OPA842 datasheet(13/32 Pages) TI2

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+5V

-5V

50W

Power-supply decoupling not shown.

V =

V )

when

OPA842

Power-supply decoupling not shown.

500W

+5V

-5V

500W

OPA842

OPA2822

OPA842

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SBOS267D – NOVEMBER 2002 – REVISED SEPTEMBER 2010

SINGLE OP AMP DIFFERENTIAL AMPLIFIER

THREE OP AMP DIFFERENCING

(Instrumentation Topology)

The voltage-feedback architecture of the OPA842,

with its high Common-Mode Rejection Ratio (CMRR),

The

primary

drawback

the

single

amp

will provide exceptional performance in differential

differential

amplifier

its

relatively

low

input

amplifier configurations. Figure 39 shows a typical

impedance. Where high impedance is required at the

configuration. The starting point for this design is the

differential input, a standard instrumentation amplifier

selection of the RF value in the range of 200Ω to 2kΩ.

(INA) topology may be built using the OPA842 as the

Lower values reduce the required RG, increasing the

differencing stage. Figure 40 shows an example of

load on the V2 source and on the OPA842 output.

this, in which the two input amplifiers are packaged

Higher values increase output noise and exacerbate

together as a dual voltage-feedback op amp, the

the

effects

parasitic

board

and

device

OPA2822. This approach saves board space, cost,

capacitances. Following the selection of RF, RG must

and power compared to using two additional OPA842

be set to achieve the desired inverting gain for V2.

devices, and still achieves very good noise and

Remember

that

the

bandwidth

will

set

distortion performance due to the moderate loading

approximately by the Gain Bandwidth Product (GBP)

on the input amplifiers.

divided by the noise gain (1 + RF/RG). For accurate

differential operation (that is, good CMRR), the ratio

R2/R1 must be set equal to RF/RG.

Figure 39. High-Speed, Single Differential

Amplifier

Figure 40. Wideband Three-Op Amp Differencing

Amplifier

Usually, it is best to set the absolute values of R2 and

R1 equal to RF and RG, respectively; this equalizes

the divider resistances and cancels the effect of input

In this circuit, the common-mode gain to the output is

bias currents. However, it is sometimes useful to

always 1, due to the four matched 500

Ω resistors,

scale the values of R2 and R1 in order to adjust the

whereas the differential gain is set by (1 + 2RF1/RG),

loading on the driving source V1. In most cases, the

which is equal to 2 using the values in Figure 40. The

achievable low-frequency CMRR will be limited by the

differential

single-ended

conversion

still

accuracy of the resistor values. The 85dB CMRR of

performed

the

OPA842

output

stage.

The

the OPA842 itself will not determine the overall circuit

high-impedance inputs allow the V1 and V2 sources to

CMRR unless the resistor ratios are matched to

be terminated or impedance matched as required. If

better than 0.003%. If it is necessary to trim the

the V1 and V2 inputs are already truly differential,

CMRR, then R2 is the suggested adjustment point.

such as the output from a signal transformer, then a

single matching termination resistor may be used

between them. Remember, however, that a defined

dc signal path must always exist for the V1 and V2

inputs; for the transformer case, a center-tapped

secondary connected to ground would provide an

optimum dc operating point.

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