An amplifier module has a 10Hz low pass filter.

When I obtain the gain vs frequency graph by applying different freq. signals to the inputs I get a consistent plot for the gain. I use a function generator and between 0.001Hz and 100Hz I get very consistent decrease in Bode plot.

But if I apply a constant DC offset to the amplifier inputs, the gain I obtain is lower than the gain at 1Hz. The difference is around 1dB. The 1Hz gain is 1dB more than the DC gain.

If I also apply 0.001Hz the gain is less comparing to 1Hz.

But if I apply 25mV DC input, the gain is less comparing to 1Hz and 0.001Hz. I would expect max gain at DC inout but it is not.

When I measure DC gain I even tried to take offsets for input and output and subtract them but still it is lower than the gain at 1Hz.

Is it possible DC gain is less than the gain at 1Hz or this is due to measurement error?

  • \$\begingroup\$ How are you measuring the gain? \$\endgroup\$ Nov 24, 2017 at 12:04
  • \$\begingroup\$ I apply 50mV pkpk input and measure at lets say 10Hz then at 100Hz ect. Then I divide the output swing to input swing at each freq. and obtain gains, a code does this part very well. But the function generator has min 0.001Hz freq. So for DC gain I set the function generator to offset (0mV) and then 25mV DC input and obtain corresponding outputs, then I measure the DC gain after subtracting the offsets like (Voutat25mV-Voutat0mV)/(Vin0mV-Vinat0mV). I use a data acquisition to sample the input and output channels \$\endgroup\$
    – floppy380
    Nov 24, 2017 at 12:29
  • \$\begingroup\$ Sorry I should have written (Voutat25mV-Voutat0mV)/(Vinat25mV-Vinat0mV) \$\endgroup\$
    – floppy380
    Nov 24, 2017 at 12:56
  • \$\begingroup\$ So when you apply +-25mVDC and 50mVppAC at 0.001Hz the AC peaks are 1dB (ie. 12%) higher and lower? Did you measure the input to make sure the generator was producing the same voltages for AC and DC? \$\endgroup\$ Nov 24, 2017 at 13:22
  • \$\begingroup\$ For DC gain I set and apply around 0mVDC input by function gen. and obtain output Vout_off and then I apply 25mVDC and obtain output Vout. Then I calculate the DC gain as (Vout-Vout_off)/(Vin-Vin_off). 50mVppAC at 1Hz the AC peaks are 1dB (ie. 12%) higher than the DC. DC gain is not max I mean. I also did the same now with 0.001Hz AC gain is less than the AC gain at 1Hz. So I don t know why at very low freq. the gain doesnt increase.. \$\endgroup\$
    – floppy380
    Nov 24, 2017 at 13:32

1 Answer 1


I think you are in a rathole. It doesn't make any sense what you are seeing. It must be some type of measurement anomaly. I suggest you use two voltmeters to measure DC gain. First, check for voltemter offset by connecting them to the same voltage at the same time. Note the offset, if any.

Then, connect one voltmeter to the input of your filter, and another one to the output. Apply a stable DC voltage to the input. Measure the output. Correct for any offset if necessary.

Then calculate Vout/Vin using voltmeter data. Compare with gain at 1 Hz, etc.

  • \$\begingroup\$ I already sample Vin and Vout by a data acquisition device and take the mean values. I also subtract the offsets. \$\endgroup\$
    – floppy380
    Nov 24, 2017 at 18:50
  • \$\begingroup\$ G{dB} = 20*log10([Vo_mean- Vo_off_mean]/[(Vi_mean - Vi_off_mean)]) This is what I do for the dc gain \$\endgroup\$
    – floppy380
    Nov 24, 2017 at 18:51
  • 1
    \$\begingroup\$ Try something new. What you are doing is not providing any new information. Using a DC voltmeter would be something new. \$\endgroup\$
    – user57037
    Nov 25, 2017 at 2:32

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