By the circuit shown bellow I wanna read the ripple current by the 0.33 resistor and the ripple voltage of the capacitor by adding a low pass filter to reduce the spikes and a high pass filter to extract the AC part of the capacitor's voltage. Now my question is how I'm supposed to read the differential voltage shown with the blue indicators and add off-set to it as the ground of the circuit is different from the differential pair's. Because of the set up of the system I cant read the voltage between the capacitor and ground.enter image description here

  • \$\begingroup\$ Are you asking how to measure the voltage or how to interpret the measurement? \$\endgroup\$
    – Frog
    Feb 27, 2021 at 8:48
  • \$\begingroup\$ I wanna read the voltage by the ADC \$\endgroup\$ Feb 27, 2021 at 8:50
  • \$\begingroup\$ Depending on your ADC you may be able to do direct differential measurements (this is ideal). Otherwise you could measure one point and then the other, although with an AC signal the time difference could be important. If you can sample at a much higher speed than the AC frequency then this may be small enough to ignore. Perhaps it would be better to have two ADCs and trigger conversions simultaneously. \$\endgroup\$
    – Frog
    Feb 27, 2021 at 8:57
  • \$\begingroup\$ It would be better to phrase your question purely as a question. Do not include your assumptions about the problem within the question. State the problem you see as clearly as possible and then ask a direct question. \$\endgroup\$ Feb 28, 2021 at 1:50

1 Answer 1


how I'm supposed to read the differential voltage shown with the blue indicators

Use an op-amp configured as a differential amplifier: -

enter image description here

Picture from here.

You may need to create a small negative voltage for the op-amp negative rail or, if you choose your op-amp carefully (inputs capable of working close to 0 volts on your schematic), you should be good to go.

Another thing to think about is the frequency of the ripple you are trying to measure; because your circuit is a boost converter, it will be running at (probably) several tens of kHz so, choose an op-amp with reasonable bandwidth in order to measure your signal without high frequency attenuation.

If you need the ripple voltage centred at some DC offset suitable for your ADC, the ground connection of resistor \$R_g\$ above can be changed to that offset voltage because \$V_{out}\$ is referenced to that voltage.

  • \$\begingroup\$ I did the exact thing as you mentioned but I had problem with the negative voltage and through simulation and adding negative supply I had an unwanted offset on my signal. Is there any way to circumvent this issue? \$\endgroup\$ Feb 27, 2021 at 9:34
  • \$\begingroup\$ Numbers, numbers, numbers. Microvolts, millivolts or volts? @عرفانمیرحسینی \$\endgroup\$
    – Andy aka
    Feb 27, 2021 at 9:39
  • \$\begingroup\$ the output voltage is about 100 mv \$\endgroup\$ Feb 27, 2021 at 9:45
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    \$\begingroup\$ Well @عرفانمیرحسینی I can only tell you that maybe you mis-connected your circuit or tried to engineer too-high a gain using an op-amp with poor input offset voltage specifications. Using a diff amp (as per my answer) is a tried and tested way of achieving what you want. \$\endgroup\$
    – Andy aka
    Feb 27, 2021 at 10:14
  • \$\begingroup\$ @عرفانمیرحسینی if we're done here you should probably consider formally accepting an answer or raise another comment if there is still something you need clarification on. \$\endgroup\$
    – Andy aka
    Mar 9, 2021 at 11:22

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