I'm trying to visualize an infrared signal using a IR receiver diode but my oscilloscope is showing triangle waves instead of square for the carrier frequency:

oscilloscope picture

One of the guys at EEVblog said I need an op-amp because the signal is too fast (38 kHz) but I'm still learning electronics and haven't used any op-amps before.

Can anybody let me know what I need or point me in the right direction?

(I already have a proper 38 kHz IR receiver with demodulation, etc, but I'm trying to visualize the signal for learning purposes using a "simple" photodiode).

EDIT: Some other questions/answers also seem to indicate I need an op-amp with a sufficient slew rate?

EDIT2: Here is my schematic per request: (I'm probing across the resistor) schematic

  • 2
    \$\begingroup\$ What circuit did you use to connect the receiver diode to the oscilloscope? \$\endgroup\$ – The Photon Jun 18 '19 at 18:16
  • \$\begingroup\$ I used the 3.3v (and also tried 5v) to diode cathode, and then diode anode to oscilliscope and 100 Ohm resistor and finally ground. \$\endgroup\$ – SofaKng Jun 18 '19 at 18:25
  • \$\begingroup\$ What kind of light are you shining on the photodiode?? \$\endgroup\$ – Scott Seidman Jun 18 '19 at 18:40
  • \$\begingroup\$ I'm using a generic infrared remote control. The 3-pin IR receiver shows perfect square (demodulated) wave forms. \$\endgroup\$ – SofaKng Jun 18 '19 at 18:47
  • \$\begingroup\$ Post a schematic please. \$\endgroup\$ – TimWescott Jun 18 '19 at 19:27

The problem is in the parasitic capacitance of the circuit you have implemented. It forms an RC timeconstant with the gain resistor which filters out high frequency signals. There are plenty of resources to help you with this problem

You will need to minimize the capacitance and possibly get a faster opamp with more gain bandwidth product (GBWP). Breadboards are terrible for implementing circuits like this, even on a PCB traces can have enough capacitance (pFs) to make a difference, so short trace sizes are best between sensor and transimpedance amplifier.

  • \$\begingroup\$ Thanks so much. There are quite a lot of options for op-amps and many different circuits from what I can find. Can you recommend anything specific (i.e. op amp model and circuit) for what I'm trying to do? This website (outsidescience.wordpress.com/2012/11/03/…) seems to recommend an LTC1050? \$\endgroup\$ – SofaKng Jun 19 '19 at 3:15

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