AD8276BRMZ-R7 datasheet(18/20 Pages) AD

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AD8276/AD8277

Rev. A | Page 18 of 20

resistor is determined by the characteristics of the coupler or

transformer and desired input voltage ranges to the AD8276.

Table 8. Low Power Op Amps

Op Amp (A1, A2)

Features

AD8506

Dual micropower op amp

AD8607

On the voltage monitoring side, potential transformers (PTs)

are used to provide coupling and galvanic isolation. The PTs

present a load to the power line and also step down the voltage

to a measureable level. The AD8276 helps to build a robust

system because it allows input voltages that are almost double

its supply voltage, while providing additional input protection

in the form of the integrated ESD diodes.

Precision dual micropower op amp

AD8617

Low cost CMOS micropower op amp

AD8667

Dual precision CMOS micropower op amp

It is preferable to use dual op amps for the high impedance inputs

because they have better matched performance and track each

other over temperature. The AD8276 difference amplifiers

cancel out common-mode errors from the input op amps, if

they track each other. The differential gain accuracy of the in-

amp is proportional to how well the input feedback resistors

(RF) match each other. The CMRR of the in-amp increases as

the differential gain is increased (1 + 2RF/RG), but a higher gain

also reduces the common-mode voltage range. Note that dual

supplies must be used for proper operation of this configuration.

Not only does the AD8276 monitor the voltage and currents on

the power lines, it is able to reject very high common-mode

voltages that may appear at the inputs. The AD8276 also

performs the differential-to-single-ended conversion on the

input voltages. The 80 kΩ differential input impedance that the

AD8276 presents is high enough that it should not load the

input signals.

RSH

ISH

AD8276

VOUT = ISH × RSH

Refer to A Designer’s Guide to Instrumentation Amplifiers for

more design ideas and considerations.

RTD

Resistive temperature detectors (RTDs) are often measured

remotely in industrial control systems. The wire lengths

needed to connect the RTD to a controller add significant

cost and resistance errors to the measurement. The AD8276

difference amplifier is effective in measuring errors caused by

wire resistance in remote 3-wire RTD systems, allowing the

user to cancel out the errors introduced by the wires. Its

excellent gain drift provides accurate measurements and stable

performance over a wide temperature range.

Figure 51. AD8276 Monitoring Current Through a Shunt Resistor

Figure 51 shows how the AD8276 can be used to monitor the

current through a small shunt resistor. This is useful in power

critical applications such as motor control (current sense) and

battery monitoring.

INSTRUMENTATION AMPLIFIER

REF

AD8276

RL2

RL1

VOUT

Σ-Δ

ADC

RL3

IEX

The AD8276/AD8277 can be used as building blocks for a low

power, low cost instrumentation amplifier. An instrumentation

amplifier provides high impedance inputs and delivers high

common-mode rejection. Combining the AD8276 with an

Analog Devices, Inc. low power amplifier (see Table 8) creates a

precise, power efficient voltage measurement solution suitable

for power critical systems.

–IN

+IN

AD8276

40kΩ

REF

VOUT

VOUT = (1 + 2RF/RG) (VIN+ – VIN–)

Figure 53. 3-Wire RTD Cable Resistance Error Measurement

Figure 52. Low Power Precision Instrumentation Amplifier

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