LM324: How to design filter to filter 450Hz/25KHz@-40dB?

Chen,

The document has time based signal waveform in it. Alos the center frequency mentioned (60Hz) is outside the lower and upper bounds ( 450Hz , 25,000Hz)

What is the desired gain in the passband and how wide is the pass band?
What is the desired attenuation in db/decade for low and high band block? [20, 40, 60 dB/decade]

Hi Ronald,

Here are my answers:

What is the desired gain in the passband and how wide is the pass band?

[] gain=0 dB, center frequency=60Hz, BW=20Hz.

What is the desired attenuation in db/decade for low and high band block? [20, 40, 60 dB/decade]

[] 40dB

System control signal is 60Hz as attached file, but the system has noise @ 450Hz and 25KHz.

It is expected that the out-band noise 450Hz and 25KHz can be solved by filter.

But the filter we designed causes the control signal to have distortion, how to design filter for this?

Tseng,

I understand better now. I will simulate a filter on Monday. 

Is the distortion you have now servere (visible on oscilloscope  waveform) or slightly higher THD+N?

If LM324 output has to change between sourcing and sinking output drivers, then there may a be a ~4us delay in the output waveform. Try adding a 1k resistor from output to GND or to 5V as a test.

This should be read by anyone using this device in a new application. Even those who use this part all the time will learn something new.

• Design Guidelines for Devices with LM324/LM358 Cores

i sir,

I use TINA and webench's simulation tool to find the right component. 25KHz noise can be solved with LPF and is easy to design. But I have not yet found a suitable 450Hz solution to meet the requirements of MEMS control system. I tried LBP, BPF and notch to reduce the 450Hz harmonic, the best simulation result is UFA42 notch filter, the results are as follows. But we can find some distortion in the waveform(vout/VF1).  

If possible, please suggest a suitable  schematic. Thank you.

450_notch.TSC

Hi Tseng,

this comes form the abrupt changes and sharp edges of your input signal. A bit input low pass filtering can help. Also, your input signal amplitude is too high and drives the output into saturation.

tseng3.TSC

Kai

Tseng,

I compared Vout with the input signal VF1 with a gain of -1. See how it overlaps. That means the notch filter didn't distort the signal.

It was distorted before it reached the filter.

450_notch+inv.TSC

Tseng,

OK, I now see that you already knew VF1 was distorted. I'm not familiar with the AUF42E but I believe it is overdriven.