In this application note, there is a circuit for temperature control which includes an NTC.
Why is there a capacitor (C5) in parallel with the inputs of the MC33201 Op Amp?
Is it a helper to the Op Amp as the Op Amp tries to make its inputs equal when there is a negative feedback?
Does it introduce an another negative feedback like this: When there is a sudden enough change in the resistance of the NTC that can pass C5, the voltage is then passed to the inverting input of the Op Amp and the output of the Op Amp does not need to change to compensate the small change on the non-inverting input due to the change of NTC resistance?
Explanation from the App Note:
The same 5 V supply is used also as reference voltage for the
Wheatstone bridge, formed by components R2 (NTC), R3, R6, R7 and C5
used for low pass filtering.
If it is for low-pass filtering, why this capacitor is not connected parallel to R5 instead of where it is now? What is the advantage of it being on the place it is now compared to it being parallel to R5?
Best Answer
The capacitor is to prevent a DC bias shift from being caused by EMI. Bipolar op-amps in particular can act as "detectors" to RF signals, and the capacitor shunts the RF to reduce the amplitude ( at the expense of increasing noise and reducing phase margin ) so it should not be too large a value capacitor.
Although all the parts are close together with minimum lead length, one could expect there is a major source of EMI nearby in the switching power supply.