The thing is, the input impedance of my fully differential amplifier is determined by the r2 and r3 resistors. So here it is 2k single impedance (differential impedance is 4 kOhm? right).
The system has a feedback structure.
(only in the non-inverting configuration, if there is no resistor or passive filter on the opamp leg, the impedance directly at the input of the opamp will be seen as high impedance).
Additionally, if parallel series resistors and passive filters are used at the input, when the input impedance is taken as equivalent, Resistance (real) - parallel impedance - parallel impedance and equivalent impedance may decrease. Here, the low resistance value always wins.
And the currents that the sensors can give are limited. If the maximum current that it can give is not enough to keep the voltage constant (the current drawn is determined by the input impedance of the ADC), not enough current can be pumped and the signal voltage amplitude starts to decrease.
FDAs have lower input impedance? I think it is used in high-speed applications and these applications can provide high current from source with low impedance. (I am still not sure in which ADC front end circuits the FDAs are used, I think a source that can provide high current is needed.)
Another issue is that if there is a passive filter, it significantly reduces the impedance. Even if we increase the input resistances of the FDA, the impedance still decreases with the logic of low gain. At the same time, as the resistance values increase in the FDA, the noise increases and makes the output noisy.
In this case I will place a high impedance INA at the Input. and then I will send it to adc with high impedance.
I will add termination resistors to the input. In this case, I can make measurements from high-speed sources that have low impedance and provide high current.
What I'm wondering is, if I add a passive differential noise filter or common mode passive filter, the impedance will decrease again. In this case, should I use an active filter?
If there are any points I think are wrong, please correct me.