# low pass filter problem

Discussion in 'The Projects Forum' started by mah, Jan 26, 2014.

1. ### mah Thread Starter Active Member

Mar 15, 2010
276
2
i am doing low pas filter in the attached file using single supply op amp Lm358 the input signal was amplified using MCP617 from mv to v but the output of the filter is also amplified however it has unity gain . i used C1=.1μf and Cin = 10 μf

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2. ### ericgibbs AAC Fanatic!

Jan 29, 2010
3,222
554
hi,
What are the values of R1 & R 2. C1 & C2, also what is your required cut off frequency.
You say Volts input, whats the signal voltage.?
E

3. ### LvW Active Member

Jun 13, 2013
674
100
Mah, what really is your question?
Is the filtering action not as desired?
Or is the amplification your problem?
Are you sure that the overall DC gain is larger than unity?
Do you consider that some lowpass characteristics (Chebyshev response) show something like a peak at the pole frequency (gain larger than at dc).

4. ### mah Thread Starter Active Member

Mar 15, 2010
276
2
R1=R2=390KΩ the required cutt off frequency is 3.4 Hz i need it pure dc signal .the signal voltage is ranging from 0-8 mv before amplification.

5. ### mah Thread Starter Active Member

Mar 15, 2010
276
2
amplification is doing well the filter is the problem . is this schematic correct ?didn't need modification ? there is a voltage drop on R3=1.4 v i think this value will affect my amplified input signal how could the filter separate between them?

6. ### LvW Active Member

Jun 13, 2013
674
100
Question: Are you feeding the whole circuit with a real VOLTAGE source whith an internal source resistance of app. zero ohms (in practice much, much smaller than the input resistance of your circuit) ?

All filter design formulas are derived under the assumption of such a low-resistive voltage source.

7. ### mah Thread Starter Active Member

Mar 15, 2010
276
2
feeding it using 5 V dc battery

8. ### ericgibbs AAC Fanatic!

Jan 29, 2010
3,222
554
hi.
Its the Signal source [your Vin symbol] being used as an input to the OPA/Filter for which we need to know the details.

eg: Is it a signal generator or some sort of sensor output signal.?

9. ### mah Thread Starter Active Member

Mar 15, 2010
276
2
it is a sensor and its output from 0-8 mv this was amplified and there is no problem with that .

10. ### LvW Active Member

Jun 13, 2013
674
100
I hope, in this case you didn`t use the coupling capacitor as shown in the figure provided by you?

...and what is the problem now?

11. ### mah Thread Starter Active Member

Mar 15, 2010
276
2
i did the circuit as it is in the figure ,. what is the problem in that?

12. ### LvW Active Member

Jun 13, 2013
674
100
In my post#6 I have asked you if you were using a low-resistive voltage source as filter input - and your answer was:
"feeding it using 5 V dc battery".

However, feeding a filter circuit with a dc voltage over a coupling capacitor seems a bit "problematic" - therfore, my question. Misunderstanding?

Moreover, if you still have problems you should describe the observed effects in detail.
Up to now - no answer to my questions in post#3.

13. ### mah Thread Starter Active Member

Mar 15, 2010
276
2
the circuit is doing well without Cin , it was blocking the dc ( original signal)