Strange sinusoidal wave showing up at Input - Page 1
Forum section: electronics / projects

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 29, 2021, 10:43:29 pm
content: I built this summing board that takes in four voltages and sums them up into different sums. It's a prototype, so I'm trying out different configurations (on separate PCBs), but I ran into a problem with my second configuration, and I need some help.For my first version of the board, I used voltage-feedback op-amps, the LM7171 and the LM7372. This version operated at +10 and -10 volts. Note that not all components were present; some were merely placeholders just in case I had to fine tune something. After some trial and lots of errors, I made each op-amp have a gain of -1x using 2k-ohm resistors for inputs to the op-amps and the feedback resistor associated with each op-amp. The one notable problem was the offset voltage adjustments. Thanks to previous help, they were more trouble than they were worth (Diff Offset #1 was injecting an unwanted signal when Input C had a signal). To bypass them, I used a wire to short "DIFF_OFFSET1" and "DIFF_OFFSET2" to ground, and I removed R12, R17, and R22 so there's no offset adjustment. (Also, the pots in parallel with the op-amps were removed due to them being unnecessary; same with RV12 and RV13, so no pots at all on the board).Now, for this second variant, I decide to try a different approach, using the THS3061 and THS3062 single and dual current-feedback op-amps. I wanted to try and increase the bandwidth with a family of parts that are all the same. Thus, I picked the two for that reason. Now, in keeping with the datasheet, I replaced all resistors with 560-ohm resistors to achieve a gain of -1x. You can see the changes in the PDF for variant 2. I learned from my mistakes from variant 1, so there's no offset adjustment (no pots), no feedback capacitor, just simple summing op-amp circuits. Also, with the new op-amps, the tantalum capacitors are all 22 uF. The datasheet specified 10 uF, but I didn't have any.However, I'm running into a problem with this variant. Somehow, when I put a square wave at Input B (1 MHz, 0.1 Vpp), I saw that somehow, a sine wave was superimposed over the square wave at the SUM_1 output, and there's no signal in Output #3. When I inject my test signal, the square wave mentioned before, into other inputs, there were no problems at all, and the circuit behaved as it should. It's strange that it's behaving this way. Viewing the schematic, there should be no way for any foreign signal to interfere with the test signal. I don't understand why problems occur with input B and none of the other inputs. I was wondering if I could get some insight as to where I should look at? Perhaps there something that I'm missing that others may see.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: October 30, 2021, 01:06:32 am
content: 1. Doublecheck all diodes and resistors in the B input circuit compared to the others.2. What is the part number of the tantalum capacitors?3. What happens at 100 kHz, 10 kHz, and 1 kHz?4. Does the signal generator have a DC offset?5. Do you have a picture of the oscilloscope signal?

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 30, 2021, 03:44:00 am
content: Quote from: Tomorokoshi on October 30, 2021, 01:06:32 amThanks for the reply.1. I double checked the diodes and resistors, and they were exhibiting normal behavior.2. Its a T354K226K Radial cap. I had a lot of them, and it was all I had for the board that was large enough. I bent the leads and soldered them onto my 1206 footprint as best as I could.https://www.digikey.com/en/products/detail/kemet/T354K226K035AT/44763483. Ill double check, but if i recall correctly, at low frequencies, the same behavior is exhibited. Inputs A, C, and D are correct (giving correct outputs), but not B. Output 3 when B is the input is 0, no signal at all. Also, to note, SUM 1 shows that sine wave imposed on my signal. All other inputs are terminated to ground via a 50 ohm load terminator.4. No, there is no DC offset on my generator.5. I have no images of my waveforms yet, but I'll try and get them when I can.Its strange how everything works except for B. What are the chances of a faulty part? Though

author: magic
author_rank: Super Contributor
timestamp: October 30, 2021, 05:54:12 am
content: No idea, but the thing that input B and output 3 have in common is the DIFF1 stage.BTW, isn't the input impedance of ports A,C different from B,D due to the diff stages?Output 3 could be realized as (A+D)-(B+C) and then the input would all be the same.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 31, 2021, 02:27:06 am
content: That's a good catch. I never even thought of that combination. That would make sense, so all sums in stage 1 would use inverting summers while stage 2 remains the same as inverting sum/difference op-amps. I wish I thought of that earlier. Definitely would have saved a lot of trouble. Thank you for your insight.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: November 01, 2021, 03:30:19 pm
content: Quote from: Tomorokoshi on October 30, 2021, 01:06:32 amSo, it turns out that after examining the B channel, the problem may have been with a resistor at DIFF1. Apparently, looking at the B channel, I went to check the resistors, and after taking them off, it turns out that one resistor had a bad leg. The metal contact on the resistor was detached from the resistor body, so I replaced the two resistors in B's channel for the DIFF1 op-amp, and now, everything seems to be working fine....at least until I try to use probes to examine each individual op-amp output.It appears that I saw a ringing voltage superimposed onto my test signal when I tried to use BNC-mini alligator clips to examine the outputs of each op-amp. This occurred even when I was just touching the output signal pins that I had at each op-amp output. Once the alligator clip was no longer touching it, the ringing went away. I was hesitant about pursuing the matter further, as this was debugging, but upon further inspection as I was writing this, I used a 51-ohm resistor to put in series with my alligator clip and touched the other end of my resistor to the output signal pin. The ringing was much much subdued now. Seeing as how a 50-ohm resistor in series between the output and the alligator clip solved the issue (or at least reduced the ringing), could this be an issue of impedance mismatching, and the ringing I saw was simply reflections?I've attached two images. Yellow waveform is my test signal. Pink waveform is my board output. Blue waveform is my signal measured with an alligator clip (in this case, OUT 1; test signal goes to B, so I should see an inverted waveform of my test signal). The ringing image is when I had my probes directly connected to my op-amp output test pin. For the 'no ringing' image, I used a 51-ohm resistor connected to the alligator probe so it's in series between the output pin and clip. Obviously, you can see the ringing is much subdued.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 02, 2021, 01:15:03 pm
content: Check that the parsitic capacitances on the inv inputs are are low- ie not fed from long tracks or wiring.