DC removable subtractor insted of filter

Discussion in 'General Electronics Chat' started by Goverdhan R, Mar 10, 2016.

  1. Goverdhan R

    Thread Starter New Member

    Mar 10, 2016
    i want know why this circuit is not working at all. i am trying to remove dc from signal( which has dc in terms of volts and ac in terms of mill volts) and i need to amplify ac signal. my assumption is that only dc is available for certain interval of time, so i will charge capacitor. After changing input some ac will be added to dc . so i tried to subtract with opamp and amplify ac as my interest is on ac.
    Suggest me what to do?
  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    Why can't you pick the AC signal off the DC with a capacitor?
  3. Goverdhan R

    Thread Starter New Member

    Mar 10, 2016
    ac signal i am interested is in frequency range of 1-2Hz and capacitor will offer high impedence for this frequency range and ac is of very low magnitude, so i was doubtful about using capacitor and made use of this circuit but not getting output. Any alternatives so i can remove dc and amplify ac signal of frequency 1- 2hz as i am using this for biomedical application.
  4. dl324

    Distinguished Member

    Mar 30, 2015
    You are essentially feeding the same signals to a difference amplifier and you expect to get an output?

    What is the amplitude of the AC and the magnitude of the DC offset? Is the DC offset fixed?
  5. alfacliff

    Well-Known Member

    Dec 13, 2013
    why the 1000 mfd cap to ground in the input? kind of defeats the ac dosnt it?
  6. Goverdhan R

    Thread Starter New Member

    Mar 10, 2016
    @dl324 this is for biomedical project. so dc offset varies for each patient( so not fixed) but remain same as long as it is connected to one patient.
    Dc offset is available while connecting to each patient.after some time signal will be mixture of ac and dc. Magnitude of dc in 3-4 volts and ac in millvolts
    @alfacliff So i can get dc which i want to subtract with ac+dc signal. But it is not happening

    at t=0 input available is dc
    at t=to input available is dc+ac
    so i want to remove dc. AC is in terms of millvots and low frequency(1-2 HZ)
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    Try something like this:
  8. crutschow


    Mar 14, 2008
    You just make the capacitor large enough so that RC time-constant is long enough to pass the low frequency AC.
    For example with a 100kΩ input resistance, a 10μF capacitor in series will give a low frequency corner of 0.16Hz.
    That will readily pass your low level 1-2Hz signal but block the DC.
    Here's a simulation of the circuit.
    As you can see, the -3dB point is .16Hz and the rolloff at 1Hz is only -109mdB (-0.109dB) or an attenuation of 0.988.

    Last edited: Mar 10, 2016
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    Yup, that's much simpler ;).