TIA Ground Plane: Better to just remove it? - Page 1
Forum section: electronics / beginners

author: LoveLaika
author_rank: Frequent Contributor
timestamp: May 11, 2021, 02:56:09 pm
content: I'm working on a design for a transimpedance amplifier on a 2-layer board, with both layers being the ground plane. I may be overthinking this, but according to the article below, while a ground plane is best when not broken, one exception is for high-speed op-amps. They recommend removing the ground plane from the op-amp's input/output pads. My signal pulse has a frequency of 1-2 MHz (and from what I can tell, that's not considered high frequency, which is 3-30 MHz looking at radio frequency), but does that just refer to the op-amp itself, or is it better to surround the entire traces for input/output? Or, perhaps it is unnecessary?My schematic and top copper plane with the cut-out are shown below. My no-fill zone to remove the copper plane encompasses the op-amp and its input/output connections.Despite the conventional practice of removing the copper, the the surface area that is cut out seems to be quite big (for a 2 inch by 2 inch board).https://www.analog.com/en/analog-dialogue/articles/high-speed-printed-circuit-board-layout.html

author: tggzzz
author_rank: Super Contributor
timestamp: May 11, 2021, 03:14:10 pm
content: A few points, not complete answers...Your signal's fundamental frequency isn't that important. The edge rate is more important, as is the frequency/phase response of your circuit.The importance, or otherwise, of parasitic capacitance and inductance can be understood in terms of the poles and zeroes they insert into your circuit's transfer function. That kind of ting has been well described elsewhere, and I couldn't do any better.One starting point w.r.t. TIAs would be Phil Hobb's works. He pushes TIAs to their limits as part of his day job. See his postings on sci.electronics.design and has websitehttps://hobbs-eo.com/Building_ElectroOptical_Systems

author: awallin
author_rank: Frequent Contributor
timestamp: May 11, 2021, 03:52:46 pm
content: For 1-2 MHz that design should work OK. The op-amp has maybe 3pF of input capacitance anyway and you may need a larger C1 cap for stability.There's usually a reverse bias circuit close to the photodiode, to reduce the photodiode capacitance. Hanging the photodiode off a long wire (as maybe suggested by your PCB?) is not a good idea.more ground vias where you do have 2 gnd-zones don't hurt.BNC or SMA output from the board would be a suggestion, close to the output-impedance resistor.If you want to go high speed the board-capacitance matters more. And also capacitive loading of the op-amp. I've used BUF602 as an output buffer after the op-amp (some designs there:https://github.com/aewallin/One-Inch-Photodetector)

author: David Hess
author_rank: Super Contributor
timestamp: May 11, 2021, 04:59:04 pm
content: Quote from: LoveLaika on May 11, 2021, 02:56:09 pmThey are referring to the bandwidth of the circuit and not the signal. Parasitic capacitance at the inverting input combined with the impedance of the feedback network creates phase lag which lowers the phase margin. A compensating capacitance can be added across the feedback resistor however this lowers the bandwidth so for maximum bandwidth, the ground plane needs to be cut away from the feedback network and inverting input.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: May 11, 2021, 09:01:34 pm
content: Oh, believe me, I got plenty of ground vias all over the board. Thanks for the suggestions. Got them all taken care of, though I'm definitely not hanging my photodiode off-board via wire. Otherwise, this whole thing's a bust.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: May 11, 2021, 09:02:49 pm
content: Thanks. I guess I'll have to do this for every design from now on if I can. This seems like a good practice.

author: StillTrying
author_rank: Super Contributor
timestamp: May 11, 2021, 10:00:30 pm
content: The main place needing the capacitance as low as possible is the -Ve input, which is all of the CAT1 track including the other side of the board under that track.Are you sure that gain is correct, 50R to 17R seems very low.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: May 11, 2021, 10:10:01 pm
content: That 50-ohm value is just a place holder. The actual values will vary.I made sure to remove the copper along the CAT 1 track on both sides.