Opamp Oscillation - Page 1
Forum section: electronics / projects

author: Evan.Cornell
author_rank: Regular Contributor
timestamp: November 12, 2020, 10:48:12 pm
content: I have a amplifier+filter signal chain comprised of AD4807-1 opamps. Singled ended supply of +3.3V & reference of 1.65V.My original simulation was done with TINA-TI and didn't see any problems, but now that I've got the prototypes back, I'm getting a ~12-13MHz oscillation on the output of the first amplifier stage. I tried to re-run my TINA-TI simulation and wasn't able to get it to run fully. Possibly the original simulation was suspect.I re-drew it up in LTSpice (attached) and do observe a ~35MHz oscillation in simulation with no input signal. Any ideas on how I can resolve this?Thanks!

author: KT88
author_rank: Frequent Contributor
timestamp: November 13, 2020, 12:04:07 am
content: Hi Evan,it looks lie a bug in LTSpice or the model of the ADA4807. If you delete the rest of the circuit, it's stable...CheersAndreas

author: Andy Watson
author_rank: Super Contributor
timestamp: November 13, 2020, 12:33:16 am
content: Too much phase shift in the feedback - is always the answerQuote from: Evan.Cornell on November 12, 2020, 10:48:12 pmIs it genuine?I note that the datasheet says "Capacitive Load = 15pF" for a 30% overshoot. I read that as suggesting that more than 15pF will push the op-amp into oscillation. A few pF due to the following circuit, plus a scope-probe could easily reach 15pF. When probing such sensitive circuits I add a 100resistor at the tip of the scope probe to "isolate" it from the op-amp.Any chance you could provide the schematic in a more universal form.

author: Zero999
author_rank: Super Contributor
timestamp: November 13, 2020, 12:55:17 am
content: Quote from: Andy Watson on November 13, 2020, 12:33:16 amLTSpice is a free download, but here's a PDF of the OP's file. I haven't done any calculations or simulated those filters, but presumably they're stable and shouldn't oscillate. It looks very convoluted. There's probably a way to simplify it.

author: Andy Watson
author_rank: Super Contributor
timestamp: November 13, 2020, 03:03:20 am
content: Quote from: Zero999 on November 13, 2020, 12:55:17 amMay be, but as I don't normally use it ...QuoteIndeed!I've not done the calculations but a cursory glance at the component values would suggest filters operating at audio-frequencies, but the op-amps boast 180MHz banwidth! I would suggest that the stage one op-amp is not going to like driving 500pF (C6 & C8) into the output of U2.

author: Pulsepowerguy
author_rank: Regular Contributor
timestamp: November 13, 2020, 03:26:06 am
content: The layout might be the problem. Please share an image of that.

author: KE5FX
author_rank: Super Contributor
timestamp: November 13, 2020, 03:26:33 am
content: Tip: don't use rail-rail amps unless you actually need them. I had to learn this thehard waymyself.Does the oscillation look like this, or does it dominate the whole waveform?http://www.ke5fx.com/1677_osc_sine.png

author: mikerj
author_rank: Super Contributor
timestamp: November 13, 2020, 10:57:20 am
content: There is no supply decoupling at all in the LTSpice netlist, does your real circuit have some? These are fast op-amps, stiff supply rails are going to be important. How is your Vref voltage being generated?

author: Evan.Cornell
author_rank: Regular Contributor
timestamp: November 13, 2020, 06:00:43 pm
content: Quote from: KE5FX on November 13, 2020, 03:26:33 amNo, it dominated the entire waveform.Usinghttps://www.analog.com/media/en/technical-documentation/application-notes/an148fa.pdf,I calculated that the feedback resistor network along with assumed 4pF parasitic capacitance works out to be right around the 12-13MHz mark.I've changed the simulation to use ADA4805-1 instead of ADA4807 and things seem to behave. I will prototype it and see if that helps.