Filter stage of amplifier circuit

Papabravo

Joined Feb 24, 2006
17,260
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.
Here is another problem. A power supply CANNOT independently control both current and voltage. Given any particular load you can control the current OR the voltage but not both. A consequence of this is that you cannot force a load to take more current than it requires. So you are monitoring control pins, and why exactly do you expect noise that you wish to filter out on these control pins?
 

Thread Starter

pinkyponky

Joined Nov 28, 2019
194
Here is another problem. A power supply CANNOT independently control both current and voltage. Given any particular load you can control the current OR the voltage but not both. A consequence of this is that you cannot force a load to take more current than it requires. So you are monitoring control pins, and why exactly do you expect noise that you wish to filter out on these control pins?
Not independently monitor, Im thinking that may be their will have some noise when amplifier is used and also may be some noise will be there from the SMPS. That's why I asked this question.

So, don't we get any noise from the control pins of the SMPS?. Please clear this question first, then we will understand either need to be used filter or not.
 

BobTPH

Joined Jun 5, 2013
4,035
Can you explain what it means to monitor the voltage and current if an SMPS other than measuring these on the output?

Bob
 

Papabravo

Joined Feb 24, 2006
17,260
Not independently monitor, Im thinking that may be their will have some noise when amplifier is used and also may be some noise will be there from the SMPS. That's why I asked this question.

So, don't we get any noise from the control pins of the SMPS?. Please clear this question first, then we will understand either need to be used filter or not.
If there were noise on the control pins wouldn't you expect the output voltage or current to be all over the place. The single ended wires running from the power supply will act as antennas to pickup whatever they can. The control circuits are purposely bandwidth limited so that frequencies in the neighborhood of the switching frequency, and above, are heavily attenuated.
 

DickCappels

Joined Aug 21, 2008
7,963
Let's remember that this is Homework Help, not Homework Done For You. So far the thread starter has been asked to show us his best attempt, or schematic at least three time. @pinkyponky if you cannot show us an attempt at a solution we may not be able to help you.
 

dcbingaman

Joined Jun 30, 2021
497
Since we know nothing about the input impedance and output impedance before and after the filter it is making it hard to answer your question. We also do not know the required roll off of the filter. Given that situation you need at least a opamp voltage follower to provide high input impedance followed by the RC filter (1st order filter) followed by another opamp voltage follower. You should know how to calculate the cutoff using the RC time constant. The inverse of the RC time constant is the frequency in radians per second. Can't tell you more without giving away a possible answer.
 

Ramussons

Joined May 3, 2013
1,099
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?.
A single stage LPF gives only 6dB/Octave cutoff slope. Maybe that will be good enough for your application. But, from experience I know that, when dealing with such low frequencies, one needs a sharper cutoff filter - may be 2 stage or even 3 to get a 12 dB or 18 dB / octave. Designing a Multi stage Passive filter is complex and the filter is lossy. That is why you need Active filters.
 
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