ADT70

REV. 0

–12–

because the input voltage range of the on-board op amp only ex-

tends to about 1 V above the negative rail. If the application re-

quires the inputs of either the on-board amp or instrumentation

amplifier to operate within 1 V of ground, it will be necessary to

generate a “pseudo-ground.” Figure 31 illustrates a typical

ADT70 “pseudo-ground” application. The Analog Devices’

ADR290, a 2.048 V reference, is being used to generate the

“pseudo-ground.” The ADR290 was selected for the following

reasons: low noise, ability to drive the required 5 mA in this

application, good temperature stability, which is usually impor-

tant in a PRTD application. However, one undesired effect of

introducing the pseudo-ground is the loss in voltage range at

high temperature. In our example, the PRTD will only operate

from –200

°C to +400°C corresponding to an input voltage

range of 1 V to 4 V.

100

PRTD Application Circuit

A 1000

Ω PRTD sensor scales by 3.85 Ωs/°C, which is exactly

ten times the scale of the 100

Ω PRTD sensor. The ADT70

has been designed to allow for 1000

Ω or 100 Ω PRTD sen-

sors. Only the gain setting resistor RG needs to be altered. For

a 100

Ω PRTD 0.00385 sensor, change RG to 4.99 kΩ as illus-

trated in Figure 32. In single supply application, with a 100

Ω

PRTD sensor, a “pseudo-ground” will be necessary because the

inputs of the instrumentation amplifier will be within 1 V of the

negative rail. See the section on single supply applications for

more information.

INST

AMP

RG

RG

4.99k

GND

SENSE

OUT

Figure 32. 100

Ω 0.00385 PRTD Application Showing

New Value for RG

INST

AMP

SHUT-

DOWN

GND

SENSE

DGND

SHUTDOWN

2.5V

REF

MATCHED

CURRENT

SOURCES

NULLA

NULLB

BIAS

1k

PRTD

RG

RG

49.9k

OUT

IN

GND

ADR290

0.1 F

0.1 F

10 F

OUT

AGND

1k

REF

RESISTOR

NODE D

NODE C

+5V

+5V

ADT70

Figure 31. Single Supply Application with an ADR290 “Pseudo-Ground”