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If i have a Buck Regulator switching at 200KHz and i want to sample the Voltage and current using OpAmp in tha path connected to ADC(12 bit).

  1. What would be the sampling frequency to measure the signal by ADC?

  2. what shall be the Bandwidth of the OpAmp for Voltage and for current sense?

If somebody could provide some rule of thumb i could work further

P.S. : In my understanding sampling freq be >2 the switching freq. voltage sense would require lower BW OpAmp as it is slow, i also get confused either to choose the Opamp based on cross over freq or switching freq.

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  • \$\begingroup\$ Sampling, what for? You want to view the waveform like from scope or you want just measure the actual value? \$\endgroup\$ – Marko Buršič Jul 15 '19 at 6:32
  • \$\begingroup\$ just to measure the actual value..Let's say ADC be part of Voltage control loop of the Buck \$\endgroup\$ – seeker Jul 15 '19 at 6:42
  • \$\begingroup\$ If we assume that what you are measuring is (nominally) a DC value, it depends on your application. What are you doing with the sampled value? You say voltage control - so you are doing a somewhat slow tuning of output voltage? for what purpose? Battery charging? A DC motor that you are varying speed of? What and why? It depends on how quickly you need the output voltage to vary according to the various control inputs. \$\endgroup\$ – danmcb Jul 15 '19 at 7:04
  • \$\begingroup\$ digital control of the power supply would be the application. The controller and compensation shall be part of lets say FPGA i just need to sense the voltage and current. I get your point that anti-aliasing needs to be done before sampling at lets say 1MS/s. My question remains regarding the OpAmp selection for the voltage and current..Do i need to consider only the loop BW to select the OpAmps for Eg if my loop BW is 20KHz then should i select OpAmps with x times 20KHz BW?what would be 'x' times to consider? also regarding sampling i could sample it at 500KS/s with filter at 200KHz ? suggest. \$\endgroup\$ – seeker Jul 15 '19 at 7:15
  • \$\begingroup\$ "just need to sense the voltage and current" <-- we're turning in circles. You don't "just" need to do that, you need to do that to achieve a specific dynamic goal, and that's what you're being asked for. \$\endgroup\$ – Marcus Müller Jul 15 '19 at 7:54
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So you want to measure the mean value and not the waveform. At first place you have to determine what are the capabilities of your MCU. You will neeed an analog low pass filter on input of ADC to elliminate aliasing effect, then you can oversample and do a digital low pass filter. For example: 1M sampling, 330kHz RC low pass filter, then a digital low pass filter - slighty higher cutoff frequency than the entire execution loop, let's say 20kHz.

BW of opamp is not an issue, you will hardly find an opamp with BW less than several Mhz. The high BW is desiderable an not viceversa, you do filtering with passive components RC, choosing a low BW product with the aim to eliminate high frequencies is a wrong way.

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