Achieving ultra-low probe tip capacitance - Page 1
Forum section: electronics / projects

author: jmw
author_rank: Frequent Contributor
timestamp: August 30, 2023, 10:34:51 pm
content: What kind of black magic do the big names use to get their probe input capacitance down to extremely low levels? I'm seeing figures of 0.4 - 0.6 pF for Keysight's InfiniiMax and some of LeCroy's active probes. With some of the solder-in heads, the probe system is rated at less than 0.5 pF in single-ended configuration!I've been experimenting with making my own single-ended probe and having a very hard time getting the capacitance below 0.9 pF in post-layout simulation. The capacitance of the input buffer is right about 0.5 pF after 5x attenuation. Then the capacitance of the tip to ground and PCB copper is another 0.4 pF using a 3D field solver - that's after I have the sockets separated and the ground plane pulled away far from the tip. Of course, more distance = less capacitance, but also more inductance. With their capacitance ratings, these probes can't push much more than 10-15 nH without wrecking probe bandwidth - the tip network must stay compact.My best effort: The distance between the tips is 12 mm and the clearance from the tip to the ground plane is 13 mm. Still about 400 fF just for this structure alone!

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 30, 2023, 10:55:05 pm
content: I think low-C probes are active and voltage limited, so you could try modeling a guard-band layout, which is a buffered unity gain signal placed between the input and any ground areas.

author: srb1954
author_rank: Super Contributor
timestamp: August 30, 2023, 10:55:46 pm
content: Bob Pease showed a design for an ultra-low capacitance FET probe in his book Troubleshooting Analog Circuits.He used a strip of PCB laminate with holes drilled in it, minimising the extra capacitance, to support the probe tip. The gate lead of the FET was direct wired to the probe pin for minimum extra capacitance.

author: jmw
author_rank: Frequent Contributor
timestamp: August 30, 2023, 11:14:00 pm
content: Quote from: PCB.Wiz on August 30, 2023, 10:55:05 pmRight, but that buffer would have its own input capacitance that appears in parallel with everything else, no? If it had less than 0.4 pF input C at unity gain and wide bandwidth, I'd use that instead of the main buffer amp (BUF802).

author: Someone
author_rank: Super Contributor
timestamp: August 31, 2023, 01:04:36 am
content: Quote from: PCB.Wiz on August 30, 2023, 10:55:05 pmUS5512838A "Probe with reduced input capacitance"

author: David Hess
author_rank: Super Contributor
timestamp: August 31, 2023, 02:12:54 am
content: Quote from: srb1954 on August 30, 2023, 10:55:46 pmAs shown below. I wonder how Bob Pease measured the input capacitance.Note that the input and output capacitance of the input transistor (Q1) are bootstrapped by the transistor itself configured as a source follower, so only the small reverse transfer capacitance remains. The low input capacitance and 90 MHz bandwidth mean input series inductance from non-ideal construction is irrelevant.I believe Q1 limits bandwidth to G/(2 Pi C) where G is the transconductance and C is the input capacitance so about the claimed 90 MHz. A different JFET would need to be used for higher bandwidth, so the slow 2N3904 and 2N3906 are not limiting performance.

author: antenna
author_rank: Frequent Contributor
timestamp: August 31, 2023, 02:38:01 pm
content: Quote from: David Hess on August 31, 2023, 02:12:54 amI've wondered that too. When I built mine, I went with the BF998 Dave Jewsberry Poor Mans Probe. On my Uniden Washington CB, if I have a SSB signal coming into the radio (from sig generator) and I use the homemade probe to look at the 10MHz PLL crystal probing right on the PLL pins for the crystal, I observe a 20Hz change in the received SSB audio tone. I am not sure how much capacitance it takes for a crystal to shift by 20Hz, but I dont think it is much. Maybe a similar crystal pulling method could be used to measure the capacitance?Edit: Maybe see how much it shifts an SSB receiver crystal like I did, then remove the probe and see how many mm of a kmown capacitance twisted pair it takes to match the shift trimming little at a time?

author: David Hess
author_rank: Super Contributor
timestamp: September 01, 2023, 01:23:56 am
content: I think the capacitance could be measured in the same way that an oscilloscope input normalizer is calibrated. Connect a trimmer in series with the input and adjust until the measured voltage is 1/2, and then measure the capacitance of the trimmer.

author: antenna
author_rank: Frequent Contributor
timestamp: September 01, 2023, 01:32:14 am
content: Quote from: David Hess on September 01, 2023, 01:23:56 amIf the trimmer capacitance is the same as the probe capacitance, it will be just as hard to measure directly on any device.Edit: i think the issue is how to confidently measure less than a pF.

author: schmitt trigger
author_rank: Super Contributor
timestamp: September 01, 2023, 02:25:56 am
content: Subscribing to thread.I also have some ideas on how to measure such small capacitance. Which I’ll post later.

author: ch_scr
author_rank: Frequent Contributor
timestamp: September 01, 2023, 06:31:14 am
content: I'd try and use a VNA to measure that capacitance. Thankfully, with the "Nano" "Tiny" and "Lite" devices, now the home gamer can play too.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 01, 2023, 07:41:57 am
content: I don't think there's more than SED threads on it, but Phil Hobbshttps://electrooptical.net/made a wafer microprobe with bootstrapping, in the I think 100MHz BW and (10s? 100s?) fF range. He might have some background or tips via email.I think you're going to have a hard time bootstrapping at very high frequencies, but generally just keep in mind what you're doing: you're making an impedance network, so the length, width and geometry in general gives its characteristics.The only thing you can do is minimize size. Don't make it bigger, make it tinier!Saying that frequency is inversely proportional to length seems so obvious when put in such basic terms, but sometimes we don't appreciate how often it can be applied.About probes in general: we want high impedance. So we need wide spacing of thin wires (high transmission line impedance). We also want high bandwidth, so we want a structure that acts like a transmission line teeing off the circuit node in question, loading it a minimal amount (that's where the high Zo comes in). If we could get Zo high enough, we might not mind the length of this line at all (just add a source termination resistor [i.e. at the tip] to eliminate stub resonance). But we probably can't, and where mismatch is inevitable, length must also be minimized.Add to that the loading effect of a FET follower, and -- well, if we can get some inductance, we might get a little peaking into it, but that's it, and certainly we don't want so much it reduces bandwidth overall.Consider PHEMTs and SiGe:C HBTs. It was the combination of these that Phil found was particularly effective; the PHEMT I think had remarkably strong channel-length modulation (or equivalent effect; drain curves looking like a triode(!)), but its low input capacitance and high gain, combined with a bootstrapped drain (using an HBT, which is essentially free of Early effect, or it's even negative(!??)), and some other details, got it working right.Just a note there, "plate resistance" reduces the gain of a "cathode" follower. So, bootstrapping that away, keeps the follower gain very close to 1. Basically between the CCS load below, and bootstrap above, the follower sees almost no change in operating conditions; its current changes a bit (particularly at HF, into node capacitance), but because gm is so extraordinary on these parts, it's a small droop easily compensated.Or maybe I'm remembering different parts of something else he was working on (TIA with input structure made from a mix of parts?); and also something about one of these two kinds having just ludicrous 1/f noise below rather high breakpoints (like 10s of MHz; practically DC for these 50GHz+ types, right, I mean c'mon?..), but using one or the other, or together, to beat that. It's been a long time, I forget which is which...Quote from: antenna on August 31, 2023, 02:38:01 pmA crystal is a bit complex to start with (you could indeed use a transformation of it), but a basic LC is easily solved for:https://www.seventransistorlabs.com/Calc/RLC.html#cxIn this case, C is known and you're adding Cx, but it's the same thing.Tim

author: jonpaul
author_rank: Super Contributor
timestamp: September 01, 2023, 07:47:46 am
content: plot Zo vs F for the probe capacitance.These low C probes have very limited use at high freq.They are also typically easy to blow on even very short overvoltage.See the Tektronix Circuits Concepts books "Oscilloscope probe Circuits"Jon

author: David Hess
author_rank: Super Contributor
timestamp: September 01, 2023, 10:54:45 am
content: Quote from: antenna on September 01, 2023, 01:32:14 amI have done it before without issues using an LCR meter or amazingly enough, even my Tektronix DM916 handheld multimeter's capacitance mode. Zero out the fixture capacitance and then add the trimmer. I used it for measuringgimmicks.Quote from: T3sl4co1l on September 01, 2023, 07:41:57 amA little bit of AC bootstrapping is common on old Tektronix active probes, but the small levels used make me think it was for peaking the response.Discrete designs for oscilloscope high impedance inputs have trouble with bootstrapping above 100 MHz. The circuits tend to oscillate in the aircraft band so careful physical design is needed for shielding and isolation.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 01, 2023, 11:02:29 am
content: Quote from: David Hess on August 31, 2023, 02:12:54 amNot too hard - set up a high frequency LC oscillator.See how much the frequency changes when the probe is connected, compare to connecting known fixed capacitors

author: dietert1
author_rank: Super Contributor
timestamp: September 01, 2023, 01:52:19 pm
content: I have been using a voltage divider made with two in series 0603 resistors at the input side and 10:1 or so division. Vishay have a paper where they model the parasitics and estimate 0.04 pF for 0603. They test the parts up to 40 GHz.With two resistors in series one gets roughly 0.02 pF or a little more depending on pad geometry. This can result in a compensated divider 10:1 - at the same time staying close to the 0.4 pF minimum of the OP.https://www.vishay.com/docs/60107/freqresp.pdfRegards, Dieter

author: Marco
author_rank: Super Contributor
timestamp: September 01, 2023, 04:47:15 pm
content: At high frequency those Infiniimax probes are just a differential resistive divider probe, a differential transmission line connected with resistors at the probe tips.With a little magic to increase the impedance at lower frequencies.

author: Marco
author_rank: Super Contributor
timestamp: September 02, 2023, 08:58:55 am
content: I think regardless of active/resistive tip, a differential transmission line without ground plane is the way to go for front part of the probe, then connect it to coax a little farther removed from the probing point.I wonder if they measure tip capacitance with a differential transmission line without ground plane as a source too ... it will look a lot better that way than the more realistic measurement over a PCB ground plane.