OPA820SKGD3 datasheet(20/30 Pages) TI1

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OP A820

+5V

−5V

50Ω

Power−supply decoupling not shown.

O =

−V )

when

OPA820-HT

SBOS587

– DECEMBER 2011

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Single Operational Amplifier Differential Amplifier

The voltage-feedback architecture of the OPA820, with its high common-mode rejection ratio (CMRR), will

provide exceptional performance in differential amplifier configurations. Figure 7 shows a typical configuration.

The starting point for this design is the selection of the RF value in the range of 200 Ω to 2 kΩ. Lower values

reduce the required RG, increasing the load on the V2 source and on the OPA820 output. Higher values increase

output noise as well as the effects of parasitic board and device capacitances. Following the selection of RF, RG

must be set to achieve the desired inverting gain for V2. Remember that the bandwidth will be set approximately

by the gain bandwidth product (GBP) 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 7. High-Speed, Single Differential 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 bias currents. However, it is sometimes useful to scale the

values of R2 and R1 in order to adjust the loading on the driving source, V1. In most cases, the achievable

low-frequency CMRR will be limited by the accuracy of the resistor values. The 85-dB CMRR of the OPA820

itself will not determine the overall circuit CMRR unless the resistor ratios are matched to better than 0.003%. If it

is necessary to trim the CMRR, then R2 is the suggested adjustment point.

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