ADT70GN Analog Devices Inc, ADT70GN Datasheet - Page 13

• Manufacturer Part Number: ADT70GN
• Manufacturer: Analog Devices Inc

Specifications of ADT70GN

• Operating Temperature (min): -40C
• Operating Temperature (max): 125C
• Operating Temperature Classification: Automotive
• Mounting: Through Hole
• Pin Count: 20
• Package Type: PDIP
• Lead Free Status / RoHS Status: Not Compliant

REV. 0
American PRTD Application Circuit
The majority of PRTD sensors use a scale factor of 0.00385 / / 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 / / 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 when yielding a
5 mV/ C ADT70 output.
Figure 33. Typical PRTD Application with American
0.003916
1k
PRTD
/
49.9k
1k
REF
RESISTOR
/ C Scale; 1 k
COLUMBIA RESEARCH LAB
MODEL DT3617
STRAIN SENSOR
R = 1k
NOTE: IDEAL VALUE
FOR RG
2k
R
R
I
I
OUTA
OUTB
+IN
IN
RG
RG
Figure 34. Typical Strain Sensor Application (Two Element Varying)
IA
IA
= 51k
˜
Scale
R
R
INST
AMP
RG
RG
I
I
+IN
OUTB
OUTA
GND
SENSE
IN
NULLA
IA
IA
NULLB
INST
AMP
OUT
SOURCES
MATCHED
CURRENT
BIAS
–13–
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 to 5 k impedance
range. The sensor used in this circuit has two elements varying.
If a constant current is driven into the sensor, a linear V
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.
2.5V
2.5V
REF
REFOUT
5V
DOWN
SHUT-
ADT70
DGND
SHUTDOWN
+V
OUT
+IN
IN
S
OA
OA
OA
+5V
ADT70
OUT
is