ADT70

REV. 0

–13–

American PRTD Application Circuit

The majority of PRTD sensors use a scale factor of 0.00385

Ω/Ω/°C.

This type of sensor is known as the European PRTD and is the most

common PRTD sensor. However, there is also an American PRTD

sensor that uses a scale factor of 0.00392

Ω/Ω/°C. Figure 33 illus-

trates the input section of the ADT70 configured for the Ameri-

can PRTD. The ideal value for RG is 50.98 k

Ω when yielding a

5 mV/

°C ADT70 output.

INST

AMP

RG

RG

49.9k

2k

1k

REF

RESISTOR

1k

PRTD

NOTE: IDEAL VALUE

GND

SENSE

OUT

Figure 33. Typical PRTD Application with American

0.003916

Ω/ Ω /°C Scale; 1 kΩ Scale

INST

AMP

SHUT-

DOWN

DGND

SHUTDOWN

2.5V

REF

MATCHED

CURRENT

SOURCES

NULLA

NULLB

BIAS

RG

RG

+5V

R

R

R

R

COLUMBIA RESEARCH LAB

MODEL DT3617

STRAIN SENSOR

R = 1k

5V

ADT70

Figure 34. Typical Strain Sensor Application (Two Element Varying)

Strain Gauge Sensor Application Circuit

Figure 34 illustrates a typical strain gauge bridge circuit. The

versatility of the ADT70 allows the part to be used with most

bridge circuits that are within the 50 k

Ω to 5 kΩ impedance

range. The sensor used in this circuit has two elements varying.

obtained. In addition, the ADT70 will work with most bridge

circuits whether one-, two-, or all-element varying.

Securing Additional Current from the Current Sources

Some sensor applications need a higher excitation current to in-

crease sensor sensitivity. There are two methods to increase the

current from the on-board current sources of the ADT70. The

most flexible method involves changing the voltage at the BIAS

node. The equation for determining the BIAS potential vs. Out-

put current is 2.5 V for roughly 1 mA, or in other words, to

double the current output simply put 5 V into BIAS. The BIAS

node should be driven with a low-noise source, such as a refer-

ence, because output current is directly dependent on BIAS volt-

age. Directly tying BIAS to the positive supply rail may produce

too much current noise especially if the positive rail is not well

regulated. The second method involves tying the two ADT70

current outputs together which doubles the current. Of course,

this technique is most useful if, as illustrated in Figure 34, the ap-

plication requires only one current source.