opamp output PCB layout question - Page 1
Forum section: electronics / beginners

author: K3mHtH
author_rank: Regular Contributor
timestamp: July 17, 2020, 10:14:15 pm
content: Hey everyone,So I have an audio circuit that has several op-amp stages cascaded into each other. What is the best practice for the trace connecting one op amp into another, especially if they are longer and go through board-to-board connectors/wires.Do I just run the op-amp output directly where it needs to go, hitting the next opamp's "input resistor" which would be located closest to it's respective op-amp? This would leave the trace very low impedance and less susceptible to noise right?I've seen some schematics where the op-amps also have "output resistors", low values - typically 1k. Is there any reason to add these right next to the op-amp before the long PCB trace? Is there any advantage to this? It seems like it would just load the next stage down...Thanks everyone!

author: David Hess
author_rank: Super Contributor
timestamp: July 18, 2020, 01:09:42 am
content: Quote from: K3mHtH on July 17, 2020, 10:14:15 pmThat is right.QuoteThe trace acts like a transmission line which can cause the output of the operational amplifier to oscillate so a low value dampening resistor is added in series.

author: K3mHtH
author_rank: Regular Contributor
timestamp: July 18, 2020, 03:19:03 am
content: Quote from: David Hess on July 18, 2020, 01:09:42 amThank you. Is there a guideline on when to add these output resistors and what value to use?Can this oscillation be mitigated without the dampening reisstor if the op-amp gain stage already has a low-pass capacitor added to the feedback loop?

author: David Hess
author_rank: Super Contributor
timestamp: July 18, 2020, 05:07:42 pm
content: Quote from: K3mHtH on July 18, 2020, 03:19:03 amAs a lower limit, I usually start with doubling the open loop output resistance of the operational amplifier which is often about 50 ohms.QuoteNo, the feedback capacitor does not help but there are alternatives to a series resistor like a Zobel network.The reason a series resistor works is that at low frequencies, the transmission line looks like a shunt capacitor to ground. This combined with the open loop output resistance of the operational amplifier adds phase lag within the feedback network and if the phase margin falls too low, then oscillation can result. The series resistance neutralizes part of the phase lag.Sometimes a series RC network from the output to ground is used to lower the gain at higher frequencies to prevent oscillation. Or the feedback capacitor might be replaced with a network which introduces less phase lag although I have never seen this done in audio applications. One way I have done it is shown in the last example here which limits the output bandwidth but provides correction for the added output series resistance at lower frequencies:https://www.analog.com/en/analog-dialogue/articles/ask-the-applications-engineer-25.htmlAnother option which can help is to use an operational amplifier which has a lower output resistance, or provide a buffer inside the feedback loop to do the same thing.

author: K3mHtH
author_rank: Regular Contributor
timestamp: July 18, 2020, 11:18:45 pm
content: Quote from: David Hess on July 18, 2020, 05:07:42 pmThanks David, this helps a lot - I appreciate it. Are these measures usually done on PCB layouts when a trace is long, or is this more of an after-the-fact mitigation when someone notices a problem? Seems to me like this would be ideal for situations where the output is run through long wires, etc.

author: David Hess
author_rank: Super Contributor
timestamp: July 19, 2020, 06:12:32 pm
content: Quote from: K3mHtH on July 18, 2020, 11:18:45 pmExperienced designers at least include these options in the board layout whether they are ultimately needed or not. But it does happen that problems are only caught later.The other usual design rules like separating power supply lines from "common" and using single point grounds applies. Larger designs may use differential signaling to avoid ground loops.Douglas Self wrote a book called Small Signal Audio Design which covers what it says although I do not remember a lot of detail in it about layout problems. He does mention using a series resistance at the output of a line driver and "zero impedance outputs" which is the same as the example I linked to.

author: K3mHtH
author_rank: Regular Contributor
timestamp: July 22, 2020, 06:03:08 am
content: Quote from: David Hess on July 19, 2020, 06:12:32 pmThanks David, that look sounds worth reading.I want to ask a very dumb question:If I am sending the signal from an op-amp directly into an op-amp (configured as as buffer/follower), is the trace connecting the two considered "high impedance" or "low impedance"? I feel bad for not knowing this.. the source op-amp's output is a very low impedance and the only thing it is connected to is the very high impedance of the next op-amp. Is this circuit particularly susceptible to noise in this configuration?

author: Zero999
author_rank: Super Contributor
timestamp: July 22, 2020, 10:02:44 am
content: What op-amps are you using? If it's fairly standard, low frequency op-amps, such as the NE5532 or TL072 and you've not got very high imput impedances, then don't worry about layout too much. Keeping the input resistors close to the respective op-amp is a good idea. If you go for output resistors, you'll have to account for the fact they'll reduce the gain slightly.

author: David Hess
author_rank: Super Contributor
timestamp: July 22, 2020, 06:26:31 pm
content: Quote from: K3mHtH on July 22, 2020, 06:03:08 amIt is considered low impedance because of the source. There should not be any problems with short lines and no series resistance at the source is required.

author: K3mHtH
author_rank: Regular Contributor
timestamp: July 25, 2020, 09:12:16 pm
content: Quote from: Zero999 on July 22, 2020, 10:02:44 amI am using mostly TL07x and an NE5532, you nailed it.When you say worrying about high input impedances, what do you consider "high"? Generally I use 100k as an input impedance on inverting-amps, but I have a few stages that are non-inverting buffers as well.

author: Zero999
author_rank: Super Contributor
timestamp: July 27, 2020, 08:24:05 am
content: Quote from: K3mHtH on July 25, 2020, 09:12:16 pmFor short distances and the high signal levels you'll typically have on the output of an op-amp 100k is not going to cause much of an issue with noise pick-up.The NE5532 and TL072 are two totally different types of op-amp. The NE5532 has BJT inputs and the TL072 has J-FET inputs.100k is a bit too high for the NE5532, but is fine for the TL072. The reason for this is input noise, which is a differnet topic to PCB layout and isn't related to noise pick-up, but it's worth visiting, since you've mentioned input impedance.Op-amps have inherent noise sources on thier inputs. There's the input voltage noise, which is simply a noisy voltage source in series with the inputs and input current noise, a fluctuation in the bias currents and is a current source in series with the inputs. The current noise, becomes a voltage noise, when a resistor is connected from the input to ground, or in the feedback loop of the op-amp, due to Ohm's law.The NE5532 has much less input voltage noise, than the TL072, but much higher current noise. The TL072 has higher voltage noise, than the NE5532, but much lower current noise.The NE5532 is less noisy, than the TL072, when the inputs are connected to low impedances.The TL072 is less noisy, than the TL072, when the inputs are connected to high impedances.Off the top of my head, the crossover point, in input impedance, when the TL72 becomes less noisy, is for input impedances of around 15k.I'll post some simple calculations later, if you're interested and someone else hasn't beaten me to it.

author: K3mHtH
author_rank: Regular Contributor
timestamp: July 29, 2020, 01:50:40 am
content: Quote from: Zero999 on July 27, 2020, 08:24:05 amI'd love to see this actually Zero if you get around to it. Not for this project, but something else I'm working on is a spring reverberation circuit and I'm debating what op-amp to use for the recovery amplifier. I'm using the op-amp in non-inverting amplifier configuration with a 10k or 12k resistor across the feedback path and a 390 resistor off the inverting side going to ground (gain 26.6). I have TL074 and NE5532 available on the BOM and at this point I pretty much can't hear any noise differences between the two opamps at this point. Most of the noise is from the spring reverb that is proving to be pretty susceptible to picking up noise (including buzz from the 16VAC switch that was nearby, but I since moved).

author: David Hess
author_rank: Super Contributor
timestamp: July 29, 2020, 02:46:28 am
content: Quote from: K3mHtH on July 29, 2020, 01:50:40 amIf you ignore Johnson noise from the source resistance, then it is just the voltage noise density divided by the current noise density and this is a good place to start for low to medium impedance circuits. Several hundred ohms to several kilohms is typical for bipolar operational amplifiers but low input bias current bipolar parts (descendants of the LM308) can be much higher and competitive with JFET and CMOS parts, although you will not find these bipolar parts in audio applications except as servo control loops where they are very useful.The lowest noise devices actually have problems driving their own feedback networks at low gain because the impedances have to be low to preserve their low noise performance. In this case, a power buffer should be added within the feedback loop which may only amount to a transistor operating in class-a.

author: Zero999
author_rank: Super Contributor
timestamp: July 29, 2020, 06:15:40 pm
content: Quote from: David Hess on July 29, 2020, 02:46:28 amThat would be the input noise power. The effective input voltage noise density, due to the bias current noise, is the current noise density multiplied by the source impedance.Quote from: K3mHtH on July 29, 2020, 01:50:40 amRefer to the application note linked below:https://www.analog.com/media/en/training-seminars/tutorials/MT-047.pdfAnd the data sheets for the TL072 & NE5532:https://www.onsemi.com/pub/Collateral/NE5532-D.PDFhttps://www.ti.com/lit/ds/symlink/tl072.pdf?ts=1596007990089&ref_url=https%253A%252F%252Fwww.google.com%252FExample.The input resistor is 10k and you're using the NE5532. The feedback resistor is 200k and can be ignored for the input noise calculation, because it's very high, compared to RIN.The Johnson noise will be the same, irespective of which op-amp is used, so it's not important for deciding which IC to use.Input voltage noise density for the NE5532 is 5.0nV/√Hz and the input current noise density is 0.7pA/√HzThe input resistance is 10k, so the noise voltage across RIN is.VN_RN = IN*RIN = 7nV/√HzThe total input noise is equal to the root of sum of the squares.VN(TOTAL)= √(VN2+ VN_RN2) = √(5n2+7n2) = √(25n+49n) = 8.6nV/√HzThis is obviously less than the TL072's input voltage noise density of 18nV/√Hz, so the NE5532 is the beter op-amp in this case. If you multiplty both resistor values by 10, to get 100k for RIN and 2M for RF, you'll find the NE5532 is more noisy, than the TL072.

author: K3mHtH
author_rank: Regular Contributor
timestamp: July 29, 2020, 09:06:42 pm
content: Quote from: Zero999 on July 29, 2020, 06:15:40 pmGreat thank you. If I'm using a non-inverting configuration and feeding the signal directly into the non-inverting pin, what am I using for the RIN value to calculate the noise?

author: David Hess
author_rank: Super Contributor
timestamp: July 30, 2020, 02:58:04 am
content: Quote from: Zero999 on July 29, 2020, 06:15:40 pmI have never heard it called noise power but it is the point where a different source resistance would benefit from a different operational amplifier. The Johnson noise from the resistor can be included but at medium and low resistances, it can be ignored for rough calculations. At high impedances it dominates.

author: Zero999
author_rank: Super Contributor
timestamp: July 30, 2020, 08:35:49 pm
content: Quote from: K3mHtH on July 29, 2020, 09:06:42 pmR14 & R353 can be ignored, especially if you're using the TL072, with its tiny noise current.The impedance of whatever is connected to J2, is RN, for the purpose of noise calculations. I presume it's under a tenth of R352, in which case the NE5532 will be less noisy, than the TL072.What does C5 do? It will drastically reduce the input impedance at higher frequencies and could cause the circuit driving it to oscillate, especially if it's an op-amp, which won't appreciate a 2.2nF capacitive load.You might want to consider making C6 non-polarised or use two 22µF capacitors connected back-to-back. Although polarised capacitors can take AC, at very low voltages, there will be more distortion, than with a small DC bias or a film capacitor.

author: K3mHtH
author_rank: Regular Contributor
timestamp: July 31, 2020, 11:27:28 pm
content: Quote from: Zero999 on July 30, 2020, 08:35:49 pmYes J2 is under a tenth of R352. J2 is a 2.2k impedance coil (200 ohm DC resistance, 270mH inductance - going by the datasheet). C5 is suggested by the manufacturer to accentuate higher frequencies. So far I haven't had a problem with oscillation, but I have only used TL074 for this circuit so far. Not sure if NE5532 is more susceptible to that problem?C6 is already non-polarized, sorry. I am lazy in my schematic and use the polarized symbol.