I am working with a photodetecting circuit built as follow:

enter image description here

When I drive an LED ON and OFF, I get the following waveforms:

enter image description here

Blue indicates that the LED is turned on when a voltage is high and turned off when low.

Yellow shows brightness reading after RC high pass filter.

The lower envelope of the yellow line looks like a voltage discharging pattern of an RC circuit.

The problem is, however, the upper envelope seems like a saturated signal.

I can it more clearly by reducing the capacitance to 10 uF:

enter image description here

By reducing capacitance from 47 uF to 10 uF, a discharging pattern in the upper envelope after a short saturated duration can be seen.

But I am curious that

  1. Why only the upper envelope is saturated? Why not for the lower envelope?
  2. Why the upper envelope is saturated at nearly 200-300 mV, not around VCC+, 5 V?

For your information, both the upper and the lower envelopes are not saturated if I remove the RC HPF and see voltage readings:

enter image description here

  • 2
    \$\begingroup\$ Don't expect an op-amp to arbitrarily be able to drive a 50 ohms load without giving some problems and messing up the signal. If you read the data sheet of the TL972 you'd see that the device is able to source maybe a mA or so but the good side is that it can sink up to 80 mA but that won't help fix the waveform. \$\endgroup\$ – Andy aka Jul 28 '15 at 17:07
  • 1
    \$\begingroup\$ Maybe you should have a look at the op-amp output when it's doing this. \$\endgroup\$ – Spehro Pefhany Jul 28 '15 at 17:09
  • \$\begingroup\$ And please provide a picture of your actual circuit. I suspect grounding/decoupling issues. \$\endgroup\$ – WhatRoughBeast Jul 28 '15 at 19:45
  • \$\begingroup\$ Looking at the data sheet, I'm almost certain you have decoupling problems. You're apparently using 10 uF electrolytics for decoupling, and you must use good low-ESR ceramics. This also suggests a cavalier approach to grounding, and this op amp has gain to 10 MHz. Please post a picture of your circuit. \$\endgroup\$ – WhatRoughBeast Jul 29 '15 at 12:55
  • \$\begingroup\$ Well I got it. But it is quite a mess... I will post my actual circuit with some labels and tags as soon as possible. \$\endgroup\$ – Jeon Jul 29 '15 at 13:07

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