Specifications Version 3.0

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phone: **31-76-5205353

4811 AP Breda, The Netherlands

fax: **31-76-5205354

Universal Transducer Interface (UTI)

Revolution in Sensor Interfacing

Features

Provides interfacing for many types of sensor
elements:

capacitors, platinum resistors, thermistors, resistive

bridges and potentiometers

Measurement of multiple sensor elements

Single 2.9 V - 5.5 V power supply, current
consumption below 2.5 mA

Resolution and linearity up to 14 bits and 13 bits

Continuous auto-calibration of offset and gain

Microcontroller-compatible output signal

Tri-state output

Typical measurement time 10 ms or 100 ms

2/3/4-wire measurement available for almost all
measurements

AC excitation voltage signal for all sensor elements

Suppression of 50/60 Hz interference.

Power down mode

Temperature range -40ºC to 85ºC

Die operating temperature up to 180ºC

General Description

The Universal Transducer Interface (UTI) is a complete
analog front end for low frequency measurement
applications, based on a period-modulated oscillator.
Sensing elements can be directly connected to the UTI
without the need for extra electronics. Only a single
reference element, of the same kind as the sensor, is
required. The UTI outputs a microcontroller-compatible
period-modulated signal. The UTI can provide
interfacing for:

Capacitive sensors 0 - 2 pF, 0 -12 pF, variable range
up to 300 pF

Platinum resistors Pt100, Pt1000

Thermistors 1 k

- 25 k

Resistive bridges 250

- 10 k

with maximum

imbalance +/- 4% or +/- 0.25%

Potentiometers 1 k

- 50 k

Combinations of the above mentioned

The UTI is ideal for use in smart microcontroller-based
systems. One output-data wire reduces the number of
interconnect lines and reduces the number of opto-
couplers required in isolated systems. Continuous auto-
calibration of offset and gain of the complete system is
performed by using the three-signal technique. The low-

frequency interference is removed by using an advanced
chopping technique.
The function selection can be configured in both software
and hardware.

B
C

Logic control circuit

Frequency

Divider

Driver

Voltage to

Period

Converter

AMP/

C-V/

Divider

MUX

UTI

SEL1 SEL2 SEL3 SEL4 CML SF PD

OUT

Pin-out and Ratings

The UTI is available in a 16-pin plastic dual-in-line
package (DIP) as well as a 18-lead small outline package
(SOIC). Figure 1 shows the pin configurations of DIP
and SOIC. The function of the pins is listed in

Table 1

16-pins DIL

UTI

Top View

(Not to Scale)

CML

OUT

SEL4

SEL2

SEL3

SEL1

18-pins SOIC

UTI

Top View

(Not to Scale)

CML

OUT

SEL4

SEL2

SEL3

SEL1

(a) (b)

Figure 1 Pin configuration.

Name

Function of the pin

, V

Power supply

A, B, C, D, E, F

Sensor connections

SEL1..SEL4

Mode selection (see Table 2)

OUT

Output

Slow/fast mode selection

CML

CMUX02/CMUX12 mode selection

Power down (tri-state)

Table 1. Function of the pins.

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Absolute Maximum Ratings

= +25

General specifications

4.1

Functionality

4.2

Three-signal technique and

calibration

The three-signal technique is a technique to eliminate the
effects of unknown offset and unknown gain in a linear
system. In order to apply this technique, in addition to
the measurement of the sensor signal, two reference
signals are required to be measured in an identical way.
Suppose a system has a linear transfer function of

off

. (1)

The measured three signals are

off

ref

off

. (2)

Then the measuring result is the ratio

off

ref

off

ref

(3)

When the system is linear, then in this ratio the influence of
the unknown offset M

off

and the unknown gain k of the

measurement system is eliminated. This technique has been
used in the UTI.
The implementation of the three-signal technique
requires a memory: A microcontroller is used to perform
the data storage and the calculations, and to digitize the

Power supply voltage

-0.3 V to +7 V

Power supply current (excluding connection to the sensor)

3 mA

Power dissipation

21 mW

Power dissipation at power down

Output voltage

-0.3 V to V

+0.3 V

Output current

8 mA

Output impedance

Input voltage ref. to V

-0.3 V to V

+0.3 V

Input current on each pin

20 mA

ESD rating

> 4000 V

Storage temperature range

-65

C to +150

Operating temperature range

-40

C to +85

Lead temperature (soldering, 10 sec)

+300

SEL1

SEL2

SEL3

SEL4

Mode

No. of Phases

Name

Mode No.

5 Capacitors, 0-2pF

C25

3 Capacitors, 0-2pF

C23

5 Capacitors, 0-12pF

C12

Capacitors, 0-2pF, external MUX CML=0

Capacitors, 0-12pF, external MUX CML=1

CMUX

3 Capacitors, variable range to 300pF

C300

Platinum resistor Pt100-Pt1000, 4-wire

Thermistor 1k

-25k

, 4-wire

Ther

2 or 3 platinum resistors Pt100-Pt1000

Pt2

2 or 3 thermistors, 1k

-25k

Ther2

Resistive bridge, ref. is V

bridge

, +/- 200mV

Ub2

Resistive bridge, ref. is V

bridge

, +/- 12.5mV

Ub1

Resistive bridge, ref. is I

bridge

, +/- 200mV

Ib2

Resistive bridge, ref. is I

bridge

, +/- 12.5mV

Ib1

Res. bridge and two resistors, +/- 200mV

Brg2

Res. bridge and two resistors, +/- 12.5mV

Brg1

3 Potentiometers 1k

-50k

Potm

Table 2. Modes of the UTI , including the name of the modes and the number of phases within 1 cycle.

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period-modulated signals. Such a system combining a
sensing element (sensor), a signal-processing circuit,
such as the UTI, and a microcontroller is called a
microcontroller-based smart sensor system.

4.3

The measurement of sensing

elements

As an example, Figure 2 shows two complete cycles of
the output signal from the UTI, each consisting of three
phases.

GND

1 cycle

off

ref

off

ref

Figure 2. The output signal of the UTI for a 3-phase mode.

During the first phase T

off

, the offset of the complete

system is measured. During the second phase T

ref

, the

reference signal is measured and during the last phase T

the signal itself is measured. These phases are
automatically controlled by the UTI itself.
The duration of each phase is proportional to the signal
which is measured during that phase. The duration of the
three phases is given by:

For capacitive measurement:

For resistive measurement:

NK C

off

ref

(

)

(

)

NK V

off

ref

(

)

(

)

where C

or V

is the sensor signal to be measured, C

ref

the reference signal, C

or V

a constant part

(including offset voltages etc.) and K

or K

the gain. The

factor N represents the number of internal oscillator
periods in one phase. In slow mode, N = 1024 and in fast
mode N = 128. The voltages V

and V

ref

are, for instance,

the voltage across the sensor resistor and the reference
resistor or V

and V

ref

represent the bridge output voltage

and the bridge supply voltage, respectively.
The output signal of the UTI can be digitized by counting
the number of internal clock cycles fitting in each phase.
This results in the digital numbers N

off

, N

ref

and N

. The

ratio C

ref

or V

ref

can now be calculated by the

microcontroller:

off

ref

off

ref

off

ref

off

ref

(4)

This ratio does not depend on the constant part and the
gain. In fact, the system is calibrated for offset and gain.

Even in the case of drift or other slow variations of offset
and gain, these effects are eliminated.
The three phases are time-multiplexed, as depicted in
Figure 2. The offset phase is labeled, because it consists
of two short intervals: the output frequency is temporarily
doubled. This is recognized by the microcontroller,
which guarantees that the correct calculation, as depicted
in formula (4), is made.
The number of phases in a complete cycle varies between
3 and 5, depending on the mode.

4.4

Resolution.

The output signal of the UTI is digitized by the
microcontroller. This sampling introduces quantization
noise, which also limits the resolution. The quantization
noise of a measurement phase, as given by the relative
standard deviation

, amounts to

phase

(5)

where t

is the sampling time and T

phase

the phase

duration. When the sampling time is 1

s and the offset

frequency is 50 kHz, the standard deviation of the offset
phase is 12.5 bits in the fast mode and 15.5 bits in the
slow mode.
Further improvement of the resolution can be obtained by
averaging over several values of M. When P values M

...

are used to calculate

, the value of

decreases

with a factor of P

1/2

Besides quantization noise, another limitation of the
resolution is due to the thermal noise of the oscillator
itself. In the fast mode, quantization noise is found to be
the main noise source.
As an example,
Figure

depicts the measured resolution as a function of

the measurement time, using the CMUX mode.

1E-4

1E-3

1E-2

1E-1

1E+0

1E+1

Measurement time (s)

Resolution (pF)

meas

-1

meas

-0.5

Quantization noise

Electronic white noise

1E-2

1E-3

1E-4

1E-5

Figure 3 The resolution after the calculation required for the
three-signal technique versus the total measurement time. The
measurement range equals 0 - 2 pF and C

= 50 pF (see Figure

7).

For the CMUX mode, the resolution as a function of the
parasitic capacitance C

(see Fig. 7) is shown in

Figure 4

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0.0E+00 2.0E+02 4.0E+02 6.0E+02 8.0E+02 1.0E+03 1.2E+03

Parasitic capacitance (pF)

Resolution (pF)

4.5E-4

4.0E-4

3.5E-4

3.0E-4

2.5E-4

2.0E-4

1.5E-4

1.0E-4

5.0E-5

0.0E-0

Figure 4 The resolution after the calculation required for the
three-signal technique versus the parasitic capacitance C

. The

measurement range equals 0 - 2 pF.

4.5

Linearity

Typically, the linearity of the UTI has values between 11
bits and 13 bits, depending on the mode.

For the CMUX mode, the nonlinearity as a function of
the parasitic capacitance C

(see Figure 7) is shown in

Figure 5.

-5.0E-04

0.0E+00

5.0E-04

1.0E-03

1.5E-03

2.0E-03

2.5E-03

3.0E-03

3.5E-03

0.0E+00

2.0E+02

4.0E+02

6.0E+02

8.0E+02

1.0E+03

1.2E+03

Parasitic capacitance (pF)

Nonlinearity

Figure 5 The nonlinearity versus the parasitic capacitance C

The measurement range equals 0 - 2 pF.

Output

The UTI outputs a microcontroller-compatible period-modulated signal and excitation signals to drive the sensing
elements. Table 3 shows some output specifications of the UTI.

= 5 V, T

= +25

Parameter

Value

Unit

Conditions/Comments

, Output low voltage

0.4

V max

, Output high voltage

-0.6

V min

Output resistance at OUT

Maximum load at OUT

= 5 V

Output resistance at pins B, C, D, E and F

800

The pins B

F are used as output in the capacitive

modes, 0

Maximum output current from E and F

For resistive and resistive-bridge modes

Fast mode
Slow mode

14
14

ns
ns

Fast mode
Slow mode

13
13

ns
ns

Intrinsic propagation delay time t

PLH

(PD-OUT) t

PHL

30
30

ms
ms

These values are measured for the slow mode. For
the fast mode, these values are 8 times smaller.

Intrinsic propagation delay time t

PLH

(SELi-OUT) t

PHL

30
30

ms
ms

These values are measured for the slow mode. For
the fast mode, these values are 8 times smaller.

Table 3 Some output specifications of the UTI.

Analog inputs

Various sensing elements can be directly connected to the inputs of the UTI. The connections of the sensing elements
with the UTI for various modes are described in section 8. Table 4 shows some input specifications of the UTI.

= 5 V, T

= +25

Parameter

Value

Unit

Conditions/Comments

Input capacitance

Capacitance leakage between A to B, C, D, E, F

-3

DIP package

Suppression of 50/60 Hz

Table 4 Some input specifications of the UTI.

Rise time

Fall time

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Control lines

As described in section 4.1, the UTI has 16 operating
modes. These modes are selected by using four selection
pins, SEL1, SEL2, SEL3 and SEL4. The mode selection
can be performed in a software and a hardware way. In
Table 2, a `1' corresponds to V

and `0' to GND. Some

special functions are available, such as, slow/fast
selection and power-down. These modes are set by SF
and PD, respectively.
The pin SF is used to set the measurement speed. When
SF = 1, the UTI is working in the fast mode. In this mode
the duration of one complete cycle of the output signal is
about 10 ms. When SF = 0, the UTI is working in the
slow mode and the duration of one complete cycle of the
output signal is about 100 ms.
The pin PD is used to set the power-down of the UTI.
When PD = 0, the UTI is powered down and the output
node is floating (high-impedance). This enables to
connect several UTI outputs to a single output wire,
provided that only a single UTI is selected (PD = 1).
The pin CML is always connected to GND except in
mode CMUX. In mode CMUX, the pin CML is used for
the measurement-range selection. These ranges are 0 - 2
pF (CML = 0) and 0 - 12 pF (CML = 1), respectively.

All digital and analog inputs are protected for ESD.
Floating inputs are not allowed, unless otherwise stated.

Different modes

In this section, we specify the UTI properties for all
modes. The names of these modes are the same as used
in Table 2. In this section, CML = 0 and SF = 0 unless
otherwise stated. Important parameters to be specified
are:

the accuracy,

the resolution,

the number of phases,

the specified signals in the various phases.

The sequences of all phases are referred to the first one
(phase 1). In this phase, the constant part (or offset) is
measured. This phase also contains the synchronization
for the microcontroller, since the output frequency in this
phase is doubled.
During the described measurements, an Intel 87C51FA
microcontroller with 3 MHz sampling frequency is used,
but other types of microcontrollers can be used as well.

8.1

Mode 0. C25: 5 capacitors 0-2pF

In this mode, 5 capacitors in the range of 0 - 2 pF with
one common electrode can be measured.

The connection of capacitors is depicted in Figure 6. The
parasitic capacitance C

is, for instance, the capacitance

of the cables. All measured capacitors should have a
common receiver electrode, connected to node A. The
signal at the transmitting electrodes (B to F) is a square
wave with amplitude V

. When a capacitor is not

measured, the node corresponding to this capacitor is
internally grounded. In mode C25, one cycle takes 5
measurement phases as depicted in Table 5.

UTI

Figure 6. Connection of capacitors to the UTI.

Phase

Measured capacitors

Output periods

+ C

NK C

(

)

+ C

NK C

(

)

+ C

NK C

(

)

+ C

NK C

(

)

+ C

NK C

(

)

Table 5. Measured capacitors during each phase.

In phase 1 the input capacitor C

+ C

is measured. In

this phase the output frequency is doubled, resulting in
two short periods. This enables synchronization of the
microcontroller. Generally, no capacitor is connected
between B and A. The specifications for the mode C25
(mode 0) are listed in Table 6.

Parameter

Typical value

s/pF

2 pF

Maximum capacitance C

2 pF

Linearity

13 bits

Resolution (SF = 0, C

=30 pF)

14 bits

Remaining offset

< 15

-3

Table 6. Specifications of the C25 and C23 modes.

The remaining offset capacitance is caused by the
parasitics between bonding wires, bonding pads and IC
pins. When this offset is too large, one should use the
mode CMUX. In this case, an external multiplexer is
used and offset can be as low as 20

-6

pF.

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8.2

Mode 1. C23: 3 capacitors 0-2pF

In this mode, 3 capacitors in the range of 0 - 2 pF with
one common electrode can be measured. The difference
with mode C25 is that one cycle consists of only 3
phases. The connection of the capacitors is shown in
Figure 6, where C

and C

are omitted now. The

measured capacitors during each phase are listed in
Table 7. The specifications are listed in Table 6.

Phase

Measured capacitors

Output periods

NK C

(

)

NK C

(

)

NK C

(

)

Table 7. Measured capacitors during each phase of the mode
C23.

8.3

Mode 2. C12: 5 capacitors 0-12pF

In this mode, 5 capacitors in the range of 0 - 12 pF with
one common electrode can be measured. The connection
of the capacitors to the UTI is shown in Figure 6. The
maximum value of the capacitance C

(i is B, C, D or E)

is 12 pF. The number of phases is 5. The specifications
are listed in Table 8. The measured capacitors during
each phase are indicated in Table 5. The main difference
with mode 0 is that the maximum measurable
capacitance is 12 pF.

Parameter

Typical value

1.7

s/pF

12 pF

Maximum capacitance C

12 pF

Linearity

13 bits

Resolution (SF = 0, C

= 30 pF)

14 bits

Remaining offset

< 15

-3

Table 8. Specifications of the C12 mode.

-6

pF.

8.4

Mode 3. CMUX: capacitors 0-

2pF/0-12pF, external MUX

In this mode, an arbitrary number of capacitors in the
range of 0 - 2 pF (CML = 0) or the range of 0 - 12 pF
(CML = 1) with a common electrode can be measured.
The UTI does not perform a phase selection, so an
external multiplexer should be used. Just for this
application, Smartec developed a novel multiplexer MUX
with nine outputs and four inputs. The specifications of
the CMUX mode are listed in Table 9.

Parameter

Typical value

(CML = 0)

(CML = 1)

s/pF

1.7

s/pF

2 pF

12 pF

Maximum capacitance C

2 pF

12 pF

Linearity (C

< 300 pF)

13 bits

Offset

-5

Resolution (SF = 0, C

< 30 pF)

14 bits

Table 9. Specifications of the CMUX mode.

A possible measurement setup is shown in Figure 7. An
external multiplexer, which is controlled by the
microcontroller (

C), multiplexes the signal at node B to

one (or more) of the capacitors. The UTI output appears
on the node "output". Nominal frequencies of the output
signal during an offset measurement (none of the
capacitors are selected) are 6 kHz (SF = 1) and 50 Hz (SF
= 0).

MUX

UTI

output

Figure 7. Possible measurement setup in the CMUX mode to
measure multiple capacitors.

8.5

Mode 4. C300: 3 capacitors, range

up to 300pF

In this mode, 3 capacitors in a variable range up to 300
pF with a common electrode can be measured. The
connection of sensors and external resistors is depicted in
Figure 8. These resistors set the voltage swing at the
transmitting electrode of C

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UTI

Figure 8. Connection of sensors to the UTI for the C300 mode.

The total capacitance at node A must be limited to 500
pF in order to keep the nonlinearity below 10

-3

. The

voltage swing at the transmitting electrodes equals V

which is set externally by means of three inaccurate
resistors R

, R

and R

, of which R

or R

may be zero.

The DC voltage V

should satisfy the following

condition:
V

< K

max

where the constant K

= 60 V·pF, and C

max

is the

maximum value of C

, C

and C

expressed in pF.

The total time constant of all resistors and capacitors
should be less than 500 ns. This sets the values of the
resistors.
Example. When C

= 300 pF, C

= 200 pF, C

= 0 and

= 5 V , practical values of the resistors are R

= 25

, R

= 1 k

and R

= 0. The voltage swing V

at the

transmitting electrode amounts to 0.2 V.
The system contains two time constants C

tot

·(R

//(R

))

and C

tot

·(R

//(R

)), where C

tot

= C

Both time constants must be smaller than 500 ns.
The nonlinearity and resolution in the slow mode are
depicted in Table 10. Here, the value of C

= 0 pF, C

30 pF and V

has the maximum value K

max

, as

described before. The measured capacitors during each
phase are listed in Table 11.

Capacitors

Nonlinearity

Resolution (pF)

=33 pF

1.4

-4

1.2

-3

=150 pF

1.9

-4

6.6

-3

=270 pF

9.0

-4

-3

=330 pF

2.6

-3

=560 pF

6.3

-3

Table 10. Values of nonlinearity and resolution in C300 mode
for different capacitor values.

Phase

Capacitor

Output periods

+ C

NK C

(

)

NK C

(

)

NK C

(

)

Table 11. The measured capacitors during each phase of the
mode C300.

8.6

Mode 5. Pt: 1 platinum resistor

Pt100/ Pt1000, 4-wire

In this mode, one platinum resistor and one reference
resistor can be measured.
The connection of the resistors to the UTI is depicted in
Figure 9. Because of the use of force/sense wires, both
resistors R

and R

ref

are measured in a 4-wire setup,

thereby completely eliminating the effect of lead
resistances. The driving voltage V

is a square wave

with amplitude V

at 1/4 of the internal oscillator

frequency. Resistors R

BIAS1

and R

BIAS2

are used to set the

current through the chain. When these two resistor
values are equal, due to the symmetry, the highest
accuracy is achieved. When one of these two resistors is
zero, this will not affect the linearity, however, the
accuracy will be decreased. For instance, for a
measurement of a Pt100, this inaccuracy amounts to

One measurement cycle consists of 4 phases. These
phases contain the information for a 2-, 3- or 4-wire
measurement.

Phase

Measured voltages

Output periods

NK V

off

NK V

(

)

NK V

(

)

NK V

(

)

Table 12. Measured voltages during the measurement of
platinum resistors

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ref

BIAS2

BIAS1

UTI

ref

BIAS1

ref

BIAS1

UTI

BIAS2

Figure 9. Connection of platinum resistors to the UTI in a 4-
wire (a), 3-wire (b) and a 2-wire (c) connection.

To calculate the ratio as in (4), we have to make different
calculations for the 2-, 3- and 4 wire measurement:

wire

phase

ref

wire

phase

ref

- -

-
-

. (6)

The linearity is better than 13 bits provided that the
amplitude of the voltages V

and V

is below 0.7 V

for V

= 5V. For V

= 3.3 V, these voltages should be

less than 0.4 V. This limits the current through the
platinum resistor.
Current limitation is also required to limit the error due
to self-heating. For instance, for a thermal resistance of
200 K/W (still air) at V

= 0.7 V and 0ºC, the self-

heating effect of a Pt100 causes an error of 1 K. If this
self-heating error is too large, R

BIAS

(= R

BIAS1

+ R

BIAS2

)

must be increased to limit the current through the Pt100.
For V

= 0.2 V, the temperature error due to self-heating

would amount to 80 mK. This is two times less than the
initial inaccuracy of a class A Pt100. In this case, the
current through the Pt100 amounts to 2 mA which
requires R

BIAS

= R

BIAS1

+ R

BIAS2

= 2.2 k

The relative sensitivity of a Pt100 is 3.9

-3

/K. When

the current through the Pt100 is 2 mA, this sensitivity
corresponds to 780

V/K. The resolution in this mode is

V, corresponding to 9 mK. This holds for the slow

mode.

Table 13 lists the specifications of the UTI in the Pt
mode.

Parameter (V

= 5 V)

Typical value

s/V

0.36 V

BIAS

(Pt100, self-heating for

200K/W = 80 mK)

2.2 k

(5%), I = 2

BIAS

(Pt1000, self-heating for

200K/W = 80 mK)

6.2 k

(5%), I =

600

Excitation current from E and F

20 mA

Offset

Linearity

13 bits

Resolution (SF = 0) (Pt100, 2 mA)

14 bits (9 mK)

Table 13. Specifications of the Pt mode.

Amplitudes of V

and V

up to 2.5 V peak-to-peak are

allowed, but self-heating effects and nonlinearity have to
be taken into account. Very good resolutions can be
obtained in this case.
However, the linearity can decrease to 8 bits for peak-
to-peak amplitudes in the range of 0.7-2.5 V.
Platinum resistors can also be measured using mode 11.

8.7

Mode 6. Ther: 1 thermistor, 4-wire

In this mode, one thermistor and one reference resistor
can be measured. The connection of the thermistor and
the reference resistor is shown in Figure 10.

ref

UTI

ref

Figure 10. Connection of the thermistor to the UTI in a 4-wire
(a), 3-wire (b) and 2-wire (c) connection.

The driving voltage V

is a chopped voltage with an

amplitude of V

/12.5 (0.4 V at V

= 5 V) and a DC

value V

/2.

The ratio of the thermistor and the reference resistor is
also given by (6). The signals, which are measured

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during the various phases, are listed in Table 12. The
voltage V

is not constant, but has the same temperature

information as V

, which linearises the sensor

characteristic.

Parameter (V

= 5 V)

Typical value

s/V

0.36 V

ref

//R

<5 k

ref

>1 k

Offset

Linearity

13 bits

Resolution (SF = 0)

V (1 mK)

Table 14. Specifications of the Ther mode.

For very large and very small values of R

(10 times or

0.1 times R

ref

), the resolution in voltage is still the same,

but the resolution in temperature is decreased. This is
due to the linearization method.
For a thermistor with a sensitivity of 4%/K, the
resolution is 1 mK for V

= 5 V.

8.8

Mode 7. Pt2: 2 or 3 platinum

resistors

In this mode, 2 or 3 platinum resistors can be measured.
The connection of the resistors to the UTI is shown in
Figure 11. The voltage V

is the same as in the mode Pt.

ref

BIAS1

UTI

ref

BIAS

UTI

BIAS2

Figure 11. Connection of 2 (a) or 3 (b) platinum resistors for
the Pt2 mode.

The same restrictions for the current through the resistors
as in the Pt mode holds here. The specifications are listed
in Table 13. Note that R

can be measured with a 4-wire

setup. Phase 5 can be used to measure just one lead
resistance or to measure R

The main difference with the Pt mode is that one
measurement cycle takes 5 phases, as listed in Table 15.

Phase

Measured voltages

Output periods

NK V

off

NK V

(

)

NK V

(

)

NK V

(

)

NK V

(

)

Table 15. Measured voltages during the phases of the Pt2
mode.

In the Figure 11(a), one of two bias resistors R

BIAS1

and

BIAS2

can be zero. However, in this case the accuracy

will be decreased. With the connection shown in Figure
11(b), the effect of lead resistances can not be eliminated.
Especially, when R

is measured with the connection

shown in Figure 11(b), the internal connection wires of
the UTI will cause an error of 0.9

for the Pt100 and 3

for the Pt1000, respectively. This measured error

depends on the supply current of the platinum resistors,
and temperature.

8.9

Mode 8. Ther2: 2 or 3 thermistors

In this mode, 2 or 3 thermistors can be measured. The
connection is depicted in Figure 12. The number of
phases is also 5, as listed in Table 15. The specifications
listed in Table 14 also hold for this mode.
With the connection shown in Figure 12(b), the effect of
lead resistances can not be eliminated. Especially, when
R

is measured with the connection shown in Figure

12(b), the internal connection wires of the UTI will cause
an error of 11.5

for the resistor R

with a value of 2.5

. This measured error depends on the supply current

of the thermistor, and temperature.

ref

UTI

ref

UTI

Figure 12. Connections of 2 (a) and 3 (b) thermistors to the
UTI.

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8.10

Mode 9. Ub2: resistive bridge, ref.

is V

bridge

, +/- 4% imbalance

In this mode, a resistive bridge can be measured where
the ratio of the bridge output voltage V

and the bridge

supply voltage V

represents the physical signal.

The measurement range of the bridge imbalance is +/-
4% in this mode.
The connection of the bridge to the UTI is shown in
Figure 13. The driving voltage across the bridge V

is a

square wave with amplitude V

. The frequency of this

signal is 1/4 of the internal oscillator frequency.
Because of the use of force/sense wires, the bridge is
measured in a 4-wire setup, as shown in Figure 13(a).
The signals measured in the various phases are indicated
in Table 16.

UTI

Figure 13. Connection of the resistive bridge to the UTI for the
Ub2 mode in a 4-wire setup (a) and a 2-wire setup (b).

During phase 2, the voltage across the bridge V

measured. A very accurate on-chip voltage divider
divides this voltage by 32. This divider does not need
calibration. After division, V

is processed in the same

way as V

Phase

Measured voltages

Output periods

NK V

off

/32 +V

NK V

(

)

NK V

(

)

Table 16. Measurement phases of the Ub2 mode.

To find the bridge imbalance, the microcontroller
calculates:

phase

-
-

. (7)

The specifications are listed in Table 17.

Parameter

Typical value

s/V

0.54 V

Bridge excitation

AC V

Excitation current from E and F

20 mA

Bridge resistance R

250

< R

< 10 k

Bridge output voltage

max +/- 0.2V

Accuracy

11 bits

Offset

Resolution (SF = 0)

Table 17. Specifications of the Ub2 mode.

8.11

Mode 10. Ub1: res. bridge, ref. is

bridge

, +/- 0.25% imbalance

In this mode, a resistive bridge can be measured where
the ratio of the bridge output voltage and the bridge
supply voltage represents the physical signal.
The main difference with mode Ub2 is that the
measurement range of the bridge imbalance is 0.25%.
(V

= 12.5 mV for V

= 5V).

The connection of the bridge to the UTI is the same as in
the Ub2 mode. An on-chip 15-times voltage amplifier
amplifies the small output voltage before it is processed
in the same way as the divided voltage across the bridge.
Both the amplifier and divider do not need calibration.
To calculate the bridge imbalance, Equation (7) can be
used, where 32 must be replaced by 480. Due to the use
of the force/sense wires, the bridge is measured in a 4-
wire setup.
The various voltages measured during each phase are
indicated in Table 18. The specifications are listed in
Table 19.

Phase

Measured voltages

Output periods

NK V

off

/32 + V

NK V

(

)

15V

+ V

(

)

Table 18. Measured voltages during each phase of the Ub1
mode.

Parameter

Typical value

s/V

0.54 V

Bridge excitation

AC V

Excitation current from E and F

20 mA

Bridge resistance R

250

< R

< 10 k

Bridge output voltage

max +/- 12.5 mV

Accuracy

10 bits

Offset

Resolution (SF = 0)

700 nV

Table 19. Specifications of the Ub1 mode.

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fax: **31-76-5205354

8.12

Mode 11. Ib2: resistive bridge, ref.

is I

bridge

, +/- 4% imbalance

In this mode, a resistive bridge can be measured where
the physical signal is represented by the output voltage of
the bridge and the current through the bridge. This
current I is converted into a reference voltage. The
connection of the bridge and the reference element is
shown in Figure 14(a).
The value of R

ref

should be chosen such that V

between 0.1 V and 0.2 V.

This mode can also be used to measure platinum resistors
in a 4-wire setup. This is shown in Figure 14(b). The
advantage in comparison with mode Pt is that now only
three phases have to be measured.

UTI

ref

Pt100

BIAS1

ref

BIAS2

Figure 14. Connection of the resistive bridge and a reference
resistor to the UTI (a) and connection of a platinum resistor in
4-wire setup (b).

Phase

Measured voltages

Output periods

NK V

off

NK V

(

)

NK V

(

)

Table 20. Measured voltages for each phase of the Ib2 mode.

Parameter

Typical value

s/V

0.54 V

Bridge excitation

AC V

Excitation current from E and F

20 mA

Bridge resistance R

250

< R

< 10 k

Bridge output voltage

max +/- 0.2 V

Accuracy

12 bits

Offset

Resolution (SF=0)

Table 21. Specifications of the Ib2 mode.

8.13

Mode 12. Ib1: resistive bridge, ref.

is I

bridge

, +/- 0.25% imbalance

This mode is similar to mode 11. The connection of the
bridge and the resistor is shown in Figure 14.
The difference with mode 11 is that the bridge
imbalance range is +/- 0.25%. The voltage across the

reference resistor should be between 0.1 V and 0.2 V, as
in mode 11.
The bridge output voltage is amplified 15 times before it
is processed in the same way as the reference voltage.
The voltages measured during each phase are indicated
in Table 22. The specifications of the Ib1 mode are listed
in Table 23.

Phase

Measured voltages

Output periods

NK V

off

+ V

NK V

(

)

15V

+ V

(

)

Table 22. Measured voltages during each phase of the Ib1
mode.

To find the bridge imbalance, the microcontroller
calculates

phase

ref

-
-

. (8)

Parameter

Typical value

s/V

0.54 V

Bridge excitation

AC V

Excitation current from E and F

20 mA

Bridge resistance R

250

< R

< 10 k

Bridge output voltage

max +/- 12.5 mV

Accuracy

10 bits

Offset

Resolution (SF = 0)

700 nV

Table 23. Specifications of the Ib1 mode.

8.14

Mode 13. Brg2: resistive bridge +/-

4% and 2 resistors

In this mode, a resistive bridge with a maximum
imbalance of +/-4% and two resistors can be measured.
One of the resistors can be temperature dependent, so the
bridge output can be digitally corrected for temperature
effects.
Both the voltage across the bridge and the current
through the bridge are measured. The connection of the
elements to the UTI is shown in Figure 15.
The voltage V

is a square wave with an amplitude

at 1/4 of the oscillator frequency. The voltage

across R

ref

should be between 0.1 V and 0.2V.

The voltages to be measured are indicated in Table 24.

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fax: **31-76-5205354

UTI

ref

Figure 15. Connections of the sensors to the UTI.

Phase

Measured voltages

Output periods

NK V

off

+ V

NK V

(

)

+ V

NK V

(

)

+ V

NK V

(

)

/32 + V

NK V

(

)

Table 24. Signals during the various phases of the mode Brg2.

The voltage across the bridge V

is divided by 32 before

it is processed in the same way as the other measured
voltages. The bridge imbalance V

is obtained from:

phase

-
-

. (9)

The specifications of this mode are listed in Table 25.

Parameter

Typical value

s/V

0.54 V

Excitation V

AC V

Excitation current from E and F

20 mA

Bridge resistance R

250

< R

< 10 k

Bridge output voltage

max +/- 0.2 V

Accuracy V

11 bits

Linearity V

12 bits

Offset V

or V

Resolution (SF = 0)

Table 25. Specifications of the Brg2 mode.

For the measurement of the signal V

, due to the effect

of the internal connection wires of the UTI, an error of
1.2% will be caused on the result of V

. This

measured error depends on the supply current of the
resistive bridge and temperature.

8.15

Mode 14. Brg1: resistive bridge +/-

0.25% and 2 resistors

This mode is similar to mode 13. The connection is
shown in Figure 15. The difference with mode 13 is that
the measurement range of the bridge imbalance is 0.25%.
The bridge output voltage V

is amplified 15 times

before it is processed further.
The voltages measured during each phase are indicated
in Table 26. The specifications are listed in Table 27.

Phase

Measured voltages

Output periods

NK V

off

+ V

NK V

(

)

15V

+ V

(

)

+ V

NK V

(

)

/32 + V

NK V

(

)

Table 26. Measured voltages during each phase of the Brg1
mode.

Parameter

Typical value

s/V

0.54 V

Excitation V

AC V

Excitation current from E and F

20 mA

Bridge resistance R

250

< R

< 10 k

Bridge output voltage

max +/- 12.5 mV

Accuracy V

10 bits

Linearity V

12 bits

Offset V

Resolution V

(SF = 0)

700 nV

Resolution V

(SF = 0)

Table 27. Specifications of the mode Brg1.

For the measurement of the signal V

, due to the effect

of the internal connection wires of the UTI, an error of
1.2% will be caused on the result of V

. This

measured error depends on the supply current of the
resistive bridge and temperature.

8.16

Mode 15. Potm: 3 potentiometers,

-25k

In this mode, 3 potentiometers in the range of 1 k

to 50

can be measured. The connection of potentiometers is

depicted in Figure 16. When only a single potentiometer
is measured with its slide connected to, for instance, node
B, nodes C and D should be connected to F. The voltage
across the potentiometers is a square wave with
amplitude V

and frequency 1/4 of the internal

oscillator frequency.

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UTI

Figure 16. Connection of potentiometers to the UTI.

It is not possible to compensate for the effect of lead
wires in this mode. Therefore, the use of low-ohmic
potentiometers should be avoided.
The measured voltages during each phase are indicated
in Table 28.

Phase

Measured Voltages

Output periods

NK V

off

NK V

(

)

NK V

(

)

NK V

(

)

NK V

(

)

Table 28. Measured voltages for each phase during
measurement of potentiometers.

The relative position M for each potentiometer is given
by:

phase

3 4 5

, ,

(10)

Parameter

Typical value

s/V

5 V

Potentiometer value R

1 k

< R

< 25 k

Accuracy

-3

Resolution (SF = 0)

14 bits

Table 29. Specifications of the Potm mode.

Chip Size

Figure 17 shows the pad configuration of the UTI chip.
The size of the die amounts to 3.1 mm

2.1 mm.

F CML OUT

SEL4

B SEL1 SEL2 SEL3

UTI

Figure 17 The pad configuration of UTI chip.

10. Development Kit

For actual development purposes, a development kit is
available. This kit can be connected directly to a personal
computer. Additional practical information can be found
in the UTI application note.

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