OPA1632: OPA1632 Input Impedance Single Ended Design

Hello,

The proper operation of an FDA requires the pairs of RG's (R1/R2 in figure 14) and Rf's (R3/R4 in figure 14) to be balanced. So, when the datasheet is mentioning a low source impedance it is referring to the input source driving the FDA, typically another amplifier. The combination of that amplifier's output impedance and R1 can make an imbalance. You have to maintain symmetry between the two sides to allow for proper operation of the FDA.

You can imagine an FDA as just two parallel inverting gain amplifiers. Just as in an inverting amplifier the biggest negative impacts of using large resistor values is that the input bias or offset current can cause large current noise. Not only this but a large resistor value will also cause a greater amount of resistor noise. However, you should know that there are some ways to compensate for this noise. Is there a strong reason you have to use the 7.5kOhm resistors?

This video dwells into using large resistor values, its impacts, and compensation technique's: training.ti.com/ti-precision-labs-op-amps-fully-differential-amplifiers-noise-analysis-advanced-compensation

This video will help understanding the workings of an FDA : training.ti.com/ti-precision-labs-op-amps-fully-differential-amplifiers-input-and-output-common-mode-and