Huge noise of TIA circuit  - Page 1
Forum section: electronics / projects

author: Folnia
author_rank: Contributor
timestamp: June 22, 2025, 02:33:05 pm
content: Hello,I'm working on a TIA circuit to detect 100pA current, and bandwidth need to be around15kHz3kHz. The test current is generated by a voltage buffer on the PCB and a 10G resistor connected by coaxial wire.The actual current to detect is not APD or other photo-diode, but a fluid chip, which can be taken as a variable resistor in parallel with a small capacitor(currently have no ideas of the actual value). A constant dc voltage (1V) will add on the chip and then go into TIA:buffer(1v) -> fluid chp -> TIAAnd I found:1) When there's nothing connect to the buffer connector and TIA input connector, the noise was available, around ±60pA ( calculated using the gain resistor 10Meg)2) When I connected the resistor using coxial wire (around 20cm, and twisted), the noise was super large, >±15nA (sometimes obvious sine wave around 8kHz)3) When I weilded the resitor directly on the PCBA, the noise got much better, >±300pASo I think the axial wires and the pins of the resistor involve the noise from EMI in the space... so I plan to do some shieldingsIs there any good idea to solve it? (In the application there would be wires connected to a variable resistor/impedance )

author: Kleinstein
author_rank: Super Contributor
timestamp: June 22, 2025, 07:08:25 pm
content: The coaxial cable at the input would be quite some capacitance and the TIA may not be that happy with it and may oscillate. One could consider some series resistor (e.g. 50 ohms range) also for the input. It should not make a significant difference for the small current to measure, but reduce the effect of capacitance at the input and give some dampening to possible RF resonance with the input cable.

author: moffy
author_rank: Super Contributor
timestamp: June 23, 2025, 12:17:19 am
content: +1 for Kleinstein, the inverting input is a sensitive node.

author: Folnia
author_rank: Contributor
timestamp: June 23, 2025, 01:35:30 am
content: Thanks! I will try it. Does the buffer output need the iso-resistor as well？

author: moffy
author_rank: Super Contributor
timestamp: June 23, 2025, 03:07:06 am
content: Quote from: Folnia on June 23, 2025, 01:35:30 amIf talking about U10 the datasheet, page 14, recommends a 10R to 20R resistor for driving capacitive loads.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 23, 2025, 09:51:11 am
content: Some resistance at the output of U10 is a good idea.There is no absolute need to have an extra buffer at the output. The OPA365 is a rather fast amplifier - I don't think it is needed here.The DAC internal buffer already has quite some noise and limited bandwidth. So one should get away with a slower (e.g. 2 MHz GBW) and less power hungry amplifier. The 1 K resistors for the gain after the DAC are a bit on the low side.

author: tszaboo
author_rank: Super Contributor
timestamp: June 23, 2025, 10:07:14 am
content: I've also found that the type of coax cable matters a lot. Some are super noisy.I went through about a dozen different types for my PRP1 probe before finding one that had a good compromise between noise, usability, bandwidth and price.

author: dietert1
author_rank: Super Contributor
timestamp: June 23, 2025, 10:32:20 am
content: Our pulse oximeters have 470 Ohm input resistors for the photometer TIA. In the beginning we used inductors (beads) as EMI filters but resistors work, too. The oximeter photodiode runs at some uA of current, so it may get some extra millivolts of forward voltage. That adds to the opamp input offset voltage. Depends on the application whether that matters. The frequency limit of 470R with a 10 pF input capacitance is about 7 MHz.Regards, Dieter

author: electron_plumber
author_rank: Regular Contributor
timestamp: June 23, 2025, 02:25:38 pm
content: Have you measured the frequency of the noise content?Remember, the TIA's input impedance is low (ideally, 0) until the TIA starts to run out of bandwidth, and then it rises with frequency (like an inductor). So, from the perspective of the diode (or whatever you connect to the TIA input), the impedance lookingintothe TIA is a tiny resistor in series with an inductor (and a small parasitic capacitor).That inductor creates a complex conjugate pole with the input capacitance. By modifying what you have connected to the input, you're changing the input capacitance and therefore shifting that pole frequency. And by changing the input resistance (all other things the same), you're adjusting it's damping factor.It helps to model this and then measure the frequency of the noise as you change input impedances. If you have a bode plotter like the bode 100 you can also measure the input impedance -- although, the resistive asymptote is not always easy to resolve.

author: Folnia
author_rank: Contributor
timestamp: June 23, 2025, 02:28:41 pm
content: Hi Kleinstein，Thanks a lot for your reply. I planned to add a RC filter (100k,1uF) between the DAC and BUFFER but forgot the place for the capacitor. And would you mind explaining what '1k gain resistor' refers to? I had assumed that the larger resistor might generate more noise..

author: Kleinstein
author_rank: Super Contributor
timestamp: June 23, 2025, 04:03:20 pm
content: The resistors in mind are R20, R21. The datasheet suggests 10 K for this use and even this is relatively low. Noise would be mainly from the DAC, especially the low power buffer inside.

author: Terry Bites
author_rank: Super Contributor
timestamp: June 28, 2025, 02:57:26 pm
content: +Kleinstein has pointed out that the inv input is very (very) sensitive and C52 needs to be calculated to compensate for the pcb parasitics, connector, cable as well as the pd it's self.Plenty of help on this calculation from TI and AD app notes. If you get this wrong the amp may sit on the edge of oscillation and that can appear as noise.Attached is a noise analysis note.Ground noise?In low level PV circuits its posible to get some ground isolation by connecting the anode to ground with a resistor of the same value as R3.

author: David Hess
author_rank: Super Contributor
timestamp: June 29, 2025, 01:14:27 am
content: A more thorough solution is to bootstrap the unwanted capacitance at the inverting input by driving the ground of RF1 with a buffered version of the voltage at the inverting input. This can easily divide the input capacitance by at least a couple orders of magnitude, but it means that the coaxial cable shield cannot be externally grounded.

author: Folnia
author_rank: Contributor
timestamp: June 30, 2025, 01:32:23 am
content: Quote from: David Hess on June 29, 2025, 01:14:27 amHi David,Thanks for your suggestions. It seems I may use the triaxil connector and cable.

author: electron_plumber
author_rank: Regular Contributor
timestamp: July 02, 2025, 01:38:03 pm
content: Quote from: David Hess on June 29, 2025, 01:14:27 amAre you referring to something like Figure 4 in this doc?https://e2e.ti.com/cfs-file/__key/communityserver-discussions-components-files/14/Hoyle_2D00_TIA-Bootstrapping.pdf

author: Folnia
author_rank: Contributor
timestamp: July 02, 2025, 02:44:11 pm
content: Quote from: Kleinstein on June 22, 2025, 07:08:25 pmHi Kleinstein,I added 50ohm iso-resistor on the TIA input and it works much better. The oscilation frequency is around 7-8kHz by FFT. Now the oscilation is much smaller (decreased from 6000pAp-p to 600pAp-p) but it still exists. I tried change the Cf (paralled with the 10Mohm gain resistor) to 10pF, and also tried increasing the iso-resistor tp 1.6Kohm, but there's still oscilation...

author: David Hess
author_rank: Super Contributor
timestamp: July 02, 2025, 04:25:11 pm
content: Quote from: electron_plumber on July 02, 2025, 01:38:03 pmYes, and that is probably what I would do with an adjustment to trim the alpha of the buffer. At higher speeds, simple transistor buffers are used which have lower alpha.I would add some provision to drive the capacitance rather than using the output of an operational amplifier directly, although some operational amplifiers can drive any capacitance so maybe one of those would be good.Bootstrapping the photodiode makes a huge difference if the photodiode is located at the end of a coaxial cable.

author: electron_plumber
author_rank: Regular Contributor
timestamp: July 04, 2025, 12:59:57 am
content: Quote from: David Hess on July 02, 2025, 04:25:11 pmThere's so much literature on this I found it a bit confusing during my last TIA design. I ended up using this cascode approach, but I still struggled stabilizing the OPA855.

author: David Hess
author_rank: Super Contributor
timestamp: July 04, 2025, 12:02:59 pm
content: Quote from: electron_plumber on July 04, 2025, 12:59:57 amFrom the looks of the datasheet, the OPA855 is very finicky. Texas Instruments did not include a separate isolated output for feedback without reason.

author: Folnia
author_rank: Contributor
timestamp: July 04, 2025, 02:34:52 pm
content: Quote from: electron_plumber on July 02, 2025, 01:38:03 pmThanks for your documentation!Actually I'm not dealing with photo-diode. My application is to use a voltage buffer to drive a fluid-chip and use TIA to amplify the current. The fluid-chip peforms like a large variable resistor with a small paralled capacitor.I read the doc and I understand the bootstrap is to eliminate the AC voltage accross the photo-diode so the current noise decrease. But for my circuit, would you mind showing me a bit how to implement it? I did some simulation like what shown in the attached pic but I haven't seen any improvements.

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 04, 2025, 09:25:12 pm
content: No surprise you see a great deal of noise/interference when the capacitor is sitting so far away from the board. Generally the input node of a TIA should be kept as compact as possible, and connecting it via a cable is a recipe for problems.To put things into perspective, that 15nA of noise at 8kHz is equivalent to what you'd get if you injected 1V across 66.6Mohm, which could be due to just 0.3pF of stray capacitance at 8kHz. The actual stray capacitance from that big resistor to the outside world is many times that.Enclosing the entire resistor inside a shield (basically an extension of the coax shield around it) would likely help a lot. But depending on what your real sensor is like, that may not be feasible (especially if the fluid itself can couple to the sensor).

author: David Hess
author_rank: Super Contributor
timestamp: July 05, 2025, 04:20:35 am
content: For the unshielded but slow TIA that I designed and built for measuring leakage in diodes, I placed the feedback resistance in close proximity to the ground plane to reduce coupling from other sources. If I shielded it to remove stray coupling, then the resolution could be down to the 100 attoamp level. A higher performance design would solder directly to the inverting input pin without the socket, but I am regularly using this to test down to the 10 femptoamp level.Photo below is one of my experiments in focus bracketing.

author: Folnia
author_rank: Contributor
timestamp: July 05, 2025, 04:24:31 am
content: Hi Mtweig,I'm always concerned with followings:1) If the phase margin of the op-amp circuit is enough, will the noise pick up by cables/from environment (if not shielding well) cause the oscillation?2) If there's oscillation on the op-amp, assume it's very small, is it available? Should any oscillation, regardless of magnitude, be eliminated for proper circuit functionality?

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 05, 2025, 01:07:50 pm
content: Quote from: Folnia on July 05, 2025, 04:24:31 amActual oscillation due to instability in the amplifier circuit is completely different from external interference. The presence of external interference will not cause oscillation to happen. If the amplifier is unstable it will oscillate without any external interference.If the stability margins of your circuit are poor, this causes gain peaking near the crossover frequency. If there's an interference source near that frequency, then it could be boosted by the amplifier, making it look like the circuit itself is oscillating.Generally there are two empirical approaches to distinguish a real oscillation from interference:1. Try making changes to reduce interference without affecting the circuit itself. For example, by putting everything in a shielded box. Or just powering off other devices nearby which might be creating the interference. If the output signal persists despite cutting off interference sources, then it's likely a true oscillation.2. Adjust the gain/phase margins of the circuit while observing the output. For example, gradually improve stability by increasing feedback capacitance or reducing feedback resistance. If the unwanted output signal amplitude gradually decreases towards zero, then it was likely due to external interference. If at some point the unwanted output signal abruptly disappears entirely, then it was a true oscillation.QuoteIMO there's no reason to tolerate any sort of instability in the amplifier circuit.

author: mjs
author_rank: Regular Contributor
timestamp: July 06, 2025, 08:03:17 am
content: One noise source that was indirecly mentioned in the previous discussion is the e_n*Cd noise. The voltage noise of the opamp is multiplied in the input node capacitance (calculate the gain by feedback network and input network). You see this when you add a small capacitance (from your cable) to the input or remove it.Put the amplifier as close to the source as possible (no cables if feasible) and minimize input node capacitance.OPA855 has 8 GHz GBW, so no wonder it's difficult to stabilize. What is your GBW need ?Building Electro-Optical Systems.. by Phil Hobbs has good overview of TIAs and their noise sources (pp. 634 in the newest edition).

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 07, 2025, 04:38:06 pm
content: Quote from: mjs on July 06, 2025, 08:03:17 amI guess this will be true for frequencies above the corner frequency given by 1/(2*pi*Rs*Cin), where Rs is the sensor resistance.But even so I'd be surprised if the opamp's e_n is relevant. I'm assuming the sensor has a resistance on the same order as the feedback resistor (10Mohm), in which case its own noise will be orders of magnitude higher.Overall I don't think that components and design principles meant for high speed optical systems are suitable for this sensor....