I am using the MC33171N as a differential op amp. (I’ve attached the schematic of that section.)
The general design is based in part on the IC’s product sheet (which I’ve attached) as well as similar designs on the net. One, in particular, includes a 4.7uF electrolytic cap in series with each input.
Would someone tell me what purpose those caps serve?

Also, from examples on the net I saw that the feedback resistor was equal to the resistor put to ground on the non-inverting side. Is this actually correct? My understanding is that for a gain (factor) of 10 I would use a feedback resistor ten times the value of the non-inverting side grounded resistor. (What is that resistor called, and what does it do?)

 

I would call it the bias or reference resistor. It establishes GND as the reference point for differential operations and allows a path for DC bias currents. In closed loop operation both inputs are constrained to be at the same DC potential.

 

The simple answer about the purpose of the two capacitors is that they are to prevent adjustments of the input pots from affecting the DC bias conditions. So the DC level of the output does not change as the input signal is adjusted. If the DC level of the output moved toward the limits of the output voltage range, flat-top distortion could happen. So the capacitors avoid distortion.

 

I would call it the bias or reference resistor. It establishes GND as the reference point for differential operations and allows a path for DC bias currents. In closed loop operation both inputs are constrained to be at the same DC potential.

thank you for your reply.

 

The simple answer about the purpose of the two capacitors is that they are to prevent adjustments of the input pots from affecting the DC bias conditions. So the DC level of the output does not change as the input signal is adjusted. If the DC level of the output moved toward the limits of the output voltage range, flat-top distortion could happen. So the capacitors avoid distortion.

thank you for your reply.