AD627ARZ-R7 datasheet(23/25 Pages) AD

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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 VREF pin of

an analog-to-digital converter (ADC) whose input range is

(VREF ± VIN). With an available output swing on the AD627 of

(−VS + 100 mV) to (+VS − 150 mV), the maximum programmable

gain is simply this output range divided by the input range.

VOUT

VDIFF

+VS

–VS

VREF

0.1µF

AD627

RG = 200kΩ

GAIN–5

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 (RG =

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.

VOUT

VREF

0.1µF

AD627

2.1kΩ

J-TYPE

THERMOCOUPLE

REF

Figure 51. Amplifying Bipolar Signals with Low Common-Mode Voltage

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