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

 

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

 

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).

 

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

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.

 

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).

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?

 

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?

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.

 

feeding it using 5 V dc battery

 

feeding it using 5 V dc battery

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.?

 

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.?

it is a sensor and its output from 0-8 mv this was amplified and there is no problem with that .

 

feeding it using 5 V dc battery

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

.it is a sensor and its output from 0-8 mv this was amplified and there is no problem with that .

...and what is the problem now?

 

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

i did the circuit as it is in the figure ,. what is the problem in that?

 

i did the circuit as it is in the figure ,. what is the problem in that?

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.

 

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.

the circuit is doing well without Cin , it was blocking the dc ( original signal)