Question regarding unexpected op amp output - Page 1
Forum section: electronics / projects

author: Digibin
author_rank: Regular Contributor
timestamp: January 10, 2016, 12:11:07 am
content: I'm prototyping the attached circuit. At first I omitted the two 10R series resistors and tied the op amp outputs directly to the SPDT switch. With no input signal and the switch control pin tied high or low (same behavior in both cases), the output from the switch looked like the attached screenshot. The switch used is theTS3A24159. With either op amp output removed from the switch inputs, the oscillation dies and the output is a clean DC signal at Vref. I'd like to know what the cause of this is. I thought it might be a DC biasing issue so considered DC blocking caps on the outputs but the switch cannot handle negative voltages. In any case the op amp outputs shouldn't interfere with each other since they are never connected.With the switch position not changing, even removing the op amp output driving the open circuit switch pin kills the oscillation. There must be something subtle going on inside the switch to cause this?Including the two 10R series resistors cleans up the signal sufficiently, but I'm not sure why!

author: Audioguru
author_rank: Super Contributor
timestamp: January 10, 2016, 12:52:47 am
content: Your opamps are trying to drive very high value capacitors to ground that cause phase shift and oscillation. Are your capacitors 100nF? They are 1000 times more than the 100pF shown in the datasheet.If you want a lowpass filter then make a proper lowpass filter, not a simple high frequency short circuit at the output of an opamp.The Texas Instruments sales sheet for the TL07x and TL08x opamps say the minimum supply is 7V (then the output will do almost nothing). Your opamps are shown with no supply voltage (maybe +5V?). The input voltages of only 1.66V are too close to ground (4V is the minimum allowed) and some opamps will cause the output to be as high as is possible due to a problem called "Opamp Phase inversion".

author: Digibin
author_rank: Regular Contributor
timestamp: January 10, 2016, 12:40:28 pm
content: Quote from: Audioguru on January 10, 2016, 12:52:47 amThe op amps areTLC072 datahsheethere. The circuits work fine when disconnected from the switch, even driving the 100nF, it's only when I connected both outputs to the switch.Quote from: blueskull on January 10, 2016, 01:02:54 amThe switch is powered from 3.3V - I didn't have any 3.3V op amps to hand so I had to use a 5V part.

author: fivefish
author_rank: Frequent Contributor
timestamp: January 10, 2016, 01:06:55 pm
content: What's this circuit supposed to do? Polarity reverse?The switch according to datasheet has analog input range of 0 to Vcc. Doesn't mention anything about negative going signals capability.Also, the OFF line isn't completely disconnected...just -72db attenuated.

author: Audioguru
author_rank: Super Contributor
timestamp: January 10, 2016, 02:46:10 pm
content: Sorry, I did not see that you are using TLC072 opamps. Why are you causing them to oscillate by connecting the very high capacitance of 100,000pF at their outputs? They are spec'd with only 50pF.We still do not know what is the supply voltage for the opamps, 4.5V is their minimum allowed.

author: Digibin
author_rank: Regular Contributor
timestamp: January 10, 2016, 03:50:13 pm
content: Quote from: fivefish on January 10, 2016, 01:06:55 pmYes, switch between 0deg and 180deg phase. The op amp output is biased at 1.6V so there is no negative swing, otherwise I would have included blocking caps at the outputs.Quote from: Audioguru on January 10, 2016, 02:46:10 pmSorry, they're running from 5V supply. Where did you get 50pF from? Figure 19 shows plenty of phase margin at even 100pF. Admittedly I'm pushing it considerably beyond that, but as I said, the circuits are stable when driving the 100nF directly, it's only when they drive the switch inputs that the oscillation occurs. The inputs of the switch only add ~225pF when in the ON state so I can't believe this is pushing it into oscillation.The oscillation persists (although jumps to into the MHz) even if I completely remove the output capacitance. So there must be an issue with the op amp driving the switch inputs?

author: Kalvin
author_rank: Super Contributor
timestamp: January 10, 2016, 04:11:35 pm
content: - The TS3A24159 is specified for 1.65-V to 3.6-V Single-Supply Operation. If you are powering it from 5V you are violating the specs. If you are running it from 3.6V just make sure that the signal level will be below 3.6V so that you are not overdriving the device and activating the on-chip clamp diodes.- The TLC072 datasheet recommends minimum of 20 ohm resistor connected to op amp output when driving capacitive loads. You are violating that recommendation both in the main op amp circuit and the circuit generating pseudo-ground.

author: Audioguru
author_rank: Super Contributor
timestamp: January 10, 2016, 04:30:25 pm
content: I think the datasheet for the TLC072 says to use 20 ohms as an output resistor when driving "a reasonable capacitance of 100pF", not the dead short from 100,000pF. The capacitor is causing a huge phase shift and oscillation. Why is the capacitor value so high or why is the resistor value feeding it so low?Maybe the 5V power supply is also oscillating because it might be missing high frequency capacitors. Maybe the power supply or overloaded by the opamps trying to drive the high value capacitors.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: January 10, 2016, 08:59:18 pm
content: Here, this will save some trouble for people who might want to look at the OP's circuit easily.

author: Digibin
author_rank: Regular Contributor
timestamp: January 10, 2016, 09:47:50 pm
content: Quote from: blueskull on January 10, 2016, 09:05:02 pmThe input signal was 50kHz 1Vpp, biased around 1.6V, so within the analogue signal range of the switch.It works fine using 22R series resistors - I was only curious really

author: dom0
author_rank: Super Contributor
timestamp: January 10, 2016, 09:52:44 pm
content: Sometimes when connecting output capacitance to op amp output that by far exceeds the capabilities of it, very strange things can happen. I doubt the 20R are effective in this circuit. One thing you can check is if there is lots of current circulating in the circuit, this often hints at oscillations internal to amplifiers or oscillations that are loaded down, but still persist.

author: Digibin
author_rank: Regular Contributor
timestamp: January 10, 2016, 10:47:01 pm
content: The oscillation (screenshot in OP) is only 1Vpp - if it was caused by excess output capacitance would you not expect it to be saturating at the supply rails?

author: Audioguru
author_rank: Super Contributor
timestamp: January 11, 2016, 01:12:43 am
content: The input is 50kHz. The output capacitor is 100,000pF (100nF or 0.1uF) which has a reactance of only 32 ohms at 50kHz.The output impedance of the opamp at 50kHz is 1 ohm so there is a lot of phase shift into the negative feedback.Why are those huge capacitors there?

author: dom0
author_rank: Super Contributor
timestamp: January 11, 2016, 09:46:17 am
content: QuoteI very much doubt it. To look at the phase shift introduced into the control loop you need to consider the impedance at the output with the loop open. Most op amps have values of 50-200 ?. The output characteristics graphs in the TLC272 hint at something around 200-250 ? from the high side and ~100 ? from the low side (asymmetry to be expected, it is an old linear CMOS process after all), statically.

author: Audioguru
author_rank: Super Contributor
timestamp: January 11, 2016, 04:34:34 pm
content: The reason of the huge capacitors at the outputs of the opamps was never told to us. Trying to filter a high frequency squarewave into a low impedance triangle wave?