high impedance single die amplifier - Page 1
Forum section: electronics / projects

author: coppercone2
author_rank: Super Contributor
timestamp: July 15, 2025, 04:49:28 am
content: If you have a multiple high Z input opamp IC, and you get a problem where one signal is 'overriding' the other signal(s). Say two non inverting amps on one chip at AC.What trouble shooting can you do in such a design? If it already has a ground planeThis is for a Apex PA76 power amp chip.The first thing I did is put a 50 ohm load to ground on each of the inputs, but I am not sure if that would even theoretically help anything, since the traces (or die?) are/is still coupled together.Does it make sense to think about the problem in terms of cross talk? Because I am not even sure if that is the correct methadology. It's like one signal completely takes over both circuits, so they output the same thing.I suspect that it might work if I use two chips and place them away from each other. Are there any other solutions for getting rid of this problem when its a dual chip?I took a long break from this circuit idea. I did have it work before on some other dual power op amp IC (I think it was a discontinued PA74) when it was on a protoboard with basically the same feedback circuit. I wonder if the reason why it could have worked is because it did not have a ground plane.Oscilloscope shows the input is correct when I probe on each input pin. That makes me think that its something with the chip itself.I thought to make shields around just the feedback loops, even though the amplifiers share one can.The only thing I think I did substantially differently is that I had jelly bean op amps to buffer the input signal to the PCB. In this case I am using multiple function generators. But since I probe the signal and it looks OK at the input, can this be some how related? I can't see what a voltage follower could do for it, if I already see the correct signals on the input of the chip.'Anyway what is the term for this phenomena? Because crosstalk to me feels like it would be if you probe the input of the chips and the signal is shitty/confusing. But if you have good looking signals at the input to the chip but they are outputting like they have the same input. To me thats like something is happening inside or maybe the feedback loops are talking to each other?

author: MariuszD
author_rank: Regular Contributor
timestamp: July 15, 2025, 08:31:21 am
content: Everything can be measured. If this is capacitive coupling will depend on frequency. Parasitic couplings inside IC are well below 1 pF maybe even below 100fF. Is the IC package grounded? Capacitances to the package can be higher.The input impedance of the amplifier is not an issue. The impedance of the circuit connected to the input is important.This is the power amplifier. Feedback resistors can be in the kiloohm range. Show us the schematic and layout.I made transimpedance amplifiers with resistors in the hundreds of megaohms range, capacitive coupling at the level of 100fF mattered, but it shouldn't for you.If the signal leaks where it shouldn't, it's called crosstalk, regardless of whether waveform looks clean. If it has the same level, I suspect a short circuit and not a crosstalk.Impedance of 1 pF at 1 MHz is 160 kOhm It can't be considered a short circuit in the kiloohm range feedback divider.

author: coppercone2
author_rank: Super Contributor
timestamp: July 15, 2025, 09:26:51 pm
content: OK yeah I was thinking of the package. Maybe its a poor ground on the can through a bolt. I was thinking to make a short to ground the package like a impedance analyzer short, by making a sheet metal spring thing that connects it to the ground.. the big difference is the prototype that worked had a different package and also no socket.

author: MariuszD
author_rank: Regular Contributor
timestamp: July 16, 2025, 10:14:55 am
content: QuoteTest it at DC. If it is connected, it should work.I don't know your schematic or PCB, I don't know how the signal path looks in it, so I won't tell you how to fix it. The problems you describe cannot be caused solely by the fact that the two amplifiers are next to each other.

author: temperance
author_rank: Super Contributor
timestamp: July 16, 2025, 11:59:26 am
content: QuoteThis doesn't make any sense.QuoteSorry, but I can't make anything of that either: "I was thinking to make a short to ground the package like a impedance analyzer short".You are thinking about cross talk and impedance as if the circuit is an RF circuit while the op amps has a GBW of only 1.4 MHz. Or nearly DC.Cross talk is a matter ofsourceimpedance and mutual capacitance and inductance between the two signals. The mutual inductance can be removed from the equation in this case because you are working with very low frequencies and short connections. The input impedance of the op amp is irrelevant.https://psec.uchicago.edu/blogs/lappd/wp-content/uploads/2011/07/Anode-presentation-Crosstalk.pdfIf two op amps in a single package are as you wrote "'overriding' the other" at the frequencies you are working with it can be:-Unsufficent decoupling, the circuit might experiance oscillation.-Driving a load for which the op amp was not made and the thing bursts into oscillation.-The feedback loops share a common ground trough which some current is flowing. The circuit is now amplifying its own output signal and probably oscillating. But you have a GND plane so that should not be an issue unless you did something very stupid with the feedback. (Feedback must be placed close the op amp input. Or in other words: no long connections must be made between the op amp neg. input and the feedback circuit)https://www.analog.com/media/en/technical-documentation/application-notes/AN-202.pdfFor your PCB traces, stray capacitance between two traces:https://e2e.ti.com/cfs-file/__key/communityserver-discussions-components-files/14/Parasitics-in-Precision-PCB-Layouts_5F00_Feb2013.pdfPage 34Plug in some numbers and calculate the stray capacitance between two adjecent traces. From there calculate the impedance between those two traces at some frequency of interest. Then take the source impedance and calculate the expected crosstalk between two adjacent traces.

author: coppercone2
author_rank: Super Contributor
timestamp: July 16, 2025, 02:01:49 pm
content: yeah its a little weird circuitI was thinking that maybe its oscillation too. I thought to look at it with a spectrum analyzer to see if there is some weird frequencies. The circuit is a weird idea. But it basically did work with some other apex amplifier chip that is discontinued, for a while at least... it gave me exactly what I wanted to see on the current and voltage waveform.But IIRC I did look at this with a 500MHz scope (TDS3054) and nothing stood out as oscillating to me, but I did not even do a FFT at the time.I think I have an idea of what I can try to look through nowI need to get rid of the local regulators completely because I did determine their sequencing was causing a big problem. I think I bypassed it before but I should remove all the parts from the PCB and just make sure its connected to tracking lab supply and some decoupling capacitors

author: temperance
author_rank: Super Contributor
timestamp: July 16, 2025, 02:46:40 pm
content: QuoteWithout a schematic or anything I and probably anyone else can't make much of what you are writing. Simple analogue circuits almost never need power supply sequencing. I also think you are looking at all this from the wrong angle and overcomplicating matters. Those op amps are way too slow to oscillate at very high frequencies, for which you would need a Ghz spectrum analyser or scope with a bandwidth higher than let's say 2 MHz. The amplifier you've got have a gain of 1 at 1.4 MHz. Nothing fancy. You are very far away from RF frequencies where a piece of wire is no longer a piece of wire.The problem(s) you are seeing should be easy to solve with basic knowledge.

author: coppercone2
author_rank: Super Contributor
timestamp: July 16, 2025, 07:57:27 pm
content: everyone but me seemed to know that bipolar power amps have power sequencing requirements, enough that they blow upthere was another thread on that matter, In the end I put low voltage drop diodes but I think I did determine how to power sequence it 'properly' on a breadboard, but it had a janky response.It 'latches up' if the power rails do not follow within some marginI heard of it but thought it would be 'rare' to get a problem. Instead it turned out to be guaranteed.If you don't know, making custom non tracking supplies for high power dual supply op amps has a good chance of burned expensive chips.And I had another pure signal PCB for temperature related control with a lot of low power op amps that had a power sequencing requirement also! long ago. But I just solved it by putting a small RC delay circuit on the power rail of some thing I eventually found to be responsible. It had about a 30% chance of entering a 'fault' condition when it did not power up the different circuit elements in the correct order.IMO its defiantly NOT something you should gloss over when there are different blocks in the design.

author: temperance
author_rank: Super Contributor
timestamp: July 17, 2025, 12:05:02 am
content: QuoteAgain, without schematics, we have no idea of what you might be doing. Low voltage drop diodes, but it had a Janky reponse,... Are we supposed to guess what you are doing? Low voltage drop diodes are probably schottky diodes. What is "a janky response"?Circuits with op amps powered by typical voltage regulators will start up fine without tracking power supplies unless a low resistance load is attached to the output which might overload the output stage if the output is driven towards the active rail while the input stage isn't working properly yet. This is not a property of bipolar op amps per se, and this applies to any circuit. It is something you must think about. What will happen if? Or maybe you are applying an input voltage from a low output resistance source with no power applied.