ADT70GR-REEL7 Analog Devices Inc, ADT70GR-REEL7 Datasheet - Page 10

• Manufacturer Part Number: ADT70GR-REEL7
• Manufacturer: Analog Devices Inc

Specifications of ADT70GR-REEL7

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

ADT70
If PRTD has a tempco resistance of 0.00385 / / C or sensi-
tivity of 3.85 / 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 gain resistor. It can be changed by changing the
gain resistor using the following equation.
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 k PRTD,
sured temperature is greater than 0 C using a 1 k 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 SHUTDOWN pin becomes high (+V
If SHUTDOWN is not used in the design, it should be con-
nected to +V
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 potentiometer connected between
NULLA (Pin 5) and NULLB (Pin 6) with the center-tap of the
potentiometer connected to +V
ADT70’s Bias Pin (Pin 4) is generally connected to the
V
reference if different output current is preferred.
–V
REFOUT
S
has to be at least –1 V. –V
Instrumentation Amp Gain
(Pin 3), but it can be connected to an external voltage
S
.
NODE D
NODE A
NODE B
PRTD
1k
RESISTOR NODE C
S
1k
REF
S
can be grounded when the mea-
(Pin 20). In Figure 27, the
Figure 28. 4-Wire Lead-Wire Resistance Cancellation Circuit
1 30
I
+IN
OUTB
R
IN
NULLA
GAIN RESISTOR
IA
IA
49 9
I
RGA
OUTA
50k
k
NULLB
INST
AMP
MATCHED
SOURCES
CURRENT
RGB
GND
SENSE
S
BIAS
).
–10–
OUT
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.
In Figure 28, I
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 PRTD sensor. The
PRTD typically changes from 230
800 C
output of –1 V to +4 V respectively, which is impossible to
IA
2.5V
2.5V
REF
REFOUT
AGND
1
5V
V
error. This impedance range results in an ADT70
S
OUTA
ADT70
DOWN
SHUT-
DGND
and I
OUTB
+V
OUT
+IN
SHUTDOWN
IN
S
OA
OA
5V
OA
provides matching excitation to
at –200 C to 4080
REV. 0
at