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I am trying to simulate this Photoplethysmography circuit in Pspice.But I am getting the following errors: ERROR -- Convergence problem in transient analysis at Time = .193 Time step = 296.9E-15, minimum allowable step size = 1.000E-12 enter image description here I am trying to give a sine input and expecting a sine output. Is it possible to create a noisy signal in pspice and see the output. How do i visualize the filters.

Thank You very much

Anupam

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  • \$\begingroup\$ You should put a high value resistor (100M) between the output and ground, and measure across that, rather than measuring a floating point. That may be the cause of the timestep error. \$\endgroup\$
    – DerStrom8
    Commented Aug 1, 2016 at 21:30
  • \$\begingroup\$ Thanks ! It helped me to get rid of the error. But I am getting a output of 160 mV when i/p sine wave is of 5v with freq 1 hz I get the same kind of o/p with a DC input also. \$\endgroup\$
    – ANUPAM BISHT
    Commented Aug 2, 2016 at 18:04
  • \$\begingroup\$ For those problems look at Andy's answer below. \$\endgroup\$
    – DerStrom8
    Commented Aug 2, 2016 at 18:50

1 Answer 1

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Well I can't help you with the PSice error message but I can point out problems: -

  • Pin 3 of U1A is not dc biased to any voltage reference - it will be just floating and this will certainly cause "real-life issues
  • You won't get a sinewave through the opto-isolator the way you are driving it - driving the LED with a sinewave will mean that the LED only conducts on the positive half cycle.
  • R3 is in series with the power feed to U1A and this looks problematic
  • No decouplers on the op-amp power supplies.
  • U2A's power rail seems to be cut

Some spice sims can allow noise and, if you want to add it to a sinewave, use two generators (one for noise and one for the sinewave) and put them in series i.e. they become additive. Some sims allow "real noise" signals to be inputted via something called a "user list".

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  • \$\begingroup\$ Thanks Andy !Can you give me more light on the 1st and 4th point you have mentioned. \$\endgroup\$
    – ANUPAM BISHT
    Commented Aug 5, 2016 at 16:43
  • \$\begingroup\$ Point 1 - op-amp inputs need to be at a DC voltage within the common-mode input range - pin 3 is only coupled by a capacitor and is therefore invalid. Op-amps (99.9% of them) require a 100nF from positive (and/or negative) rail to ground. Some also like an extra 10uF on those rails too and some will ask for both plus a 100pF capacitor. \$\endgroup\$
    – Andy aka
    Commented Aug 5, 2016 at 16:48

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