB. A unique circuit
technique is employed to interpolate between these tap-points,

indicated by the "slider" in Figure 1, thus providing continuous
attenuation from 0 dB to 42.14 dB. It will help, in understanding
the AD603, to think in terms of a mechanical means for moving
this slider from left to right; in fact, its "position" is controlled
by the voltage between Pins 1 and 2. The details of the gain-
control interface are discussed later.

The gain is at all times very exactly determined, and a linear-in-
dB relationship is automatically guaranteed by the exponential
nature of the attenuation in the ladder network (the X-AMP
principle). In practice, the gain deviates slightly from the ideal
law, by about

0.2 dB peak (see, for example, Figure 16).

Noise Performance

An important advantage of the X-AMP is its superior noise per-
formance. The nominal resistance seen at inner tap points is
41.7

(one third of 125

), which exhibits a Johnson noise-

spectral density (NSD) of 0.83 nV/

Hz (that is,

4kTR) at 27

which is a large fraction of the total input noise. The first stage
of the amplifier contributes a further 1 nV/

Hz, for a total input

noise of 1.3 nV/

Hz. It will be apparent that it is essential to use

a low resistance in the ladder network to achieve the very low
specified noise level. The signal's source impedance forms a
voltage divider with the AD603's 100

input resistance. In

some applications, the resulting attenuation may be unaccept-
able, requiring the use of an external buffer or preamplifier to
match a high impedance source to the low impedance AD603.

The noise at maximum gain (that is, at the 0 dB tap) depends
on whether the input is short-circuited or open-circuited: when
shorted, the minimum NSD of slightly over 1 nV/

Hz is achieved;

when open, the resistance of 100

looking into the first tap

generates 1.29 nV/

Hz, so the noise increases to a total of

1.63 nV/

Hz. (This last calculation would be important if the

AD603 were preceded by, for example, a 900

resistor to allow

operation from inputs up to 10 V rms.) As the selected tap
moves away from the input, the dependence of the noise on
source impedance quickly diminishes.

Apart from the small variations just discussed, the signal-to-
noise (S/N) ratio at the output is essentially independent of the
attenuator setting. For example, on the 11 dB/+31 dB range
the fixed gain of

35.8 raises the output NSD to 46.5 nV/

Hz.

Thus, for the maximum undistorted output of 1 V rms and a
1 MHz bandwidth, the output S/N ratio would be 86.6 dB, that
is, 20 log (1 V/46.5

V).

SCALING

REFERENCE

GAIN

CONTROL

INTERFACE

AD603

PRECISION PASSIVE
INPUT ATTENUATOR

FIXED GAIN

AMPLIFIER

*NORMAL VALUES

R = 2R LADDER NETWORK

VPOS

VNEG

GPOS

GNEG

VINP

COMM

0dB

6.02dB 12.04dB 18.06dB 24.08dB 30.1dB 36.12dB 42.14dB

20 *

694 *

6.44k *

OUT

FDBK

Figure 1. Simplified Block Diagram of the AD603

AD603

REV. C

The Gain-Control Interface

The attenuation is controlled through a differential, high-
impedance (50 M

) input, with a scaling factor which is

laser-trimmed to 40 dB per volt, that is, 25 mV/dB. An internal
bandgap reference ensures stability of the scaling with respect to
supply and temperature variations.

When the differential input voltage V

= 0 V, the attenuator

"slider" is centered, providing an attenuation of 21.07 dB. For
the maximum bandwidth range, this results in an overall gain of
10 dB (= 21.07 dB + 31.07 dB). When the control input is
500 mV, the gain is lowered by 20 dB (= 0.500 V

40 dB/V),

to 10 dB; when set to +500 mV, the gain is increased by 20 dB, to
30 dB. When this interface is overdriven in either direction, the
gain approaches either 11.07 dB (= 42.14 dB + 31.07 dB) or
31.07 dB (= 0 + 31.07 dB), respectively. The only constraint on
the gain-control voltage is that it be kept within the common-mode
range (1.2 V to +2.0 V assuming +5 V supplies) of the gain
control interface.

The basic gain of the AD603 can thus be calculated using the
following simple expression:

Gain (dB) = 40 V

+ 10

(1)

where V

is in volts. When Pins 5 and 7 are strapped (see next

section) the gain becomes

Gain (dB) = 40 V

+ 20 for 0 to +40 dB

and

Gain (dB) = 40 V

+ 30 for +10 to +50 dB

(2)

The high impedance gain-control input ensures minimal loading
when driving many amplifiers in multiple channel or cascaded
applications. The differential capability provides flexibility in
choosing the appropriate signal levels and polarities for various
control schemes.

For example, if the gain is to be controlled by a DAC providing
a positive only ground-referenced output, the "Gain Control
LO" (GNEG) pin should be biased to a fixed offset of +500 mV,
to set the gain to 10 dB when "Gain Control HI" (GPOS) is at
zero, and to 30 dB when at +1.00 V.

It is a simple matter to include a voltage divider to achieve other
scaling factors. When using an 8-bit DAC having an FS output
of +2.55 V (10 mV/bit), a divider ratio of 2 (generating 5 mV/bit)
would result in a gain-setting resolution of 0.2 dB/bit. The use
of such offsets is valuable when two AD603s are cascaded, when
various options exist for optimizing the S/N profile, as will be
shown later.

Programming the Fixed-Gain Amplifier Using Pin Strapping

Access to the feedback network is provided at Pin 5 (FDBK).
The user may program the gain of the AD603's output amplifier
using this pin, as shown in Figure 2. There are three modes: in
the default mode, FDBK is unconnected, providing the range
+9 dB/+51 dB; when V

OUT

and FDBK are shorted, the gain is

lowered to 11 dB/+31 dB; when an external resistor is placed
between V

OUT

and FDBK any intermediate gain can be achieved,

for example, 1 dB/+41 dB. Figure 3 shows the nominal maxi-
mum gain versus external resistor for this mode.

GPOS

GNEG

VINP

COMM

VPOS

OUT

VNEG

FDBK

AD603

VC1

VC2

VPOS

OUT

VNEG

a. 10 dB to +30 dB; 90 MHz Bandwidth

GPOS

GNEG

VINP

COMM

VPOS

OUT

VNEG

FDBK

AD603

VC1

VC2

VPOS

OUT

VNEG

2.15k

5.6pF

b. 0 dB to +40 dB; 30 MHz Bandwidth

GPOS

GNEG

VINP

COMM

VPOS

OUT

VNEG

FDBK

AD603

VC1

VC2

VPOS

OUT

VNEG

18pF

c. +10 dB to +50 dB; 9 MHz Bandwidth

Figure 2. Pin Strapping to Set Gain

EXT

DECIBELS

100

10k

100k

1:VdB (OUT)

2:VdB (OUT)

VdB (OUT)

Figure 3. Gain vs. R

EXT

, Showing Worst-Case Limits

Assuming Internal Resistors Have a Maximum Tolerance
of 20%

REV. C

AD603

Optionally, when a resistor is placed from FDBK to COMM,
higher gains can be achieved. This fourth mode is of limited
value because of the low bandwidth and the elevated output off-
sets; it is thus not included in Figure 2.

The gain of this amplifier in the first two modes is set by the
ratio of on-chip laser-trimmed resistors. While the ratio of these
resistors is very accurate, the absolute value of these resistors
can vary by as much as

20%. Thus, when an external resistor

is connected in parallel with the nominal 6.44 k

20% inter-

nal resistor, the overall gain accuracy is somewhat poorer. The
worst-case error occurs at about 2 k

(see Figure 4).

EXT

1.2

DECIBELS

100

10k

100k

1.0

0.8

0.6

0.4

0.2

0.0

0.2

0.4

0.6

0.8

1.0

1:VdB (OUT) (1):VdB (O

REF

)

VdB (OUT) VdB (O

REF

)

Figure 4. Worst-Case Gain Error, Assuming Internal Resis-
tors Have a Maximum Tolerance of 20% (Top Curve) or
+20% (Bottom Curve)

While the gain-bandwidth product of the fixed-gain amplifier is
about 4 GHz, the actual bandwidth is not exactly related to the
maximum gain. This is because there is a slight enhancing of the
ac response magnitude on the maximum bandwidth range, due
to higher order poles in the open-loop gain function; this mild
peaking is not present on the higher gain ranges. Figure 2 shows
how optional capacitors may be added to extend the frequency
response in high gain modes.

CASCADING TWO AD603S

Two or more AD603s can be connected in series to achieve
higher gain. Invariably, ac coupling must be used to prevent the
dc offset voltage at the output of each amplifier from overload-
ing the following amplifier at maximum gain. The required high
pass coupling network will usually be just a capacitor, chosen to
set the desired corner frequency in conjunction with the well-
defined 100

input resistance of the following amplifier.

For two AD603s, the total gain-control range becomes 84 dB
(two times 42.14 dB); the overall 3 dB bandwidth of cascaded
stages will be somewhat reduced. Depending on the pin-strapping,
the gain and bandwidth for two cascaded amplifiers can range
from 22 dB to +62 dB (with a bandwidth of about 70 MHz) to
+22 dB to +102 dB (with a bandwidth of about 6 MHz).

There are several ways of connecting the gain-control inputs in
cascaded operation. The choice depends on whether it is impor-
tant to achieve the highest possible Instantaneous Signal-to-Noise
Ratio (ISNR), or, alternatively, to minimize the ripple in the gain
error. The following examples feature the AD603 programmed
for maximum bandwidth; the explanations apply to other gain/
bandwidth combinations with appropriate changes to the arrange-
ments for setting the maximum gain.

Sequential Mode (Optimal S/N Ratio)
In the sequential mode of operation, the ISNR is maintained at
its highest level for as much of the gain control range possible.
Figure 5 shows the SNR over a gain range of 22 dB to +62 dB,
assuming an output of 1 V rms and a 1 MHz bandwidth; Figure
6 shows the general connections to accomplish this. Here, both
the positive gain-control inputs (GPOS) are driven in parallel by
a positive-only, ground-referenced source with a range of 0 V to
+2 V, while the negative gain-control inputs (GNEG) arc biased
by stable voltages to provide the needed gain-offsets. These volt-
ages may be provided by resistive dividers operating from a
common voltage reference.

S/N RATIO dB

0.2

2.2

0.2

0.6

1.0

1.4

1.8

Figure 5. SNR vs. Control Voltage--Sequential Control
(1 MHz Bandwidth)

The gains are offset (Figure 7) such that A2's gain is increased
only after A1's gain has reached its maximum value. Note that
for a differential input of 600 mV or less, the gain of a single
amplifier (A1 or A2) will be at its minimum value of 11.07 dB;
for a differential input of +600 mV or more, the gain will be at
its maximum value of 31.07 dB. Control inputs beyond these
limits will not affect the gain and can be tolerated without dam-
age or foldover in the response. This is an important aspect of
the AD603's gain-control response. (See the Specifications sec-
tion of this data sheet for more details on the allowable voltage
range) The gain is now

Gain (dB) = 40 V

+ G

(3)

where V

is the applied control voltage and G

is determined

by the gain range chosen. In the explanatory notes that follow,
we assume the maximum-bandwidth connections are used, for
which G

is 20 dB.

AD603

REV. C

31.07dB

42.14dB

GPOS

GNEG

31.07dB

42.14dB

GPOS

GNEG

40.00dB

51.07dB

8.93dB

INPUT

0dB

= 0V

= 0.473V

= 1.526V

OUTPUT
20dB

31.07dB

42.14dB

GPOS

GNEG

31.07dB

0dB

GPOS

GNEG

0dB

11.07dB

31.07dB

INPUT

0dB

= 1.0V

= 0.473V

= 1.526V

OUTPUT
20dB

31.07dB

2.14dB

GPOS

GNEG

31.07dB

0dB

GPOS

GNEG

0dB

28.93dB

31.07dB

INPUT

0dB

= 2.0V

= 0.473V

= 1.526V

OUTPUT
60dB

Figure 6. AD603 Gain Control Input Calculations for Sequential Control Operation

+31.07dB

+10dB

+31.07dB

+28.96dB

11.07dB

0.473

1.526

8.93dB

0.5

1.0

1.50

2.0

(V)

20

62.14

22.14

GAIN

(dB)

GAIN OFFSET OF 1.07dB, OR 26.75mV

Figure 7. Explanation of Offset Calibration for Sequential
Control

With reference to Figure 6, note that V

refers to the differen-

tial gain-control input to A1 and V

refers to the differential

gain-control input to A2. When V

is zero, V

= 473 mV and

thus the gain of A1 is 8.93 dB (recall that the gain of each indi-
vidual amplifier in the maximum-bandwidth mode is 10 dB
for V

= 500 mV and 10 dB for V

= 0 V); meanwhile, V

1.908 V so the gain of A2 is "pinned" at 11.07 dB. The over-
all gain is thus 20 dB. This situation is shown in Figure 6a.

When V

= +1.00 V, V

= 1.00 V 0.473 V = +0.526 V,

which sets the gain of A1 to at nearly its maximum value of
31.07 dB, while V

= 1.00 V 1.526 V = 0.526 V, which sets

A2's gain at nearly its minimum value 11.07 dB. Close analysis
shows that the degree to which neither AD603 is completely
pushed to its maximum or minimum gain exactly cancels in the
overall gain, which is now +20 dB. This is depicted in Figure 6b.

When V

= +2.0 V, the gain of A1 is pinned at 31.07 dB and

that of A2 is near its maximum value of 28.93 dB, resulting in
an overall gain of 60 dB (see Figure 6c). This mode of operation
is further clarified by Figure 8, which is a plot of the separate
gains of A1 and A2 and the overall gain versus the control voltage.
Figure 9 is a plot of the gain error of the cascaded amplifiers versus
the control voltage. Figure 10 is a plot of the gain error of the
cascaded stages versus the control voltages.

30

0.2

2.2

0.2

OVERALL GAIN dB

0.6

0.1

1.4

1.8

10

20

COMBINED

Figure 8. Plot of Separate and Overall Gains in Sequential
Control

REV. C

AD603

S/N RATIO dB

0.2

2.2

0.2

0.6

1.0

1.4

1.8

Figure 9. SNR for Cascaded Stages--Sequential Control

2.0

0.5

2.0

0.2

GAIN ERROR dB

1.5

0.0

1.0

1.5

1.0

0.5

0.0 0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

Figure 10. Gain Error for Cascaded Stages--Sequential
Control

Parallel Mode (Simplest Gain-Control Interface)
In this mode, the gain-control of voltage is applied to both inputs
in parallel--the GPOS pins of both A1 and A2 are connected to
the control voltage and the GNEW inputs are grounded. The
gain scaling is then doubled to 80 dB/V, requiring only a 1.00 V
change for an 80 dB change of gain:

Gain (dB) = 80 V

+ G

(4)

where, as before G

depends on the range selected; for example,

in the maximum-bandwidth mode, G

is +20 dB. Alternatively,

the GNEG pins may be connected to an offset voltage of
+0.500 V, in which case, G

is 20 dB.

The amplitude of the gain ripple in this case is also doubled, as
shown in Figure 11, while the instantaneous signal-to-noise ratio
at the output of A2 now decreases linearly as the gain increased
(Figure 12).

2.0

0.5

2.0

0.2

GAIN ERROR dB

1.5

0.0

1.0

1.5

1.0

0.5

0.0 0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

Figure 11. Gain Error for Cascaded StagesParallel
Control

0.2

IS/N RATIO dB

0.2

0.4

0.6

0.8

1.0

1.2

Figure 12. ISNR for Cascaded StagesParallel Control

Low Gain Ripple Mode (Minimum Gain Error)
As can be seen from Figures 9 and 10, the error in the gain is
periodic, that is, it shows a small ripple. (Note that there is also
a variation in the output offset voltage, which is due to the gain
interpolation, but this is not exact in amplitude.) By offsetting
the gains of A1 and A2 by half the period of the ripple, that is,
by 3 dB, the residual gain errors of the two amplifiers can be
made to cancel. Figure 13 shows that much lower gain ripple
when configured in this manner. Figure 14 plots the ISNR as a
function of gain; it is very similar to that in the "Parallel Mode."

AD603

REV. C

3.0

0.1

GAIN ERROR dB

2.5

2.0

1.5

1.0

0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0 1.1

Figure 13. Gain Error for Cascaded StagesLow Ripple
Mode

0.2

IS/N RATIO dB

0.2

0.4

0.6

0.8

1.0

1.2

Figure 14. ISNR vs. Control VoltageLow Ripple Mode

THEORY OF THE AD603
A Low Noise AGC Amplifier

Figure 15 shows the ease with which the AD603 can be connected
as an AGC amplifier. The circuit illustrates many of the points
previously discussed: It uses few parts, has linear-in-dB gain,
operates from a single supply, uses two cascaded amplifiers in
sequential gain mode for maximum S/N ratio, and an external
resistor programs each amplifier's gain. It also uses a simple
temperature-compensated detector.

The circuit operates from a single 10 V supply. Resistors R1,
R2, R3, and R4 bias the common pins of A1 and A2 at 5 V.
This pin is a low impedance point and must have a low impedance
path to ground, here provided by the 100

F tantalum capacitors

and the 0.1

F ceramic capacitors.

The cascaded amplifiers operate in sequential gain. Here, the
offset voltage between the pins 2 (GNEG) of A1 and A2 is
1.05 V (42.14 dB

25 mV/dB), provided by a voltage divider

consisting of resistors R5, R6, and R7. Using standard values,
the offset is not exact, but it is not critical for this application.

The gain of both A1 and A2 is programmed by resistors R13
and R14, respectively, to be about 42 dB; thus the maximum
gain of the circuit is twice that, or 84 dB. The gain-control
range can be shifted up by as much as 20 dB by appropriate
choices of R13 and R14.

The circuit operates as follows. A1 and A2 are cascaded.
Capacitor C1 and the 100

of resistance at the input of A1

form a time-constant of 10

s. C2 blocks the small dc offset

voltage at the output of A1 (which might otherwise saturate A2
at its maximum gain) and introduces a high-pass corner at about
16 kHz, eliminating low frequency noise.

A half-wave detector is used, based on Q1 and R8. The current
into capacitor C

is just the difference between the collector

current of Q2 (biased to be 300

A at 300 K, 27

C) and the col-

lector current of Q1, which increases with the amplitude of the

AD603

10V

0.1 F

R14

2.49k

10V

R4
2.49k

C6
0.1 F

C5
100 F

2.49k

AD603

10V

0.1 F

R13

2.49k

10V

R2
2.49k

C4
0.1 F

C3
100 F

2.49k

RT
100

3.48k

1.05k

5.49k

1V OFFSET FOR

SEQUENTIAL GAIN

5.5V

6.5V

NOTES

PROVIDES A 50 INPUT IMPEDANCE

C3 AND C5 ARE TANTALUM

10V

AGC LINE

0.1 F

THIS CAPACITOR SETS
AGC TIME CONSTANT

AGC

1.54k

806

2N3904

2N3906

R10
1.24k

R11
3.83k

R12
4.99k

C11
0.1 F

C9
0.1 F

10V

C10

0.1 F

Figure 15. A Low Noise AGC Amplifier

REV. C

AD603

output signal. The automatic gain control voltage, V

AGC

, is the

time-integral of this error current. In order for V

AGC

(and thus

the gain) to remain insensitive to short-term amplitude fluctuations
in the output signal, the rectified current in Q1 must, on average,
exactly balance the current in Q2. If the output of A2 is too small
to do this, V

AGC

will increase, causing the gain to increase, until

Q1 conducts sufficiently.

Consider the case where R8 is zero and the output voltage V

OUT

is a square wave at, say, 455 kHz, which is well above the corner
frequency of the control loop.

During the time V

OUT

is negative with respect to the base voltage

of Q1, Q1 conducts; when V

OUT

is positive, it is cut off. Since

the average collector current of Q1 is forced to be 300

A, and

the square wave has a duty-cycle of 1:1, Q1's collector current
when conducting must be 600

A. With R8 omitted, the peak

amplitude of V

OUT

is forced to be just the V

of Q1 at 600

typically about 700 mV, or 2 V

peak-to-peak. This voltage,

hence the amplitude at which the output stabilizes, has a strong
negative temperature coefficient (TC), typically 1.7 mV/

Although this may not be troublesome in some applications, the
correct value of R8 will render the output stable with temperature.

To understand this, first note that the current in Q2 is made
to be proportional to absolute temperature (PTAT). For the
moment, continue to assume that the signal is a square wave.

When Q1 is conducting, V

OUT

is now the sum of V

and a

voltage that is PTAT and which can be chosen to have an equal
but opposite TC to that of the V

. This is actually nothing more

than an application of the "bandgap voltage reference" principle.
When R8 is chosen such that the sum of the voltage across it
and the V

of Q1 is close to the bandgap voltage of about 1.2 V,

OUT

will be stable over a wide range of temperatures, provided,

of course, that Q1 and Q2 share the same thermal environment.

Since the average emitter current is 600

A during each half-

cycle of the square wave a resistor of 833

would add a PTAT

voltage of 500 mV at 300 K, increasing by 1.66 mV/

C. In prac-

tice, the optimum value will depend on the type of transistor
used and, to a lesser extent, on the waveform for which the
temperature stability is to be optimized; for the inexpensive
2N3904/2N306 pair and sine wave signals, the recommended
value is 806

This resistor also serves to lower the peak current in Q1 when
more typical signals (usually, sinusoidal) are involved, and the
1.8 kHz LP filter it forms with C

helps to minimize distortion

due to ripple in V

AGC

. Note that the output amplitude under

sine wave conditions will be higher than for a square wave, since
the average value of the current for an ideal rectifier would be
0.637 times as large, causing the output amplitude to be
1.88 (=1.2/0.637) V, or 1.33 V rms. In practice, the somewhat
nonideal rectifier results in the sine wave output being regulated
to about 1.4 V rms, or 3.6 V p-p.

The bandwidth of the circuit exceeds 40 MHz. At 10.7 MHz,
the AGC threshold is 100

V (67 dBm) and its maximum gain

is 83 dB (20 log 1.4 V/100

V). The circuit holds its output at

1.4 V rms for inputs as low as 67 dBm to +15 dBm (82 dB),
where the input signal exceeds the AD603's maximum input
rating. For a 30 dBm input at 10.7 MHz, the second harmonic
is 34 dB down from the fundamental and the third harmonic is
35 dB down.

CAUTION

Careful component selection, circuit layout, power-supply
decoupling, and shielding are needed to minimize the AD603's
susceptibility to interference from radio and TV stations, etc. In
bench evaluation, we recommend placing all of the components
in a shielded box and using feedthrough decoupling networks
for the supply voltage. Circuit layout and construction are also
critical, since stray capacitances and lead inductances can form
resonant circuits and are a potential source of circuit peaking,
oscillation, or both.

AD603

REV. C

GAIN VOLTAGE Volts

GAIN ERROR dB

2.50

0.5

2.00

1.50

1.00

0.50

0.00

0.50

1.00

1.50

0.4 0.3 0.2 0.1 0.0 0.2 0.3 0.4

0.5 0.6

45MHz

70MHz

10.7MHz

455kHz

70MHz

Figure 16. Gain Error vs. Gain Control
Voltage at 455 kHz, 10.7 MHz, 45 MHz,
70 MHz

FREQUENCY Hz

GAIN dB

100k

10M

100M

Figure 19. Frequency and Phase
Response vs. Gain (Gain = +30 dB,
P

= 30 dBm, Pin 5 Connected to

Pin 7)

10dB/DIV

Figure 22. Third Order Intermodula-
tion Distortion at 455 kHz (10

Probe

Used to HP3585A Spectrum Analyzer,
Gain = 0 dB, P

= 0 dBm, Pin 5 Con-

nected to Pin 7)

FREQUENCY Hz

GAIN dB

100k

10M

100M

Figure 17. Frequency and Phase
Response vs. Gain (Gain = 10 dB,
P

= 30 dBm, Pin 5 Connected to

Pin 7)

GAIN CONTROL VOLTAGE Volts

GROUP DELAY ns

7.60

0.6

7.40

7.20

7.00

6.80

6.60

6.40

0.4

0.2

0.4

0.6

Figure 20. Group Delay vs. Gain
Control Voltage

10dB/DIV

Figure 23. Third Order Intermodula-
tion Distortion at 10.7 MHz (10

Probe

Used to HP3585A Spectrum Analyzer,
Gain = 0 dB, P

= 0 dBm, Pin 5 Con-

nected to Pin 7)

FREQUENCY Hz

GAIN dB

100k

10M

100M

Figure 18. Frequency and Phase
Response vs. Gain (Gain = +10 dB,
P

= 30 dBm, Pin 5 Connected to

Pin 7)

DATEL

DVC 8500

HP3326A

DUAL

CHANNEL

SYNTHESIZER

100

+5V

0.1 F

5V

511

AD603

PROBE

HP3585A

SPECTRUM

ANALYZER

0.1 F

Figure 21. Third Order Intermodula-
tion Distortion Test Setup

LOAD RESISTANCE 

NEGATIVE OUTPUT VOLTAGE LIMIT Volts

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

100

200

500

1000 2000

3.2

3.4

Figure 24. Typical Output Voltage
Swing vs. Load Resistance (Negative
Output Swing Limits First)

REV. C

AD603

FREQUENCY Hz

INPUT IMPEDANCE 

100k

102

100

10M

100M

Figure 25. Input Impedance vs.

Frequency (Gain = 10 dB)

200ns

100

Figure 28. Gain-Control Channel
Response Time

3.5V

500mV

1.5V

44ns

50ns

456ns

GND

INPUT

500mV/DIV

OUTPUT

500mV/DIV

Figure 31. Transient Response,
G = 0 dB, Pin 5 Connected to Pin 7
(Input is 500 ns Period, 50% Duty-
Cycle Square Wave, Output Is

Captured Using Tektronix 11402

Digitizing Oscilloscope)

FREQUENCY Hz

INPUT IMPEDANCE 

100k

102

100

10M

100M

Figure 26. Input Impedance vs.

Frequency (Gain = +10 dB)

4.5V

500mV

49ns

50ns

451ns

INPUT GND

1V/DIV

OUTPUT GND

500mV/DIV

Figure 29. Input Stage Overload
Recovery Time, Pin 5 Connected to
Pin 7 (Input Is 500 ns Period, 50%
Duty-Cycle Square Wave, Output Is
Captured Using Tektronix 11402
Digitizing Oscilloscope)

3.5V

500mV

1.5V

44ns

50ns

456ns

INPUT GND

100mV/DIV

OUTPUT GND

500mV/DIV

Figure 32. Transient Response,
G = +20 dB, Pin 5 Connected to Pin 7
(Input is 500 ns Period, 50% Duty-
Cycle Square Wave, Output Is

Captured Using Tektronix 11402

Digitizing Oscilloscope)

FREQUENCY Hz

INPUT IMPEDANCE 

100k

102

100

10M

100M

Figure 27. Input Impedance vs.

Frequency (Gain = +30 dB)

2V

49ns

50ns

451ns

INPUT GND

100mV/DIV

OUTPUT GND

1V/DIV

Figure 30. Output Stage Overload
Recovery Time, Pin 5 Connected to
Pin 7 (Input Is 500 ns Period, 50%
Duty-Cycle Square Wave, Output Is
Captured Using Tektronix 11402
Digitizing Oscilloscope)

FREQUENCY Hz

PSRR dB

100k

20

40

60

10M

100M

10

30

50

Figure 33. PSRR vs. Frequency (Worst
Case Is Negative Supply PSRR,
Shown Here)

AD603

REV. C

Figure 34. Test Setup Used for: Noise
Figure, 3rd Order Intercept and 1 dB
Compression Point Measurements

INPUT FREQUENCY MHz

INPUT LEVEL dBm

10

25

15

20

= 25 C

TEST SETUP
FIGURE 34

Figure 37. 1 dB Compression Point,
10 dB/+30 dB Mode, Gain = 30 dB

GAIN dB

NOISE FIGURE dB

23 24

= 25 C

= 50

TEST SETUP
FIGURE 34

70MHz

30MHz

50MHz

10MHz

Figure 35. Noise Figure in 10 dB/
+30 dB Mode

INPUT LEVEL dBm

OUTPUT LEVEL dBm

20

10

= 25 C

TEST SETUP
FIGURE 34

30MHz

40MHz

70MHz

Figure 38. 3rd Order Intercept 10 dB/
+30 dB Mode, Gain = 10 dB

GAIN dB

NOISE FIGURE dB

33 34

= 25 C

= 50

TEST SETUP
FIGURE 34

20MHz

10MHz

Figure 36. Noise Figure in 0 dB/+40 dB
Mode

INPUT LEVEL dBm

OUTPUT LEVEL dBm

40

30

20

= 25 C

= 50

= 100

TEST SETUP
FIGURE 34

30MHz

40MHz

70MHz

Figure 39. 3rd Order Intercept, 10 dB/
+30 dB Mode, Gain = 30 dB

HP3326A

DUAL

CHANNEL

SYNTHESIZER

100

+5V

0.1 F

5V

AD603

HP3585A

SPECTRUM

ANALYZER

DATEL

DVC 8500

0.1 F

Ð­Ð»ÐµÐºÑ‚Ñ€Ð¾Ð½Ð½Ñ‹Ð¹ ÐºÐ¾Ð¼Ð¿Ð¾Ð½ÐµÐ½Ñ‚: AD603AQ

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