AD627
APPLICATIONS CIRCUITS
CLASSIC BRIDGE CIRCUIT
4 TO 20 mA SINGLE-SUPPLY RECEIVER
Figure 5± shows how a signal from a 4 to 20 mA transducer can
be interfaced to the ADuC1±2, a ±2-bit ADC with an embedded
microcontroller. The signal from a 4 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 transducerꢀ adds a current
dependent offset error; therefore, the current must be sensed
differentially.
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, −±2 V, or
−±5 Vꢀ, 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 VREF pin of
an analog-to-digital converter (ADCꢀ whose input range is
(VREF ± VINꢀ. With an available output swing on the AD627 of
(−VS + ±00 mVꢀ to (+VS − ±50 mVꢀ, the maximum programmable
gain is simply this output range divided by the input range.
In this example, a 24.9 Ω shunt resistor generates a maximum
differential input voltage to the AD627 of between ±00 mV
(for 4 mA inꢀ and 500 mV (for 20 mA inꢀ. 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 1±9 and the LSB
size is 6±0 ꢁA.
+V
S
THERMOCOUPLE AMPLIFIER
0.1µF
Because the common-mode input range of the AD627 extends
0.± V below ground, it is possible to measure small differential
signals that have a low, or no, common-mode component.
Figure 5± shows a thermocouple application where one side of
the J-type thermocouple is grounded.
200kΩ
GAIN–5
R
V
G =
V
AD627
OUT
DIFF
V
REF
0.1µF
Over a temperature range from −200°C to +200°C, the J-type
thermocouple delivers a voltage ranging from −7.190 mV to
+±0.777 mV. A programmed gain on the AD627 of ±00 (RG =
2.± kΩꢀ and a voltage on the AD627 REF pin of 2 V result in the
output voltage of the AD627 ranging from ±.±±0 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 A± (see Figure 37ꢀ is not driven below
ground. This can be checked using the equations in the Input
Range Limitations in Single-Supply Applications section.
5V
–V
S
Figure ±0. Classic Bridge Circuit
0.1µF
J-TYPE
THERMOCOUPLE
R
2.1kΩ
G
V
AD627
OUT
REF
V
REF
Figure ±1. Amplifying Bipolar Signals with Low Common-Mode ꢀoltage
Rev. D | Page 22 of 24
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