Feedback Capacitor Low-Pass Filter Pitfalls

Feedback Capacitor Low-Pass Filter Pitfalls

Adding a capacitor in parallel with the feedback resistor of an op amp is an easy way of accomplishing low- pass filtering. This technique works quite well in an inverting  amplifier (see the curves below) but not necessarily in a non-inverting amplifier. If the NI amplifier has high gain, the filtering is not bad– but inferior to the inverting case.  As the NI amplifier gain is reduced, the filter effectiveness suffers. In a gain of +2V/V, there is only 6dB of stopband attenuation. In a voltage-follower   (gain of +1V/V), there is no low- pass filtering at all! In each amplifier, the value of R2 was stepped logarithmically from 100 ohms to 100k.\e(x,2) (Circuit is created by Neil P. Albaugh, TI-Tucson )

Circuit for Demonstration of Pitfalls related with the Feedback Capacitor in Low-pass Filters 
Feedback Capacitor Low-Pass Filter Pitfalls
Feedback Capacitor Low-Pass Filter Pitfalls
Demonstration of Pitfalls related with the Feedback Capacitor in Low-pass Filters circuits

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Creation and Simulation of a 1kV High Common Mode Voltage Difference Amplifier circuit with TINA or with TINACloud

High Common Mode Voltage Difference Amplifier circuit

Achieving a high common mode voltage differential amplifier requires a very high attenuation resistor network.

To minimize op amp input offset voltage and drift errors, an autozero type is used– OPA735.  An OPA335 can be used on a 5V supply. Common- mode rejection is critically dependent on the ratio matching of R3:R2 & R4:R1. These should be matched to extremely tight tolerances. Power dissipation in R3 & R4 are significant. Bypass capacitors are not shown.  (Circuit is created by Neil P. Albaugh  TI – Tucson)

High Common-Mode Voltage Difference Amplifier circuit:
1 kV High Common-Mode Voltage Difference Amplifier circuit
1 kV High Common-Mode Voltage Difference Amplifier circuit
Creation and Simulation of a 1 kV High Common-Mode Voltage Difference Amplifier circuit

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Click here  to download the FREE trial demo of TINA  Design Suite

Michael Koltai
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