TIA amplifier Layout Review - Page 1
Forum section: electronics / projects

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: August 29, 2025, 01:59:04 pm
content: Hi all,I have a basic TIA amplifier board based on OPA847 Opamp, while testing what I am seeing is peak to peak noise of 20mV at the input put.My input is from RIGOL DG922 function generator, which is very stable upto amplitudes of 1mV as well, But when I probe the input there is noise of 20mV peak which on amplification saturates the amplifier. I am using Lab bench power supplies for the same. to avoid any unwanted capacitance on the path I remove the ground under the opamp and the input line.

author: tggzzz
author_rank: Super Contributor
timestamp: August 29, 2025, 02:12:54 pm
content: If it is a TIA, the input is current, not voltage.Hence "peak to peak noise of 20mV at the input" is meaningless.

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: August 29, 2025, 03:08:06 pm
content: there is a input resistor of 100ohm I have kept the rf as 1k so as normal it should behave like a inverting amp with gain 10.

author: tggzzz
author_rank: Super Contributor
timestamp: August 29, 2025, 04:01:39 pm
content: Probably 150ohm, including the sig gen's 50ohm output.Experimental details matter, e.g. the complete schematic, a photo of the circuit and connections to test equipment.You need to experiment, to see what causes the noise to change or stay the same. That can give clues as to where the noise originates, e.g. in the opamp, in a resistor, coupled from an external source.First thing to try is removing the sig gen and grounding the input.

author: MariuszD
author_rank: Regular Contributor
timestamp: August 29, 2025, 07:25:33 pm
content: Quote from: Arsh ahmad on August 29, 2025, 01:59:04 pmThe cable capacitance is also added to the input capacitance.What cable did you use, and what is the capacitance of Cf?

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: August 30, 2025, 07:01:50 am
content: Earlier cf was 1pf now I have removed it and using the circuit as a basic inverting amplifier.The input is connectes to a fucntion generator using a twisted pair cable

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: August 30, 2025, 12:30:41 pm
content: Quote from: tggzzz on August 29, 2025, 04:01:39 pmI have verifed the circuit once again and found out that the circuit is oscillating at around 500Mhz.

author: tggzzz
author_rank: Super Contributor
timestamp: August 30, 2025, 12:54:26 pm
content: Quote from: Arsh ahmad on August 30, 2025, 12:30:41 pmTIAs tend to do that.Search this website for "Phil Hobbs" for other discussions about TIAs.

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: August 30, 2025, 12:59:12 pm
content: Quote from: tggzzz on August 30, 2025, 12:54:26 pmI am now trying use the circuit as an inverting amplifier, and grounded the input, then while seeing an output I am finding that the circuit is oscillating at 500Mhz and the output voltage is also quiet high.I have tried to put a feedback capacitor of 120nF to put high feedback compensation still I am getting the same oscillations

author: MariuszD
author_rank: Regular Contributor
timestamp: August 30, 2025, 03:05:57 pm
content: Quote from: Arsh ahmad on August 30, 2025, 07:01:50 amWith GBW= 3.9GHz even basic inverting amplifier is not easy.I hope the cable isn't connected directly to the inverting input.To make an inverting amplifier, you need to insert an SMD resistor into the cut trace between J1 and U3.

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: August 30, 2025, 04:56:35 pm
content: Quote from: MariuszD on August 30, 2025, 03:05:57 pmI have attached the schematic.For debugging the circuit, i have changed R14 to 1k to reduce the gain, and changed the c11 to 10nF to limit the bandwidth and make the system stable, still the opamp oscillates even with input ground. oscillations are upto 500mV peak to peak which is very huge and frequency is 500Mhz.

author: tggzzz
author_rank: Super Contributor
timestamp: August 30, 2025, 05:02:07 pm
content: So an APD is involved; I'm unsurprised.Save yourself a lot of time by reading about TIAs in The Art of Electronics 3 plus X-chapters, and in Phil Hobbs' bookhttps://electrooptical.net/Building_ElectroOptical_Systems/"Building electro-optical systems : making it all work" because "[it is] intended in the first instance for use by oppressed graduate students".Phil Hobbs is referred to several times in TAoE.

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: August 30, 2025, 05:07:02 pm
content: Quote from: tggzzz on August 30, 2025, 05:02:07 pmYes an APD is do involved but currently I have shorted the input of the 100 ohm resistor to ground. so circuit should work as basic inverting amplifier with gain of 10 and there should not be any oscillations. I have designed several TIAs in the past but never seen any circuit to oscillate this bad. Even with B filed probes I am able to pickup the 500Mhz oscillation

author: MariuszD
author_rank: Regular Contributor
timestamp: August 31, 2025, 01:05:47 pm
content: R11 has disappeared. Loading the output with cable capacitance also causes oscillations.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 31, 2025, 06:14:53 pm
content: I didn't get a decoupling idea on the PCB. May be I missed someting or some via.

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: September 01, 2025, 04:33:46 am
content: Quote from: Vovk_Z on August 31, 2025, 06:14:53 pmDecaps: Marked in red

author: Vovk_Z
author_rank: Super Contributor
timestamp: September 01, 2025, 05:44:25 am
content: Thank you, I was unattenive last time. Yes, there is decoupling.But, Gnd vias are plased quite far from each other for such a fast opamp.Possibly, you have to put decoupling caps close to each other, or add one cap right under an opamp (bottom side, from -5 VDC to +5 VDC).