Huge noise on the TIA output - Page 1
Forum section: electronics / projects

author: Folnia
author_rank: Contributor
timestamp: November 13, 2024, 02:59:10 pm
content: Hi,I'm testing my TIA board (OPA392 with 10MΩ gain resistor) and find that the output noise is huge, around ±50mV. Another board using TLC2201 (also 10MΩ gain resistor) has much better performance, only around ±5mV. There are both 50Hz common-mode disturbance on the both boards, but the TLC2201 one is also much better. The output noise are tested when nothing connected to the RF connector at IN-.Compared with the In of the two OPAmps, the TLC2201 do have smaller current noise 0.6fA/rtHz, when OPA392 is 70fA/rtHz. But based on a quick estimation using BW =300kHz, the OPA392 output noise should be 70fA/rtHz * √(300kHz) * 10MΩ = 0.38mV, shall be much less than what I tested.What I imagine is that the TLC2201 board uses a larger Cf(around 400pF tested by DMM), so I will try to change a larger Cf to limit the bandwidth.The sch and layout are attached. If some one can find some hints plz let me know, many thanks in advance!

author: Andree Henkel
author_rank: Regular Contributor
timestamp: November 13, 2024, 04:20:45 pm
content: Where do you get your 300kHz BW estimate from?TIA circuit bandwidth is completely different to inverting or noninverting amplifier, using same OPV.what is capacity of your photodiode? Which mode do you work? Photoconductive (reverse bias) or photovoltaic (zero-bias)?Try switching to reverse and apply a biasing voltage. This can greatly reduce capacity of photodiode.The input noise voltage of your OPV will interact with the impedance resulting from photodiode capacity.Thus generate a noise current, which will flow through your feedback resistor, resulting in noise at the output.While the input noise voltage itselv times TIA gain of one is most times negligable compared to noise of resistor and the input current noise times resistor. This noise mecahnism depending on diode capacity, input voltage noise can be the most important.Take a look at:Texas Instruments Application Note:https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.tij.co.jp/jp/lit/an/snoa942a/snoa942a.pdf&ved=2ahUKEwjS45fQ2tmJAxVy9gIHHW2rBTAQFnoECB0QAQ&usg=AOvVaw3sqPXzvmqrTXQspQWbER6T

author: Folnia
author_rank: Contributor
timestamp: November 13, 2024, 04:45:07 pm
content: Hi Andree,Thank you but as I mentioned that the noise is measured when there's nothing connect to the IN- connector. And in my application is to measure pico current flowing a resistor but not a photodiode.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 13, 2024, 06:02:07 pm
content: The feedback capacitance makes a big difference with the bandwidth. More BW with the OPA392 gives more noise.0.1 pF is really small and may even be in a range where the circuit could ring, at least with added capacitance at the input.For the noise, there is also the noise of the 10 M resistor and the voltage noise of the amplifier.It could help to loop at the spectrum of the noise, e.g. with a scope and it's FFT function.

author: TimFox
author_rank: Super Contributor
timestamp: November 13, 2024, 06:16:07 pm
content: The thermal noise current from a 10 megohm feedback resistor is 40 fA/Hz1/2.The thermal noise voltage is 400 nV/Hz1/2(You can calculate either the noise voltage or the noise current from a resistor, but you can only use one value at a time since they represent the same noise power.)

author: mtwieg
author_rank: Frequent Contributor
timestamp: November 13, 2024, 07:16:25 pm
content: With a 10Mohm feedback and no well-defined source impedance, I wouldn't expect the output noise to match back-of-the-envelope calculations. 50mV output corresponds to 5nA of input current, which over >100kHz of bandwidth could be coupling in via extremely small parasitic capacitances, not just the components soldered to the input pin.I would check that the thing is actually stable first, and post the FFT of the output, as requested above.

author: Folnia
author_rank: Contributor
timestamp: November 14, 2024, 06:54:48 am
content: Yes, now I'm pretty sure it's due to the bandwidth limitation caused by Cf. The TLC2201 board has a 1nF Cf and the BW might be under 50Hz!I switched the Cf on the OPA392 board to 100pF then the noise is almost the background of the scope except a few 50Hz interbulance.My purpose to put a 100f capacitor here is to ensure there's some capacitance here to keep stability (actually the 0402 footprint might also contribute some), but still have enough bandwidth (>100kHz). And based on the simulation, it should be OK.Phase margin when Cf = 0.1pF/5pF/100pFBW when Cf = 0.1pF/5pF/100pFLooking for a good way to have both low noise and enough bandwidth... (I alos tried T-network but the noise leve is almost the same ...)PS: The FFT of the output when Cf=100f.