Looking for everything wrong with my input buffer design. - Page 1
Forum section: electronics / beginners

author: R-802
author_rank: Contributor
timestamp: June 14, 2025, 03:12:21 pm
content: Hello, I'm designing a PCB for digital guitar effects processor and am currently working on an op-amp based input buffer to get a low impedance drive suitable for an ADC.Planning on using the OPA1642 (https://www.ti.com/lit/ds/symlink/opa1642.pdf) op-amp from TI and TAC5212 codec (https://www.ti.com/lit/ds/symlink/tac5212.pdf). See the attached screenshot for the current schematic.I have never designed a circuit like this before so any critique, guidance or suggestions for improvement would be greatly appreciated.

author: ahsrabrifat
author_rank: Regular Contributor
timestamp: June 14, 2025, 03:13:45 pm
content: Did you simulate your circuit?

author: R-802
author_rank: Contributor
timestamp: June 14, 2025, 03:17:31 pm
content: No? I will try get that setup right now. I see Texas Instruments has TINA-TI, I guess I could use that or LTSpice? I did simulate the transfer function of the bandpass filter to ensure no effect on audible frequency band, see attached.

author: MariuszD
author_rank: Regular Contributor
timestamp: June 14, 2025, 03:48:27 pm
content: Many things are wrong.Look here:https://www.diyaudio.com/community/threads/single-supply-buffer-help.274302/post-4325418This is correct, single supply, AC coupled buffer. Some components should be changed.

author: SteveThackery
author_rank: Super Contributor
timestamp: June 14, 2025, 09:00:07 pm
content: I'm not qualified to advise the OP, but I do have a question. There appears to be no feedback at DC, so presumably the output of the op-amp will sit hard against one of the supply rails?

author: TimFox
author_rank: Super Contributor
timestamp: June 14, 2025, 09:26:08 pm
content: There is no DC feedback, nor negative power connection to the amplifier.

author: R-802
author_rank: Contributor
timestamp: June 15, 2025, 01:38:14 am
content: I read in in the docs that the OPA1642 can take a single supply voltage? Guessing i'm interpreting that incorrectly.

author: ledtester
author_rank: Super Contributor
timestamp: June 15, 2025, 01:56:17 am
content: Quote from: R-802 on June 15, 2025, 01:38:14 amAll that matters is the difference between V+ and V-.The datasheet says that the min difference is 4.5V and the max difference is 36V.If you use a balanced +/- supply you need at least +/- 2.25V and it can go up to +/- 18V.But you can set V- equal to -1V and V+ to 3.5V and that will also work for the op-amp.Or you can set V- to 0V and V+ anywhere between 4.5V and 36V.At end of this page:https://beavisaudio.com/techpages/buffers/is an op-amp buffer circuits where the op-amp supply V- is set to GND and V+ to 9V. AGND is set to 4.5V (half of the supply voltage).

author: R-802
author_rank: Contributor
timestamp: June 15, 2025, 02:55:40 am
content: Ok, I've just copied that design. What do you think about C1 on the feedback loop, should there be bypass capacitors on the op-amp +/- supply?

author: TimFox
author_rank: Super Contributor
timestamp: June 15, 2025, 02:59:32 am
content: You still have no DC feedback: need some resistive path from Out to - In.You need a bypass cap on the positive power input to the op amp; negative is already grounded

author: R-802
author_rank: Contributor
timestamp: June 15, 2025, 03:21:21 am
content: Like this? I don't want to distort the output of the op-amp at all.

author: TimFox
author_rank: Super Contributor
timestamp: June 15, 2025, 03:28:43 am
content: That circuit should work, but C1 will have absolutely no effect (good or bad) on the circuit function unless the op amp has a huge differential input capacitance.

author: srb1954
author_rank: Super Contributor
timestamp: June 15, 2025, 03:52:39 am
content: The HPF cut-off frequency is actually 3.18Hz. R3 and R7 are effectively in parallel as far as AC signals are concerned so the effective load resistance seen by C4 is 500kthus doubling your calculated cut-off frequency.You need to have a very clean 9V supply feeding R7 as any noise on that supply will be fed into the input of the buffer amp. Depending on the source impedance up to 0.5x the supply noise will appear on the op-amp input. If you have a noisy supply it might be wise to add extra noise filtering to the feed to R7. This could be accomplished by using a 10kresistor in series with R7 with the junction of the 2 resistors bypassed by a 10uF or larger capacitor.

author: R-802
author_rank: Contributor
timestamp: June 15, 2025, 05:20:53 am
content: Thank you for the feedback, I'm planning on using the LT3042 (https://www.analog.com/en/products/lt3042.html) which seems like a good low noise LDO for the op-amp single supply.Input to the LDO will be from a 12V DC 3A wall adapter which can apparently be quite noisy so there will be filtering before and after the LDO into the op-amp.Potentially considering a TVS diode after the 12V 2.5mm input jack into an RLC filter, a few filter capacitors, directly to LDO, and then your RC filter suggestion into the op-amp. What do you think?

author: ledtester
author_rank: Super Contributor
timestamp: June 15, 2025, 05:22:03 am
content: Quote from: R-802 on June 15, 2025, 02:55:40 amYou need a resistor in parallel with C1 to create a low pass filter.You can use this calculator:https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-low-pass-and-high-pass-filterto determine the values for the resistor and C1.You want it to look like the attached circuit.

author: R-802
author_rank: Contributor
timestamp: June 15, 2025, 06:33:49 am
content: Ok, so I've done a simulation in TINA TI. Is this what the circuit is supposed to do, I'm assuming there is a bit more I could do to evaluate its performance?From what I understand, THD+N is a common metric to go by?

author: srb1954
author_rank: Super Contributor
timestamp: June 18, 2025, 01:54:05 pm
content: Quote from: R-802 on June 15, 2025, 05:20:53 amThe LT3042 is more than adequate for this application although rather expensive. To get the best performance out of the LT3042 you need to carefully follow the recommendations in the extensive application notes in the datasheet, particularly relating to component selection and PCB layout.QuoteA 12V 3A rated wall adapter will probably be a switching regulator based design and will likely have significant noise spikes and high-frequency ripple on its output. Preventing those noise spikes getting into your circuitry will be a major problem and will require careful attention to input filtering, PCB layout and grounding. There are some notes in the LT3042 datasheet about avoiding issues with noise spikes from switching power supplies. Incorrect PCB layout can severely degrade the achievable noise rejection of the LT3042 where it is fed from a switching PSU.Unless you really need the 3A rating for supplying other circuitry I would consider using a lower rated current wall adapter using a simple mains transformer/rectifier/capacitor combination. The low frequency ripple from such a device will be much easier to deal with than the high frequency noise from a switching PSU.QuoteProbably no need for a separate RC filter as previously suggested when using such a high performance regulator as the LT3042.N.B. My suggestion for the RC filter was only for the supply connection to the input bias resistors, not for the whole op amp. An op amp typically doesn't like much resistance in its supply line so, if you are utilising an RC filter on the positive supply pin the series resistor has to be quite low, say <10, to avoid stability problems. With such a low series resistor you can't get much useful supply filtering at lower frequencies unless you use an enormous bypass capacitor. However, a small inductor or ferrite bead, in conjunction with the usual 100n bypass cap, in place of a low value series resistor can be much more useful for suppressing higher frequency noise.

author: edavid
author_rank: Super Contributor
timestamp: June 18, 2025, 02:23:11 pm
content: 1. For a guitar amp, you should use a TS jack. (If for some reason you use TRS, you definitely don't want to ground R though!)2. What is the ADC input range? Aren't you going to be overdriving it? Can you match the op amp output voltage to the ADC input, and get rid of C3? If you retain C3, does it need to be larger?

author: R-802
author_rank: Contributor
timestamp: June 22, 2025, 04:19:47 pm
content: Have taken your improvements into account and made an update here:https://www.eevblog.com/forum/beginners/schematic-review-request-high-impedance-input-buffer-and-filter/Cost is something I didn't consider, won't be using the LT3042 as I have since seen some audio circuits that have good enough performance without these expensive IC's.From what I have read and seen online, the PCB layout is going to consist of a 4 layer stackup with signals on the top and bottom and two continuous ground planes in the center, digital and analog components will of course be physically seperated as much as possible.In the updated circuit a schottkey diode, CLC filter, and polymer capacitor bank is now on the 12V supply for the opamps, along with decoupling on each opamp, unsure if this is adequate for cleaning the switching supply though.

author: themadhippy
author_rank: Super Contributor
timestamp: June 22, 2025, 04:37:12 pm
content: for a guitar processor dont get to hung up on exotic parts,consider whats coming before and after,the internals of the guitar and any effects pedals , wont be using anything fancy and yer marshall/fender/boogie stack aint really the last word in hifi. The good ole 5532 or the tl071 is found in multiple audio products,even some that cost many thousands,