differential amp for reverse biased photodiode - Page 1
Forum section: electronics / projects

author: thodoris46
author_rank: Newbie
timestamp: October 11, 2014, 06:27:19 pm
content: Hello everyone, i designed a card to Read with an ADC the current of a photo diode InGa .I have designed something similar with the following modules-TIA (Transimbedance amp)-sallen key low pass filter-16 Bit ADC-In Another card High voltage supply for the photo diodeThe previous design had a problem with 50 Hz humming (I'm pretty sure that it passes through magnetic field) I shielded the system with copper tape and worked fineNow this photo diode has a heatsink that the photo diode fits inhttp://www.hamamatsu.com/eu/en/product/category/3100/4007/4175/A3179/index.Out of the heat sink comes a coaxial cable where the center conductor is the anode and the shield the cathode , it has another cable for TEC cooling but there is no noise there.I have not been able to shield the system and I'm going to try cutting the humming with an instrumentation amplifier.The expected signal is dc and the noise is 50 Hz with harmonics in 75 and 100 Hz.I made a design (pdf) but it must be wrong .. because when you get the signal from the first op-amp (TIA) it changes it's phase, so on the second op-amp (IN-amp) the 50 Hz noise will have a difference in phase from signal to Ground.Another approach is putting the anode-cathode of the photo diode on the inputs of an in-amp, but where am i going to put the 0,5 V reverse bias voltage and how will the 0,5 V bias voltage effect the in-amphelp guys...

author: doktor pyta
author_rank: Frequent Contributor
timestamp: October 11, 2014, 08:36:18 pm
content: HI,Try to answer few questions first:1. What is the purpose of U8? I assume You want to get stable low noise -2.5V photodiode bias voltage. But why are You using divider R35, R34? I think that better solution would be using only RC filter (with high time constant) to bias the anode of PD.2. Connected with 1. : You should decouple to ground anode of the PD. Any reason why not doing this ?3. What is the reason of using IN-AMP U12 ? As I can see You are using it as typical single ended amplifier (waste of money) and typical low noise OP-AMP + two resistors could be used.4. Question about U6. Datasheet says: "+VBD – Digital I/O power supply. Decouple with GND pin." Why are You using R9 ?

author: thodoris46
author_rank: Newbie
timestamp: October 11, 2014, 09:26:44 pm
content: Thank you for the reply !!1. I need a -0.5 Volt to bias the PD that's why i'm using the voltage divider.You mean unregulated supply with an RC filter ? I prefer using regulators and reference chips to power sensitive sensors.2. You are right a cap is needed, in my former design a used a 100 nF but i am skeptical if that capacitor effects the transfer function of the TIA. The capacitance of the PD inserts a zero in the transfer function which you have to compensate with the feedback cap. I wonder if the decoupling cap has the same effect on the transfer function of the TIA. I was thinking of putting a decoupling cap and put a buffer between the voltage divider-decoupling cap and the anode. I'd like to hear you're opinion on this3. You mean an op-amp in differential mode ? The in-amp has better input impedance than an op-amp and cost 3-5 euros more, that kind of money we have4. ..mistake from placing parts fast..the 1K resistor is connected to SDI and then to +VBD supply as seen in Figure 47

author: doktor pyta
author_rank: Frequent Contributor
timestamp: October 11, 2014, 09:55:58 pm
content: Maybe let's consider if You really need this -0,5V bias. From My experiences with InGaAs photodiodes I know that their capacitance can't be reduced by reverse biasing as much as in Si photodiodes.See datasheet of a typical InGaAs PD:http://www.hamamatsu.com/resources/pdf/ssd/g8605_series_kird1049e06.pdfSo it will slightly lower the capacitance (page 3) but it will increase dark current (page 2).It seems You want to obtain obtain bandwidth of about 30kHz (?)Decoupling of photodiode's anode could help in reducing the 50Hz hum.I'd use let's say 100k, 100uF RC lowpass filter.I really don't think this hum is magnetically coupling into Your system.Oh, and when it comes to replacing AD8221 I'd suggest using non-inverting amplifier.Often the hum while testing such systems comes from fluorescent or incandescent light sources in the lab.

author: thodoris46
author_rank: Newbie
timestamp: October 12, 2014, 08:02:27 am
content: Yeah but if i don't have a bias voltage it will lower the gain of the PD right ?The light will be DC so i don't really care about bandwidth.When i take a measurement without the PD by sort circuiting the inverting input with ground, 50 Hz is gone. The system is in a small suitcase to have dark conditions. When i ground the metal parts of the suitcase on true ground or reference ground the 50 Hz is gone. That leads me to think that the pcb card is not responsible for the humming and its because the wire-photodiode-heatsink system.I've done measurements with the suitcase closed a thick black cloth over it and the lights and windows of the lab closed. Also the signal is within the manufacture dark current. The InGaAs i'm using is G12183-105K