ADT70

REV. 0

–10–

If PRTD has a tempco resistance of 0.00385

Ω/Ω/°C or sensi-

tivity of 3.85

Ω/°C, the system output voltage scaling factor will

be 5 mV/

°C.

The gain of the instrumentation amplifier is normally at 1.30,

with a 49.9 k

Ω gain resistor. It can be changed by changing the

gain resistor using the following equation.

Instrumentation Amp Gain

k

R

GAIN RESISTOR

.

.

=

Ω









130

49 9

In Figure 2 the ADT70 is powered by a dual power supply. In

order for the part to measure below 0

°C, using a 1 kΩ PRTD,

sured temperature is greater than 0

°C using a 1 kΩ PRTD. GND

Sense (Pin 13), DGND (Pin 15), and AGND (Pin 2) are all con-

nected to ground. If desired, GND Sense could be connected to

whatever potential desired for an output offset of the instrumen-

tation amplifier. However, AGND and DGND must always be

connected to GND.

ADT70 will turn off if the

SHUTDOWN pin(GND) is low,

and will turn on when

If

SHUTDOWN is not used in the design, it should be con-

The undedicated op amp in the ADT70 can be used to transmit

measured signal to a remote location where noise might be intro-

duced into the signal as it travels in a noisy environment. It can

also be used as a general purpose amplifier in any application de-

sired. The op amp gain is set using standard feedback resistor

configurations.

Higher precision of matching the current sources can be

achieved by using a 50 k

Ω potentiometer connected between

NULLA (Pin 5) and NULLB (Pin 6) with the center-tap of the

ADT70’s Bias Pin (Pin 4) is generally connected to the

reference if different output current is preferred.

Eliminating Lead-Wire Resistance by Using 4-Wire

Configuration

In applications where the lead-wire resistance can significantly

contribute error to the measured temperature, implementing a

4-wire lead-resistance canceling circuit can dramatically mini-

mize the lead-wire resistance effect.

the reference resistor and the PRTD respectively. The lead-re-

sistance from the current source to the PRTD or reference resis-

tor is not of concern because the instrumentation amplifier is

measuring the difference in potential directly on the PRTD

(Node A) and reference resistor (Node C). Since there is almost

no current going from Node A and Node C into the amplifier’s

input, there is no lead-wire resistance error.

A potential source of temperature measurement errors is the

possibility of voltage differences between the ground side of the

reference resistor and the PRTD. Differences in lead-wire resis-

tance from ground to these two points, coupled with the 1 mA

excitation current, will lead directly to differential voltage errors

at the input of the instrumentation amplifier of the ADT70. By

connecting the ground side of the PRTD (Node B of Figure 28)

to the noninverting input of the op amp and connecting the

ground side of the reference resistor (Node D) to both the in-

verting input and the output of the op amp, the two points can

be forced to the same potential. It is not important that this po-

tential is exactly at ground since the instrumentation amplifier

rejects common-mode signals at the input. Note that all three

connections should be made as close as possible to the body of

the reference resistor and the PRTD to minimize error.

Single Supply Operation

When using the ADT70 in single supply applications a few

simple but important points need to be considered. The most

important issue is ensuring that the ADT70 is properly biased.

To bias the ADT70, first consider the 1 k

Ω PRTD sensor. The

PRTD typically changes from 230

Ω at –200°C to 4080 Ω at

800

°C ± 1 Ω error. This impedance range results in an ADT70

output of –1 V to +4 V respectively, which is impossible to

RGA

RGB

INST

AMP

SHUT-

DOWN

GND

SENSE

DGND

SHUTDOWN

2.5V

REF

MATCHED

CURRENT

SOURCES

NULLA

NULLB

BIAS

ADT70

50k

1k

PRTD

NODE D

1k

REF

RESISTOR

NODE C

NODE B

NODE A

5V

AGND

5V

Figure 28. 4-Wire Lead-Wire Resistance Cancellation Circuit