ADC0808/ADC0809
8-Bit µP Compatible A/D Converters with 8-Channel
Multiplexer

General Description

The ADC0808, ADC0809 data acquisition component is a
monolithic CMOS device with an 8-bit analog-to-digital con-
verter, 8-channel multiplexer and microprocessor compatible
control logic. The 8-bit A/D converter uses successive ap-
proximation as the conversion technique. The converter fea-
tures a high impedance chopper stabilized comparator, a
256R voltage divider with analog switch tree and a succes-
sive approximation register. The 8-channel multiplexer can
directly access any of 8-single-ended analog signals.

The device eliminates the need for external zero and
full-scale adjustments. Easy interfacing to microprocessors
is provided by the latched and decoded multiplexer address
inputs and latched TTL TRI-STATE

outputs.

The design of the ADC0808, ADC0809 has been optimized
by incorporating the most desirable aspects of several A/D
conversion techniques. The ADC0808, ADC0809 offers high
speed, high accuracy, minimal temperature dependence, ex-
cellent long-term accuracy and repeatability, and consumes
minimal power. These features make this device ideally
suited to applications from process and machine control to
consumer and automotive applications. For 16-channel mul-
tiplexer

with

common

output

(sample/hold

port)

see

ADC0816 data sheet. (See AN-247 for more information.)

Features

Easy interface to all microprocessors

Operates ratiometrically or with 5 V

or analog span

adjusted voltage reference

No zero or full-scale adjust required

8-channel multiplexer with address logic

0V to 5V input range with single 5V power supply

Outputs meet TTL voltage level specifications

Standard hermetic or molded 28-pin DIP package

28-pin molded chip carrier package

ADC0808 equivalent to MM74C949

ADC0809 equivalent to MM74C949-1

Key Specifications

Resolution

8 Bits

Total Unadjusted Error

LSB and

1 LSB

Single Supply

5 V

Low Power

15 mW

Conversion Time

100 µs

Block Diagram

TRI-STATE

is a registered trademark of National Semiconductor Corp.

DS005672-1

See Ordering

Information

October 1999

ADC0808/ADC0809

8-Bit

µP

Compatible

A/D

Converters

with

8-Channel

Multiplexer

DS005672

www.national.com

Absolute Maximum Ratings

(Notes 2, 1)

If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.

Supply Voltage (V

) (Note 3)

6.5V

Voltage at Any Pin

-0.3V to (V

+0.3V)

Except Control Inputs

Voltage at Control Inputs

-0.3V to +15V

(START, OE, CLOCK, ALE, ADD A, ADD B, ADD C)

Storage Temperature Range

-65°C to +150°C

Package Dissipation at T

=25°C

875 mW

Lead Temp. (Soldering, 10 seconds)

Dual-In-Line Package (plastic)

260°C

Dual-In-Line Package (ceramic)

300°C

Molded Chip Carrier Package

Vapor Phase (60 seconds)

215°C

Infrared (15 seconds)

220°C

ESD Susceptibility (Note 8)

400V

Operating Conditions

(Notes 1, 2)

Temperature Range (Note 1)

MIN

MAX

ADC0808CCN,ADC0809CCN

-40°C

+85°C

ADC0808CCV, ADC0809CCV

-40°C

+85°C

Range of V

(Note 1)

4.5 V

to 6.0 V

Electrical Characteristics

Converter Specifications: V

=5 V

REF+

, V

REF(-)

=GND, T

MIN

MAX

and f

CLK

=640 kHz unless otherwise stated.

Symbol

Parameter

Conditions

Min

Typ

Max

Units

ADC0808

Total Unadjusted Error

25°C

LSB

(Note 5)

MIN

to T

MAX

LSB

ADC0809

Total Unadjusted Error

0°C to 70°C

LSB

(Note 5)

MIN

to T

MAX

LSB

Input Resistance

From Ref(+) to Ref(-)

1.0

2.5

Analog Input Voltage Range

(Note 4) V(+) or V(-)

GND-0.10

+0.10

REF(+)

Voltage, Top of Ladder

Measured at Ref(+)

+0.1

Voltage, Center of Ladder

/2-0.1

/2+0.1

REF(-)

Voltage, Bottom of Ladder

Measured at Ref(-)

-0.1

Comparator Input Current

=640 kHz, (Note 6)

-2

0.5

µA

Electrical Characteristics

Digital Levels and DC Specifications: ADC0808CCN, ADC0808CCV, ADC0809CCN and ADC0809CCV, 4.75

5.25V,

-40°C

+85°C unless otherwise noted

Symbol

Parameter

Conditions

Min

Typ

Max

Units

ANALOG MULTIPLEXER

OFF(+)

OFF Channel Leakage Current

=5V, V

=5V,

=25°C

200

MIN

to T

MAX

1.0

µA

OFF(-)

OFF Channel Leakage Current

=5V, V

=0,

=25°C

-200

-10

MIN

to T

MAX

-1.0

µA

CONTROL INPUTS

IN(1)

Logical "1" Input Voltage

-1.5

IN(0)

Logical "0" Input Voltage

1.5

IN(1)

Logical "1" Input Current

=15V

1.0

µA

(The Control Inputs)

IN(0)

Logical "0" Input Current

-1.0

µA

(The Control Inputs)

Supply Current

CLK

=640 kHz

0.3

3.0

ADC0808/ADC0809

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Electrical Characteristics

(Continued)

Digital Levels and DC Specifications: ADC0808CCN, ADC0808CCV, ADC0809CCN and ADC0809CCV, 4.75

5.25V,

-40°C

+85°C unless otherwise noted

Symbol

Parameter

Conditions

Min

Typ

Max

Units

DATA OUTPUTS AND EOC (INTERRUPT)

OUT(1)

Logical "1" Output Voltage

= 4.75V

OUT

= -360µA

OUT

= -10µA

2.4
4.5

V(min)
V(min)

OUT(0)

Logical "0" Output Voltage

=1.6 mA

0.45

OUT(0)

Logical "0" Output Voltage EOC

=1.2 mA

0.45

OUT

TRI-STATE Output Current

=5V

µA

-3

µA

Electrical Characteristics

Timing Specifications V

REF(+)

=5V, V

REF(-)

=GND, t

=20 ns and T

=25°C unless otherwise noted.

Symbol

Parameter

Conditions

MIn

Typ

Max

Units

Minimum Start Pulse Width

(

Figure 5)

100

200

WALE

Minimum ALE Pulse Width

(

Figure 5)

100

200

Minimum Address Set-Up Time

(

Figure 5)

Minimum Address Hold Time

(

Figure 5)

Analog MUX Delay Time

(

Figure 5)

2.5

µs

From ALE

, t

OE Control to Q Logic State

=50 pF, R

=10k (Figure 8)

125

250

, t

OE Control to Hi-Z

=10 pF, R

=10k (Figure 8)

125

250

Conversion Time

=640 kHz, (Figure 5) (Note 7)

100

116

µs

Clock Frequency

640

1280

kHz

EOC

EOC Delay Time

(

Figure 5)

8+2 µS

Clock

Periods

Input Capacitance

At Control Inputs

OUT

TRI-STATE Output

At TRI-STATE Outputs

Capacitance

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. DC and AC electrical specifications do not apply when operating
the device beyond its specified operating conditions.

Note 2: All voltages are measured with respect to GND, unless othewise specified.

Note 3: A zener diode exists, internally, from V

to GND and has a typical breakdown voltage of 7 V

Note 4: Two on-chip diodes are tied to each analog input which will forward conduct for analog input voltages one diode drop below ground or one diode drop greater
than the V

n supply. The spec allows 100 mV forward bias of either diode. This means that as long as the analog V

does not exceed the supply voltage by more

than 100 mV, the output code will be correct. To achieve an absolute 0V

to 5V

input voltage range will therefore require a minimum supply voltage of 4.900 V

over temperature variations, initial tolerance and loading.

Note 5: Total unadjusted error includes offset, full-scale, linearity, and multiplexer errors. See

Figure 3. None of these A/Ds requires a zero or full-scale adjust. How-

ever, if an all zero code is desired for an analog input other than 0.0V, or if a narrow full-scale span exists (for example: 0.5V to 4.5V full-scale) the reference voltages
can be adjusted to achieve this. See

Figure 13.

Note 6: Comparator input current is a bias current into or out of the chopper stabilized comparator. The bias current varies directly with clock frequency and has little
temperature dependence (

Figure 6). See paragraph 4.0.

Note 7: The outputs of the data register are updated one clock cycle before the rising edge of EOC.

Note 8: Human body model, 100 pF discharged through a 1.5 k

resistor.

ADC0808/ADC0809

www.national.com

Functional Description

Multiplexer. The device contains an 8-channel single-ended
analog signal multiplexer. A particular input channel is se-
lected by using the address decoder.

Table 1 shows the input

states for the address lines to select any channel. The ad-
dress is latched into the decoder on the low-to-high transition
of the address latch enable signal.

TABLE 1.

SELECTED

ADDRESS LINE

ANALOG

CHANNEL

IN0

IN1

IN2

IN3

IN4

IN5

IN6

IN7

CONVERTER CHARACTERISTICS

The Converter

The heart of this single chip data acquisition system is its
8-bit analog-to-digital converter. The converter is designed to
give fast, accurate, and repeatable conversions over a wide
range of temperatures. The converter is partitioned into 3
major sections: the 256R ladder network, the successive ap-
proximation register, and the comparator. The converter's
digital outputs are positive true.

The 256R ladder network approach (

Figure 1) was chosen

over the conventional R/2R ladder because of its inherent
monotonicity, which guarantees no missing digital codes.
Monotonicity is particularly important in closed loop feedback
control systems. A non-monotonic relationship can cause os-
cillations that will be catastrophic for the system. Additionally,
the 256R network does not cause load variations on the ref-
erence voltage.

The bottom resistor and the top resistor of the ladder net-
work in

Figure 1 are not the same value as the remainder of

the network. The difference in these resistors causes the
output characteristic to be symmetrical with the zero and
full-scale points of the transfer curve. The first output transi-
tion occurs when the analog signal has reached +

LSB

and succeeding output transitions occur every 1 LSB later up
to full-scale.

The successive approximation register (SAR) performs 8 it-
erations to approximate the input voltage. For any SAR type
converter, n-iterations are required for an n-bit converter.
Figure 2 shows a typical example of a 3-bit converter. In the
ADC0808, ADC0809, the approximation technique is ex-
tended to 8 bits using the 256R network.

The A/D converter's successive approximation register
(SAR) is reset on the positive edge of the start conversion
(SC) pulse. The conversion is begun on the falling edge of
the start conversion pulse. A conversion in process will be in-
terrupted by receipt of a new start conversion pulse. Con-
tinuous conversion may be accomplished by tying the
end-of-conversion (EOC) output to the SC input. If used in
this mode, an external start conversion pulse should be ap-
plied after power up. End-of-conversion will go low between
0 and 8 clock pulses after the rising edge of start conversion.

The most important section of the A/D converter is the com-
parator. It is this section which is responsible for the ultimate
accuracy of the entire converter. It is also the comparator
drift which has the greatest influence on the repeatability of
the device. A chopper-stabilized comparator provides the
most effective method of satisfying all the converter require-
ments.

The chopper-stabilized comparator converts the DC input
signal into an AC signal. This signal is then fed through a
high gain AC amplifier and has the DC level restored. This
technique limits the drift component of the amplifier since the
drift is a DC component which is not passed by the AC am-
plifier. This makes the entire A/D converter extremely insen-
sitive to temperature, long term drift and input offset errors.

Figure 4 shows a typical error curve for the ADC0808 as
measured using the procedures outlined in AN-179.

ADC0808/ADC0809

www.national.com

Typical Performance Characteristics

TRI-STATE Test Circuits and Timing Diagrams

Applications Information

OPERATION

1.0 RATIOMETRIC CONVERSION

The ADC0808, ADC0809 is designed as a complete Data
Acquisition System (DAS) for ratiometric conversion sys-
tems. In ratiometric systems, the physical variable being
measured is expressed as a percentage of full-scale which is
not necessarily related to an absolute standard. The voltage
input to the ADC0808 is expressed by the equation

(1)

=Input voltage into the ADC0808

=Full-scale voltage

=Zero voltage

=Data point being measured

MAX

=Maximum data limit

MIN

=Minimum data limit

A good example of a ratiometric transducer is a potentiom-
eter used as a position sensor. The position of the wiper is di-
rectly proportional to the output voltage which is a ratio of the
full-scale voltage across it. Since the data is represented as
a proportion of full-scale, reference requirements are greatly
reduced, eliminating a large source of error and cost for
many applications. A major advantage of the ADC0808,
ADC0809 is that the input voltage range is equal to the sup-
ply range so the transducers can be connected directly
across the supply and their outputs connected directly into
the multiplexer inputs, (

Figure 9).

Ratiometric transducers such as potentiometers, strain
gauges, thermistor bridges, pressure transducers, etc., are
suitable for measuring proportional relationships; however,
many types of measurements must be referred to an abso-
lute standard such as voltage or current. This means a sys-

DS005672-16

FIGURE 6. Comparator I

vs V

REF

=5V)

DS005672-17

FIGURE 7. Multiplexer R

vs V

REF

=5V)

, t

DS005672-18

, C

= 10 pF

DS005672-19

, C

= 50 pF

DS005672-20

, t

DS005672-21

, C

= 10 pF

DS005672-22

, C

= 50 pF

DS005672-23

FIGURE 8.

ADC0808/ADC0809

www.national.com

Applications Information

(Continued)

tem reference must be used which relates the full-scale volt-
age to the standard volt. For example, if V

REF

=5.12V,

then the full-scale range is divided into 256 standard steps.
The smallest standard step is 1 LSB which is then 20 mV.

2.0 RESISTOR LADDER LIMITATIONS

The voltages from the resistor ladder are compared to the
selected into 8 times in a conversion. These voltages are
coupled to the comparator via an analog switch tree which is
referenced to the supply. The voltages at the top, center and
bottom of the ladder must be controlled to maintain proper
operation.

The top of the ladder, Ref(+), should not be more positive
than the supply, and the bottom of the ladder, Ref(-), should
not be more negative than ground. The center of the ladder
voltage must also be near the center of the supply because
the analog switch tree changes from N-channel switches to
P-channel switches. These limitations are automatically sat-
isfied in ratiometric systems and can be easily met in ground
referenced systems.

Figure 10 shows a ground referenced system with a sepa-
rate supply and reference. In this system, the supply must be
trimmed to match the reference voltage. For instance, if a
5.12V is used, the supply should be adjusted to the same
voltage within 0.1V.

The ADC0808 needs less than a milliamp of supply current
so developing the supply from the reference is readily ac-
complished. In

Figure 11 a ground referenced system is

shown which generates the supply from the reference. The
buffer shown can be an op amp of sufficient drive to supply
the milliamp of supply current and the desired bus drive, or if
a capacitive bus is driven by the outputs a large capacitor will
supply the transient supply current as seen in

Figure 12. The

LM301 is overcompensated to insure stability when loaded
by the 10 µF output capacitor.

The top and bottom ladder voltages cannot exceed V

and

ground, respectively, but they can be symmetrically less than
V

and greater than ground. The center of the ladder volt-

age should always be near the center of the supply. The sen-
sitivity of the converter can be increased, (i.e., size of the
LSB steps decreased) by using a symmetrical reference sys-
tem. In

Figure 13, a 2.5V reference is symmetrically cen-

tered about V

/2 since the same current flows in identical

resistors. This system with a 2.5V reference allows the LSB
bit to be half the size of a 5V reference system.

DS005672-7

FIGURE 9. Ratiometric Conversion System

ADC0808/ADC0809

www.national.com

Applications Information

(Continued)

4.0 ANALOG COMPARATOR INPUTS

The dynamic comparator input current is caused by the pe-
riodic switching of on-chip stray capacitances. These are
connected alternately to the output of the resistor ladder/
switch tree network and to the comparator input as part of
the operation of the chopper stabilized comparator.

The average value of the comparator input current varies di-
rectly with clock frequency and with V

as shown in

Figure 6.

If no filter capacitors are used at the analog inputs and the
signal source impedances are low, the comparator input cur-
rent should not introduce converter errors, as the transient
created by the capacitance discharge will die out before the
comparator output is strobed.

If input filter capacitors are desired for noise reduction and
signal conditioning they will tend to average out the dynamic
comparator input current. It will then take on the characteris-
tics of a DC bias current whose effect can be predicted con-
ventionally.

Typical Application

TABLE 2. Microprocessor Interface Table

PROCESSOR

READ

WRITE

INTERRUPT (COMMENT)

8080

MEMR

MEMW

INTR (Thru RST Circuit)

8085

INTR (Thru RST Circuit)

Z-80

INT (Thru RST Circuit, Mode 0)

SC/MP

NRDS

NWDS

SA (Thru Sense A)

6800

VMA

·R/W

VMA

·R/W

IRQA or IRQB (Thru PIA)

DS005672-10

*Address latches needed for 8085 and SC/MP interfacing the ADC0808 to a microprocessor

ADC0808/ADC0809

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Notes

LIFE SUPPORT POLICY

NATIONAL'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL
COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:

1. Life support devices or systems are devices or

systems which, (a) are intended for surgical implant
into the body, or (b) support or sustain life, and
whose failure to perform when properly used in
accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.

2. A critical component is any component of a life

support device or system whose failure to perform
can be reasonably expected to cause the failure of
the life support device or system, or to affect its
safety or effectiveness.

National Semiconductor
Corporation
Americas
Tel: 1-800-272-9959
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www.national.com

ADC0808/ADC0809

8-Bit

µP

Compatible

A/D

Converters

with

8-Channel

Multiplexer

National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.

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