Capacitor for filter design.

Thread Starter

The_Logician

Joined Oct 2, 2023
12
I am trying to make an amplifier-filter circuit for a signal of a few millivolts. The cut-off frequency is to be 120 Hz.(low-pass filter) Am i better off with electrolytic or ceramic capacitors?

Also do i amplify then filter or filter then amplify?
 

Audioguru again

Joined Oct 21, 2019
6,610
A simple lowpass filter is a series resistor feeding a capacitor to ground. The source impedance and the impedance it feeds are parts in the filter, then an inverting opamp integrator lowpass filter solves both problems.
The filter slope is very gradual at only -6dB per octave.
Electrolytic and ceramic capacitors have a wide range of capacitance so use a 5% film capacitor instead.
 

Thread Starter

The_Logician

Joined Oct 2, 2023
12
A simple lowpass filter is a series resistor feeding a capacitor to ground. The source impedance and the impedance it feeds are parts in the filter, then an inverting opamp integrator lowpass filter solves both problems.
The filter slope is very gradual at only -6dB per octave.
Electrolytic and ceramic capacitors have a wide range of capacitance so use a 5% film capacitor instead.
And what would you recommend to remove DC offset in an op-amp? Will a series capacitor with a resistor to ground work? I'm planning to use a 1uF capacitor with a 100k resistor. That's a cutoff of about 1.5 Hz
 

Audioguru again

Joined Oct 21, 2019
6,610
A cheap opamp can have its fairly high offset voltage adjusted to zero with a trimpot. A good opamp is rated for very low offset voltage.
A 1uF series capacitor at the opamp output feeding a 100k resistor to ground will take about 7.5 seconds to charge. It is a very high output impedance.
 

HaldorPhil

Joined Mar 8, 2019
16
I am trying to make an amplifier-filter circuit for a signal of a few millivolts. The cut-off frequency is to be 120 Hz.(low-pass filter) Am i better off with electrolytic or ceramic capacitors?

Also do i amplify then filter or filter then amplify?
Best for filtering accuracy is a film cap, or a C0G/NPO ceramic since these are the most temperature stable. I only use electrolytic caps in power supplies.

The choice of filter design is determined in large part by how much gain you need to provide. For low gain applications, I like the Multiple Feedback topology because it lets you combine the filter and gain stage into a single op-amp. For higher gain applications, I like the Sallen Key topography after the input gain stage.

Also, input requirements (ESD, very low or high impedance, differential inputs etc.) may drive you to separate the gain stage from the filter stage no matter how much gain you need.

https://www.ti.com/lit/an/sloa049d/sloa049d.pdf
 
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