Datasheet

AD627 Data Sheet
Rev. E | Page 22 of 24
APPLICATIONS CIRCUITS
CLASSIC BRIDGE CIRCUIT
Figure 50 shows the AD627 configured to amplify the signal
from a classic resistive bridge. This circuit works in dual-supply
mode or single-supply mode. Typically, the same voltage that
powers the instrumentation amplifiers excites the bridge.
Connecting the bottom of the bridge to the negative supply of
the instrumentation amplifiers (usually 0 V, −5 V, −12 V, or
−15 V), sets up an input common-mode voltage that is
optimally located midway between the supply voltages. It is
also appropriate to set the voltage on the REF pin to midway
between the supplies, especially if the input signal is bipolar.
However, the voltage on the REF pin can be varied to suit the
application. For example, the REF pin is tied to the V
REF
pin of
an analog-to-digital converter (ADC) whose input range is
(V
REF
± V
IN
). With an available output swing on the AD627 of
(−V
S
+ 100 mV) to (+V
S
− 150 mV), the maximum programmable
gain is simply this output range divided by the input range.
V
OUT
V
DIFF
+V
S
–V
S
V
REF
0.1µF
0.1µF
AD627
R
G =
200kΩ
GAIN–5
00782-048
Figure 50. Classic Bridge Circuit
4 mA TO 20 mA SINGLE-SUPPLY RECEIVER
Figure 51 shows how a signal from a 4 mA to 20 mA transducer
can be interfaced to the ADuC812, a 12-bit ADC with an
embedded microcontroller. The signal from a 4 mA to 20 mA
transducer is single-ended, which initially suggests the need for
a simple shunt resistor to convert the current to a voltage at the
high impedance analog input of the converter. However, any
line resistance in the return path (to the transducer) adds a
current dependent offset error; therefore, the current must be
sensed differentially.
In this example, a 24.9 Ω shunt resistor generates a maximum
differential input voltage to the AD627 of between 100 mV
(for 4 mA in) and 500 mV (for 20 mA in). With no gain resistor
present, the AD627 amplifies the 500 mV input voltage by a
factor of 5, to 2.5 V, the full-scale input voltage of the ADC. The
zero current of 4 mA corresponds to a code of 819 and the LSB
size is 4.88 μA.
THERMOCOUPLE AMPLIFIER
Because the common-mode input range of the AD627 extends
0.1 V below ground, it is possible to measure small differential
signals that have a low, or no, common-mode component.
Figure 51 shows a thermocouple application where one side of
the J-type thermocouple is grounded.
Over a temperature range from −200°C to +200°C, the J-type
thermocouple delivers a voltage ranging from −7.890 mV to
+10.777 mV. A programmed gain on the AD627 of 100 (R
G
=
2.1 kΩ) and a voltage on the AD627 REF pin of 2 V result in the
output voltage of the AD627 ranging from 1.110 V to 3.077 V
relative to ground. For a different input range or different
voltage on the REF pin, it is important to verify that the voltage
on Internal Node A1 (see Figure 37) is not driven below
ground. This can be checked using the equations in the Input
Range Limitations in Single-Supply Applications section.
V
OUT
5V
V
REF
0.1µF
AD627
R
G
2.1kΩ
J-TYPE
THERMOCOUPLE
REF
00782-050
Figure 51. Amplifying Bipolar Signals with Low Common-Mode Voltage