EEC136 Team Sierra: Photo diode and Transimpedance update

I began construction on a circuit to experiment with photodiode and LED. First, a circuit was constructed to flash an LED. Another circuit was constructed with a photodiode to measure response to LED. Below is an idea of what the experiment looked like. Also attached are photos of the two circuits together.

The function generator was used to drive the LED’s at a specified frequency. The top of the LED was placed above the photodiode and mounted. In this manner, the output the response of the photodiode could be analyzed and quantized.

First, DC measurements were made on the constructed circuit. The diode had an open circuit voltage of around 650 mV. When the circuit was constructed with the 300k resistor though, the voltage across the diode dropped to around 280 mV. This suggests a current of a current of 88 uA. Next, the output of the photodiode was connected to the oscilloscope. I drove the LED with a square wave and observed the waveform at the photodiode output.

One thing I noticed was that an ambient light condition, a large DC bias exists. If the box was isolated and put in the dark box, then the Bias was removed. Additionally, the signal became much less noisy and clearly resembled the driving signal. Another thing I noticed was how the modulation scheme developed. The output LED’s are 180 out of phase and are switched. The switching procedure acts as a modulator shifting cardiac signal from baseband to the carrier band. In this manner, our previous circuit with high pass filter followed by Sample and Hold implements the demodulation application.

I also started looking into the topology selection for transimpedance stage. The reason for a differential stage is to eliminate common mode (CM) noise. I will update with some more information in a later blog. Below is some information on differential opamp stage extracted from Wikipedia. The key idea with this circuit is to amplify the difference between the terminals of diode.

In my next experiments, I will be implementing the transimpedance stage and analyze the approach to remove common mode noise. Additionally, I want to perform some quantization on how hard we can drive the LED’s. As we are driving the LED’s with PWM approach we can increase current through device.

Photodiode/Transimpedance Links

1. Photodiode Technical Information

http://sales.hamamatsu.com/assets/applications/SSD/photodiode_technical_information.pdf

2. Photodiode Characteristics

http://www.optics.arizona.edu/Palmer/OPTI400/SuppDocs/pd_char.pdf

3. Designing of Photodiode Amplifier circuits with OPA128

http://focus.ti.com/lit/an/sboa061/sboa061.pdf

4. OPA350 Datasheet

http://users.ece.utexas.edu/~valvano/Datasheets/OPA4350.pdf

5. Differential Operational Amplifier explanation

http://www.ecircuitcenter.com/Circuits/opdif/opdif.htm

6. Differential Operational Amplifier Stage