Why does my buffer attenuate low frequencies? - Page 1
Forum section: electronics / beginners

author: Yuu
author_rank: Regular Contributor
timestamp: September 20, 2023, 02:21:09 am
content: Edit: Issue was cable problems and not the circuit!Hello, I recently designed a buffer+attenuator so I could play with using my USB soundcard to probe voltages. The design is one I found and modified to suit my own needs. It's not really too original. Anyway, while it's designed to attenuate, the frequency response should at least be flat but it's not.Circuit: (I ended up disconnecting signal to ring on output TRS connector)Layout: (I messed up the power-on switch layout but you live you learn)Freq Response Measurement:When I did a sweep with the function generator and looking at the output on a scope, I remember it looking just fine. It didn't seem to change magnitude. Maybe all this is a wild goose chase and it's me learning to use my measurement equipment. Either way, here is what the distortion analyzer gives meSo, any ideas? Any glaring circuit problems? I am thinking the 100nF coupling cap is forming a high-pass filter but my calculations and simulations all say that 100nF should be high enough that audio frequencies should pass just fine... I don't know what the deal is.Edit: I just double-checked. It does seem the distortion analyzer is right. It looks like I've made a high-pass filter on the input.

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 20, 2023, 02:50:49 am
content: Could it be that your 100nF capacitor isn't actually 100nF, but something much less?

author: EPAIII
author_rank: Super Contributor
timestamp: September 20, 2023, 08:29:42 am
content: Well, any series capacitor is a high pass filter. 100 nF is about 628 Ohms at 100 Hz. And you have C4, 15 pF to ground so there is an AC Voltage divider. Are you sure C4 is 15 pF?You seem to have built the circuit, so it should be simple enough to just try a larger value for C5. A 1 or 10 uF capacitor with leads can be just touched to the sides of C5.

author: nali
author_rank: Frequent Contributor
timestamp: September 20, 2023, 09:24:02 am
content: 100n is around 16k at 100Hz, but even so something is awry.I would take out C4 just in case it isn't the value you think it is. It may be worthwhile temporarily removing the output resistor R7 and linking input to output, just to check your measurement methodology/equipment is OK and give you a known working baseline to start troubleshooting.

author: Terry Bites
author_rank: Super Contributor
timestamp: September 20, 2023, 04:26:51 pm
content: It all makes perfect sense if C4 is 100p! .... For what its worth, the difference in repsonse for 900K and 1Meg is insignificant. Especially for an audio app and given typical capacitor tolerances. Good enough is fine.Simulate it to convince yourself.

author: Yuu
author_rank: Regular Contributor
timestamp: September 20, 2023, 04:52:02 pm
content: Quote from: Terry Bites on September 20, 2023, 04:26:51 pmYeah I was thinking about this earlier, the pole location and the +20 dB/decade rise, but no... I tested the cap after pulling it out and it was indeed 100nF. I then put in a really large 10uF ceramic capacitor and it just made it worse (see plot below). Also, my frequency response measurements were already made with the 15pF cap C4 taken out. That was just there to mimic an oscilloscope input and allow better matching when using oscilloscope probes.I really don't know what's up with this thing.Edit: Just used a signal gen to inject sine waves with the output of the buffer floating. I probed the input, probed after the cap, and then probed the output. It seemed fine with no attenuation. I should mention though, with my signal gen set to High Z and 1 Vpp, it did seem to, at the input, drag my signal down to like 300 mVrms according to my multimeter and like maybe 120 mVpp according to my oscope. That's weird... but at least the signal level didn't seem attenuated changing frequency.Maybe this is a loading effect on the op amp? I don't think that makes sense though because changing the input capacitor changed the frequency response and made it worse.Maybe due to the virtual ground I've setup? I don't know!

author: StillTrying
author_rank: Super Contributor
timestamp: September 20, 2023, 05:48:40 pm
content: Have you checked that the split GND, R3 100R, C3 C2 100n works at low frequency, I can imagine it doesn't very well if the output is driving some load, because of R3.

author: Yuu
author_rank: Regular Contributor
timestamp: September 20, 2023, 06:43:04 pm
content: Quote from: StillTrying on September 20, 2023, 05:48:40 pmGood idea. Will check next. I might just take R3 out, jumper it, and see what happens. The whole virtual ground part is slightly sketchy to me so if I designed something in a bad way, that's a likely place for it...

author: Yuu
author_rank: Regular Contributor
timestamp: September 20, 2023, 07:02:59 pm
content: Removed the resistor (R3) on the output of the virtual ground buffer and shorted it. Didn't help.

author: srb1954
author_rank: Super Contributor
timestamp: September 20, 2023, 09:18:39 pm
content: Input a 1Khz sine wave and probe on the node v_in with your scope and operate SW1. The signal amplitude indicated by the scope should not change noticeably as SW1 switches between the 2 positions.If it does change it indicates that there is excessive loading due to the op amp and/or its associated input circuitry and this could also account for the poor LF response. This excessive loading could be from a damaged op amp input or leaky protection diodes. The fact that yourmaximumgain barely gets better than -10dB indicates that the op amp is not functioning correctly regardless of any frequency response problems.Check all the DC voltages on all pins of the op amp are correct by directly probing the pins themselves. A missing supply rail to the op amp due to a poor solder joint could also present the symptom of excessive loading or low gain.

author: Yuu
author_rank: Regular Contributor
timestamp: September 20, 2023, 10:01:23 pm
content: So I ended up fixing it!!The issue was a cable!!I was using this TRS -> RCA cable and was using the wrong channel! I'm used this TRS to bare wire thing where the red was the tip (the left signal) so I just assumed the same for the RCA but that's wrong!Here are some accurate plots now (which I like much better!)

author: TimFox
author_rank: Super Contributor
timestamp: September 20, 2023, 10:27:50 pm
content: With the accidental use of the wrong cable channel, you did end up with a very small capacitance in series with the signal, similar to accidentally having 100 pF in place of 100 nF in series with the input impedance.