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.