# Filter stage of amplifier circuit

#### pinkyponky

Joined Nov 28, 2019
195
Hi,

I would like to use the RC filter with the Ferrite bead on the amplifier input to keep the frequency less than 1Hz and filter out the noisy signal which will go to the input of the amplifier. So, If I use the high resistance value like 1Mohm to meet my requirement (less than 1Hz). Then, the high resistance value will reduce the current flow, Right?. So, in this case, Did I get no current at the input of the amplifier?. If so, then, how do I filter out the signal to get lower than the 1Hz?.

And also, how to choose the ferrite bead?.

Last edited:

#### Ramussons

Joined May 3, 2013
1,100
A ferrite bead has no effect on frequencies less than a few 10s of Kilo Hertz.
If ytou need to pass on frequencies less than 1 Hz, you will need to use 3rd or 4th Order Active Filters using Op Amps.
This may be useful. https://www.electrical4u.com/active-low-pass-filter/

#### pinkyponky

Joined Nov 28, 2019
195
A ferrite bead has no effect on frequencies less than a few 10s of Kilo Hertz.
If ytou need to pass on frequencies less than 1 Hz, you will need to use 3rd or 4th Order Active Filters using Op Amps.
This may be useful. https://www.electrical4u.com/active-low-pass-filter/
Hi Ramussons,

May I know why shouldn't use the passive low pass filter in this case?. What will happen if I use the 100kohm and 4.7uF as RC low pass filter?.

#### LowQCab

Joined Nov 6, 2012
1,361
Do You have a Schematic that You can post ?
( along with a description of what the Circuit is intended to do ).
.
.
.

#### pinkyponky

Joined Nov 28, 2019
195
Do You have a Schematic that You can post ?
( along with a description of what the Circuit is intended to do ).
.
.
.
Hi LowQCab,

No, I don't have. Just starting the circuit so this is first step to start the circuit.

The main aim of the project is to monitor the current and voltage which are coming from the SMPS, so this signal is going to the amplifier input. In order to monitor current and voltage, If I use the resistor with the series of amplifier input then there will be a current flow reduce. Actually, Is it require to use the filter circuit before the amplifier?. If so, how can I implement this?.

Last edited:

#### BobTPH

Joined Jun 5, 2013
4,053
Why do you think you need 1MΩ for an RC filter with a cutoff of 1Hz? 1K and 160uF will do it.

Bob

#### pinkyponky

Joined Nov 28, 2019
195
Why do you think you need 1MΩ for an RC filter with a cutoff of 1Hz? 1K and 160uF will do it.

Bob
Ok, If I use that, the current flow will be reduce when use the resistor series with the amplifier input, right?. So. I shouldn't lost the current or voltage.

Last edited:

#### BobTPH

Joined Jun 5, 2013
4,053
You seem to think all of the current out of the supply is going through the RC filter. That would not be the way to do this.

Bob

#### pinkyponky

Joined Nov 28, 2019
195
You seem to think all of the current out of the supply is going through the RC filter. That would not be the way to do this.

Bob
Hi Bob,

Then, what is the other way to filter-out the noise?. Can noise be expected from the power supply?.

Last edited:

#### BobTPH

Joined Jun 5, 2013
4,053
Show us a circuit including the power supply as a block, the load as a block, and the circuit you want to insert to monitor current and voltage.

Bob

#### pinkyponky

Joined Nov 28, 2019
195
Show us a circuit including the power supply as a block, the load as a block, and the circuit you want to insert to monitor current and voltage.

Bob
Hi bob,

As I told in previous post, I don't have a circuit, this is first stage of the entire circuit. Just I stated the project now.

#### BobTPH

Joined Jun 5, 2013
4,053
Now I am confused about whether you are trying to reduce noise to the monitoring circuit or noise to the load.

Bob

#### BobTPH

Joined Jun 5, 2013
4,053
Hi bob,

As I told in previous post, I don't have a circuit, this is first stage of the entire circuit. Just I stated the project now.
We cannot help with the details if you don’t know the big picture.

Bob

#### Audioguru again

Joined Oct 21, 2019
3,869
One passive RC makes a very poor filter with a gradual cutoff slope.
If 1Hz is at -3dB (0.707 times the original level) then 10Hz is at -20dB (one tenth the original level) and 100Hz is at -40dB (one hundredth the original level).

#### LowQCab

Joined Nov 6, 2012
1,361
What is the expected maximum-Voltage from the Output of the Power-Supply ?
What will You be using to measure / monitor this Voltage ?
Why do You need to measure / monitor the Output-Voltage ?
Why is a standard Digital-Meter not going to work ?
.
.
.

#### Papabravo

Joined Feb 24, 2006
17,314
The solution to your problem(s) and you knowledge of circuits in general can hardly be advanced by prevarication on the matter of producing a schematic diagram.
It will, among other things, reveal that to measure voltage across a load you want a high impedance device in parallel the load, and to measure current you want a low impedance device in series with the load. An RC filter with a low frequency pole may help with one of those but will be a disaster for the other. Putting the matter of the filter aside for a moment you have more fundamental problems to deal with, and I can assure you that a well constructed schematic diagram will get you to the promised land far faster than any alternative that you can come up with. It's time to come to the mountain.

#### pinkyponky

Joined Nov 28, 2019
195
Now I am confused about whether you are trying to reduce noise to the monitoring circuit or noise to the load.

Bob
Just to monitor the voltage and current at the SMPS. See below pic, its descries that overall picture of the project.

#### Attachments

• 7.8 KB Views: 8

#### Papabravo

Joined Feb 24, 2006
17,314
Just to monitor the voltage and current at the SMPS. See below pic, its descries that overall picture of the project.
It is not clear from your diagram what you are looking at. It does not indicate that you are measuring current in an obvious way. It looks a though you believe that there is a voltage (with respect to ground) that you can measure which will tell you something about the current that is being supplied. Do I have this right or is there something else going on under the hood?

#### pinkyponky

Joined Nov 28, 2019
195
What is the expected maximum-Voltage from the Output of the Power-Supply ?
What will You be using to measure / monitor this Voltage ?
Why do You need to measure / monitor the Output-Voltage ?
Why is a standard Digital-Meter not going to work ?
.
.
.
Please can you see post #17.

1) It is not output, we are monitoring from the control pins of the SMPS. The max voltage is 6V.
2) By using the Amplifier, ADC and Micro controller.
3) This is project to design such a circuit to monitor the voltage and current of SMPS. it is not a output voltage.
4) If I learn that how to design the circuit to monitor the parameters of SMPS, may be there after my professor had a plan to give another assignment to control the output of voltage and current of the SMPS through these control pins as described in post #17.