Transimpedance Amplifier and Photodiode for BFSK

Question

I am working on my first transimpedence amplifier circuit and am having some trouble. I have am looking to reconstruct a BFSK signal modulated on an LED. The BFSK signal uses a 50kHz sine wave at 80mA to represent a '1' and a 30kHz sine wave at 10mA to represent a '0'. The signal has a data rate of 4.8kHz maximum. I am using a TSTA7100 for my LED. The photodiode I am attempting to use is the PC50-7-TO8.

I have constructed my transimpedence amplifier circuit according to this Texas Instruments video. I would ideally like my output to range from 0V to 5V. I have roughly estimated that my photodiode's input current at a maximum would be 10uA.

The circuit I currently have connected is shown below:

^{simulate this circuit – Schematic created using CircuitLab}

The issue I am having is that there is no measurable signal being recreated on the Op Amp's output from the transmitted LED BFSK signal. In fact, instead, I see odd waveforms that I believe are caused by the +5V switching power supply. I get the same waveform whether the LED is on or off. The output (node V_out) is shown below:

My questions are what is wrong with this circuit causing the op amp not to recreate the photodiode's signal? Also, is there any advice on how I could improve the received signal quality?

I am still a novice with op amps so this has already been quite a learning experience.

Answer 1

The easiest modification you can make to your single rail photodiode amplifier is to reverse the photodiode position and have the anode grounded like this: -

I've also marked on the direction of photodiode current with a red arrow. This circuit would now satisfy the feedback constraints of a single rail op-amp; the output will rise positively from close to 0 volts in the presence of illumination.

_{However, do not try and use this with any op-amp; only op-amps capable of working with inputs and outputs at the most negative supply rail will work and the LM324 is good for that.}

With your current circuit, the op-amp output would have to fall negatively below 0 volts to satisfy the feedback rules for an op-amp; clearly that is impossible because your circuit hasn't got a negative supply rail and, in effect, the output clamps to 0 volts with no useful signal present at the output.

Answer 2

1. You neglected the ± supply in the video. Fix that.
2. With 500kOHm your noise current is too high with BJT style OP Amps so use FET/CMOS type, Fix that. High R use CMOS.
3. You chose a PD with no daylight blocking filter so you can see the radiated 60Hz light. Fix that with a daylight blocking filter or IR pass filter or change PD to a SHarp/Vishay type with "black plastic" filter.
4. You have no design specs for input/outputs Optical input current, power source avail, light Ambient specs, light source, output range, path loss [dB], TIA gain [dB] , noise BW {v,i per root Hz}, IC parts avail? Make a list of Specs. Fix that

Any questions? My best advice.. Always do 4) first with a checklist before any design. Good job so far.

If you have no other parts to work with .... say so. You can recess PD in a tunnel with heat shrink tubing then directional. You can bias to Vcc/2 and match R for Vin+/-. If still too noisy, filter Vcc with series R , reduce Rf to < 10k then add 2nd stage AC gain.

To measure sensor current Use a load R and measure it or compute it

Then install TINA http://www.ti.com/tool/TINA-TI?keyMatch=SPICE%20DOWNLOAD&tisearch=Search-EN-everything

Then Analyse> Noise and use Vin = xxx uV based on your worst case test. include SNR and f max BW observe SNR for your design.

Bias away from supply rails.

Answer 3

Maybe this answer will help understanding the single-supplied circuit:

https://electronics.stackexchange.com/a/473488/61398