Active Probe: Have I just made an oscillator? - Page 1
Forum section: electronics / rf-microwave

author: Nitrousoxide
author_rank: Regular Contributor
timestamp: February 04, 2019, 04:47:36 am
content: Hi,I recently have been attempting to design and construct a differential probe. I constructed my first prototype to only find out that it forms a ~186 MHz oscillator.The problem area consists of an LTC6268-10 configured as a unity gain differential amplifier.Here are some photos of measurements and layout:Measurement:Construction:Layout:The question is as to what has caused this. Is this a layout issue?Would it be because the amplifier is driving a capacitance that is inherent in the transmission line, causing the amplifier to become unstable? (The capacitance would have to be quite large, and judging from the trace the paracitics would not be that high).How would I solve this? Would having a less than unity gain buffer work? I have already tried adding some artificial paracitic capacitive feedback on the amplifier, but that did not cause much change.Checking the supplies, I do see a bit of the oscillation present. Decoupling is 1nF in parallel with 10nF for both supplies. Additional 1uF capacitance across supply inputs to ground.EDIT: Forgot to mention layer stackup:Top: Signal and ground floodInner: GroundInner: Split plate for 5 and -5Bottom: Ground

author: KE5FX
author_rank: Super Contributor
timestamp: February 04, 2019, 05:33:23 am
content: The first paragraph of the data sheet says it's a "decompensated op amp that is gain-of-10 stable."

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 04, 2019, 05:54:25 am
content: Quote from: Nitrousoxide on February 04, 2019, 04:47:36 amnot layout alone. LTC6268-10 is decompensated opamp, only stable at gain 10 and higher. so unity gain setup for it is the recipe for the disaster.https://www.analog.com/media/en/technical-documentation/data-sheets/626810f.pdfQuote from: Nitrousoxide on February 04, 2019, 04:47:36 amload capacitance may throw even a unity gain stable opamp into unstability, so better watch that out during the process, regardless of the opamp used. read the opamp datasheet carefullyQuote from: Nitrousoxide on February 04, 2019, 04:47:36 amthere maybe tricks, to make decompensated opamp stable at unity gain, but i dont know never done that, better choose unity gain stable opamp in the first place. or change your circuit to bigger or equal to gain 10. here a short reading if you want... ymmv.http://www.ti.com/lit/an/sloa051a/sloa051a.pdfa piece of advice. change your power supply pins to 4 pins setup with 2nd pin cut off (blocked on the female plug), so you wont accidentally insert it in the wrong way, another recipe for disaster, opamp "poof" kind of disaster. btw, is that similar to nctnico's GHz diff probe layout/board?

author: Nitrousoxide
author_rank: Regular Contributor
timestamp: February 04, 2019, 06:23:00 am
content: Quote from: Mechatrommer on February 04, 2019, 05:54:25 amI can't believe I totally overlooked the datasheet stability criterionYeah, Im well aware that driving capacitive loads can cause op amp instability as they can shift a pole into (or into) the RHP of the Laplacian domain, or a phase shift of 180 (part of the reason an internal Cc is used to create a dominant pole). I just seemed to have potatoed and exhibited a case of not "RTFM".I was sort of hoping I could use a feedback capacitor to add some phase lead to resolve the issue whilst maintaining bandwidth.Quote from: Mechatrommer on February 04, 2019, 05:54:25 amIll add the suggestion about power supply layout on the board (especially since the op amp is about $7 AU one off). I also totally forgot to silk screen the positive and negative input pinsI'm not familiar with nctnico's GHz diff probe layout/board, but I shall look it up. Thanks!

author: Nitrousoxide
author_rank: Regular Contributor
timestamp: February 04, 2019, 06:51:17 am
content: Update: Yeah. It was the gain issue. Will post frequency sweep results. If its actually of any use i'd gladly share the source.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: February 04, 2019, 09:03:49 pm
content: Schematic?

author: tggzzz
author_rank: Super Contributor
timestamp: February 04, 2019, 09:56:43 pm
content: Quote from: Nitrousoxide on February 04, 2019, 04:47:36 amOld aphorism: "amplifiers oscillate, oscillators won't".QuoteIf it is acting as a transmission line rather than a lumped component, then it is resistive not capacitive. What's the wavelength of a 180MHz sine wave, and how long is the lead.But others have noted a more likely cause.

author: Nitrousoxide
author_rank: Regular Contributor
timestamp: February 05, 2019, 01:58:06 am
content: While I have you all here. I'd like to present the god awful frequency response (and a question):Figure 1: Worlds crappeist frequency response:Schematic:(Note: The 2p capacitors are omitted)I can distinctively observe poles at higher frequencies. Im assuming that they're due to losses and paracitics from the FR4 board.However, what's puzzling is the effect of a zero(?) at around 225 MHz. What would be causing that? Parasitic inductance? Capacitance to ground?I know constructing a very flat broadband probe is difficult, but I didnt expect it to be /this/ bad.The probe works perfectly fine in the time domain, Its just that I had the intent of achieving the flattest frequency response

author: KE5FX
author_rank: Super Contributor
timestamp: February 05, 2019, 03:02:08 am
content: Until the oscillation is tamed, there will be no way to get a meaningful frequency response plot out of it. That sharp peak, for instance, isn't something you'd be likely to see in a stable circuit.

author: Nitrousoxide
author_rank: Regular Contributor
timestamp: February 05, 2019, 04:31:56 am
content: Quote from: KE5FX on February 05, 2019, 03:02:08 amThere is no more oscillation. Although upon further analysis... It seems that shielding is an issue since the output stage seems to be amplifying local radio stations....

author: KE5FX
author_rank: Super Contributor
timestamp: February 05, 2019, 07:13:48 am
content: Quote from: Nitrousoxide on February 05, 2019, 04:31:56 amNo, it's still (very) marginally stable, it looks like. Normally you'd need high-Q components to create a sharp turning point in the frequency response like what you're seeing near 225 MHz. My guess is that your plane on layer 2 is applying too much capacitance to ground at the opamp input terminals. The effect of excessive input C is similar to excessive output C, in that it can hose your phase margin bigtime. It's a good idea to open up the ground plane under the inverting input and possibly the output pins. Often the data sheet will talk about this, but not always.Since you're using a SOIC rather than a QFN, it would be fairly easy to test that idea by lifting the input pins and their associated resistors, reconnecting them using short lengths of wire in place of the PCB traces. I'd probably try that next if faced with these symptoms.The FM broadcast stations might be coming in on your power rails, given that Vee is used to bias the source followers. PSRR and CMRR may not be great near 100 MHz. Your power rails seem reasonably well-bypassed but not ideally so -- it's best to run your power traces through the capacitor pads, rather than hanging the caps off on their own tributary traces. And if you are close to oscillation in the 100-200 MHz range, you have effectively built a Q multiplier that will make your device an even more effective antenna than it otherwise would be.

author: Nitrousoxide
author_rank: Regular Contributor
timestamp: February 05, 2019, 07:42:18 am
content: Quote from: KE5FX on February 05, 2019, 07:13:48 amWould I not need to have a ground plane immediately under the top layer to help control impedance?The datasheet did mention removing the ground plane under and around the inverting pin, but that was in a transimpedance configuration with a high the node being high impedance.Ive attempted to minimise the use of power traces by using a split plane, I suppose In the next revision I can modify the plane.

author: KE5FX
author_rank: Super Contributor
timestamp: February 05, 2019, 09:21:49 am
content: Quote from: Nitrousoxide on February 05, 2019, 07:42:18 amYour traces are too short to act as transmission lines at the frequencies you're dealing with, so theircharacteristicimpedance is not important. But the bulk impedance at various points along the trace is a different story. Consider how much of the copper area of your input traces consists of SMT pads as opposed to the traces themselves. You might have been careful to choose a trace width that yields an impedance close to the desired target value, but those pads will look like bypass capacitors if you don't open up the ground plane beneath them. The resistor pads could exhibit as much as 1 to 2 pF to ground each, a significant amount when dealing with an opamp that's built on what is probably a 10 GHz process.QuoteNot a big fan of split planes, but they can be appropriate in some instances. I just haven't run into any lately...