ADT70

REV. 0

–11–

achieve in a single supply application where the negative rail is

ground or 0 V. Therefore, to achieve full scale operation the

output of ADT70 should be shifted by 1 V to allow for opera-

tion in the 0 V to 5 V region.

The most straightforward method to shift the output voltage

incorporates the use of the GND SENSE as shown in Figure 29.

To shift output voltage range apply a potential equal to the neces-

sary shift on the GND SENSE pin. For example, to shift the out-

GND SENSE. When applying a potential to GND SENSE, care

should be taken to ensure that the voltage source is capable of driv-

ing 2 k

Ω and does not introduce excessive noise. Figure 29 uses the

on-board 2.5 V voltage reference for a low noise source. This refer-

ence is then divided to 1 V and buffered by the on-board op amp

to drive GND SENSE at a low impedance. A small 500

Ω potenti-

ometer can be used to calibrate the initial offset error to zero.

However, a voltage applied to GND SENSE is not the only

method to shift the voltage range. Placing a 768

Ω resistor in the

PRTD sensor path also shifts the output voltage by 1 V. This

second method, as shown in Figure 30, is usually not recom-

mended for the following reasons; the input voltage range of the

op amps is limited to around 1 V from the negative and positive

rails and this could cause problems at high temperature, limiting

the upper range to 600

°C; the physical location of this resistor

(if placed at a distance from the ADT70) may have an impact

on the noise performance. The method frees up the on-board op

amp for another function and achieves the lowest impedance

ground point for GND SENSE.

This brief section on ADT70 single supply operation has focused

on simple techniques to bias the ADT70 such that all output volt-

ages are within operational range. However, these techniques may

not be useful in all single supply applications. For example, in Fig-

ure 3 the additional on-board op amp is operating at near ground

potential which will create problems in a single supply application

INST

AMP

SHUT-

DOWN

GND

SENSE

DGND

SHUTDOWN

2.5V

REF

MATCHED

CURRENT

SOURCES

NULLA

NULLB

BIAS

1k

PRTD

SENSOR

RG

RG

49.9k

1k

REF

RESISTOR

TO CONTROLLER

15k

9.76k

500

POT

ADT70

Figure 29. A Single Supply Application with Shifted Ground Sense Pin

INST

AMP

SHUT-

DOWN

GND

SENSE

DGND

SHUTDOWN

2.5V

REF

MATCHED

CURRENT

SOURCES

NULLA

NULLB

BIAS

1K

PRTD

RG

RG

49.9k

768

RESISTOR

TO CONTROLLER

1K

REF

RESISTOR

TO A/D CONVERTER

+5V

ADT70

Figure 30. A Basic Single Supply Operational Diagram with Bias Resistor in Sensor Path