Inside a ionization vacuum gauge - Page 1
Forum section: electronics / general

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: August 31, 2025, 02:02:45 am
content: Here are the internals of the electrometer section of an ionization vacuum gauge. The design is a pretty standard transimpedance amplifier, with resistor values up to 100 Gohm. The input is just a BNC.I reverse engineered the schematic. The op amp used is the ICH8500A, with <10fA input current. (The part also has a surprisingly large +/- 50 mV offset, and there is a potentiometer for nulling the offset. BF244 JFETs are used for input protection. The two JFETs that are thermally coupled are for the "LOG" scale (since the voltage across a JFET is an exponential function of the current). There are a few pots for adjusting the gain of the LOG setting, and there is also a switch which is activated when LOG mode is selected, and it turns on a JFET which allows the LOG scale to be calibrated. On every other range, the circuit is basically a transimpedance amplifier, and a 741 with 100x voltage gain attached to it. On the lowest scale, the output is 100 fA/V if you do the math.For testing, I used a (dirty) 50 gigaohm resistor, and a 10mV voltage source. I'm pretty sure I've touched that resistor without wearing gloves, so the numbers might be off. The theoretical current should be 200 fA. I tried to null everything using the potentiometers, and then I turned on the 10 mV supply. The device read 220 fA, which is pretty close. The noise is pretty low, at least when the wind isn't blowing. It seems to be around +/- 1 fA when nothing is moving.I think I can replace the op amp with a LMC662, which seems to beat the ICH8500A in every aspect. The 10fA max leakage seems to only be for 25 deg C in the datasheet.

author: EC8010
author_rank: Frequent Contributor
timestamp: August 31, 2025, 11:51:52 am
content: You might want to consider LMC6001A, although I guarantee a sharp intake of breath when you see the price. I have just made a similar instrument and havered over using one of my two remaining LMC6001A or an LMC662 (of which I have ten). I decided that I wanted reliable results and I knew the LMC6001A would give them. I used LMC6001A. Various posters on this forum have stated that LMC6001A is a selected LMC662 die, with only one op-amp connected. If you read the data sheets carefully, you will see that the LMC662mightbe good, but isn't guaranteed to be good, whereas the LMC6001A is. Anyway, when it came to testing, I found a voltage offset of 35uV (entirely acceptable) and leakage current is low enough for the pA I need to measure. If you want better performance, there's the TI LMP7721 which I have used, and there's also the ADA4530-1 which I have also used where the alarm level was at 2fA, but beware that it is very finicky about 0V connections. If you get low frequency oscillation (1Hz or below), it seems to be because its likes its PCB 0V ground plane bonded firmly down to chassis at multiple points rather than just one.You need really clean power rails for electrometers; forget powering them directly from a bench supply. Local regulation is a must.Physical cleanliness everywhere is essential when you're using a 100G current conversion resistor; you cannot afford to touch anything with bare fingers. Scrupulous scrubbing with flux cleaner and rinsing with copious isopropyl alcohol is necessary. Do not breathe on anything. Or allow draughts. Or look at it in a funny way.Edit: That BNC looks dirty to me (on the PTFE). I keep caps over my low current BNCs and triax connectors.

author: SanderMTI
author_rank: Contributor
timestamp: August 31, 2025, 02:04:52 pm
content: Which ion gauge controller did you reverse engineer the schematic from ?There is a chance I have the official one.If the amplifier is actually going to be used on an ion gauge head, you don't need to go very low current wise.The "x-ray limit" of most gauge heads is around 10-10 mBar which, with a typical 10/mBar gauge head, would correspond toa nanoampere of actual current.Also, a pressure below 10-8 mBar would be called "ultra high vacuum" which is very difficult to obtain.The ion gauges for most vacuum equipment such as mass spectrometers and whatnot typically cover 10-9 mBar to 10-3 mBar.A noted exception were the ones based on the (wonderful but obsolete) 310J varactor bridge made by Analog Devices.They would go down to 10-11 mBar.In an actual ion gauge the electrons come in with more than 150 eV of energy, so I'm glad to see the input protection.Of course, if it's going to be used as a general purpose current electrometer, the comments of EC8010 apply.Sander

author: w42i
author_rank: Contributor
timestamp: August 31, 2025, 02:39:01 pm
content: Application notes about log converters typically claim a superior log conformance of bipolar transistors. Consequently I wonder what the reason for choosing JFETs in this design might be.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: August 31, 2025, 10:02:12 pm
content: Quote from: SanderMTI on August 31, 2025, 02:04:52 pmI'm actually not sure, I think it was an HP one which still had the analog meter.Quote from: EC8010 on August 31, 2025, 11:51:52 amThis might be a stupid question, but in the Keithley 617, a 7815 is used to generate the voltage. How low noise does the supply have to be?I should be fine as long as I don't use a switching power supply, right?

author: EC8010
author_rank: Frequent Contributor
timestamp: September 01, 2025, 08:37:07 am
content: It starts with the power supply rejection ratio (PSRR) of the electrometer op-amp. Quite a few are only 60dB even at low frequencies. About ten years ago, I started on a project where 1fA needed to be measured to an accuracy of about +/-10%. It was known that the signal to be measured would not be clean DC but would have random pulses on it, up to 80fA, and that these would need to be accurately measured and subtracted, and that was when the power supply hum problem was uncovered. You can attack the problem from the theory end of looking at the PSRR against frequency graph and calculating what is the maximum tolerable level of hum etc that you can tolerate, and from the practical end you shouldalwaysmonitor the raw output of an electrometer with an oscilloscope triggered from line to check for hum and other interference.Once you've realised that power supply hum gets through electrometer op-amps, you start wondering about low frequency noise on power rails. I used TL1431 shunt regulators with ADA4530-1 at 1fA and that seemed to be fine (the ADA4530-1 has very good PSRR). I've also used TL1431 with LMC6001A for much less sensitive stuff and that seems fine. However, TL1431/431 are band-gap references and quite noisy, so every so often, I wonder whether quieter supplies might reduce electrometer base line noise/drift. One possibility is to make a traditional series regulator out of op-amp, voltage reference, and series pass transistor. For such a thing, the MAX6126 reference is quieter than band-gap references down to 0.1Hz and there are any number of quiet op-amps. I would expect an old 7815 to be quite noisy. In the end, it all depends on how low in frequency you need to go, how small a current you need to measure, and what noise/drift you can tolerate.By the way, I was told by a chap at TI that electrometer input leakage current is dominated by the input protection diodes to rails rather than the MOSFETs. The ADA4530-1 takes its protection diodes to the output of its guard amplifier; presumably how it achieves its very low input leakage current and excellent PSRR.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: September 03, 2025, 03:24:51 am
content: Here are all the measurements:The first image is what my setup looks like. I put everything in a cookie tin, and I'm powering the device with a lab power supply. I also wiped the 50 gigaohm resistor with alcohol.With 10mV applied to the 50 Gohm resistor, I got the results shown in the third picture. You can see that the reading is pretty stable over 3 minutes. Apart from a few spikes probably due to cosmic rays, I got 2.06V, and the voltage deviated by about +/- 0.01V (10mV = 1fA). If I just leave the input floating (with no resistor connected, since I think connecting the resistor would introduce current noise), I get 30mV. So my measurement is 203 fA for the current (which is pretty close to the 200fA expected, and within the 2% tolerance of my resistor).The fourth picture shows the result with nothing connected on the input (over 3 minutes). The fifth picture shows the result at a smaller timescale, and with FFT enabled. You can see that there is almost no noise at 60Hz, despite powering everything with a lab supply.I think 1fA is probably the limit for this setup, since the current noise of the 100G and 50G resistors are both greater than 0.1 fA at 0.1 Hz bandwidth (set by the 15pF capacitor inside). I would probably have to go to 1 teraohm or even higher if I want to get under 1fA noise. (Also I wouldn't have any good methods of generating 1fA without introducing a ton of current noise into the system)

author: EC8010
author_rank: Frequent Contributor
timestamp: September 03, 2025, 09:12:50 am
content: As you've demonstrated, the difficulty of looking at electrometers with an oscilloscope is noise.The first trace is an ADA4530-1 with 100G current conversion resistor and capped input straight into the oscilloscope. Note that the oscilloscope is triggered from line.The second trace is via a simple LC 32Hz low-pass filter.The third trace invokes averaging to reduce noise and measures mean level, and shows an offset of 0.2fA. Had there been hum, it would have been visible because the oscilloscope is triggered from line.I'm sure your oscilloscope can be used in the same way. Although the trace here was via a 32Hz filter, even a 2kHz filter produces pretty much the same visible effect and uses far smaller components; 36mH inductor and 100nF capacitor, with 1k series resistor to produce a maximally flat (Butterworth) response.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: September 04, 2025, 03:33:31 am
content: Here are the newest results.I enabled a digital filter which adds 3 more bits of resolution. I'm pretty sure it's implemented in software on the oscilloscope as a filter.There is pretty much no 60 Hz noise in the first picture (at least I can't see any). The cutoff of the digital filter is 160 kHz so it is not being filtered out. (The scope is sampling at 20 Msps, and you can do 10 MHz / (2^3)^2 to get about 160kHz)In the second image, I did the same setup as before (input floating), but with a metal BNC dust cap on the input. It seems like the noise is quite low (around 1fA p-p). I think the theoretical noise is 0.127 fA RMS (for a 100 gigaohm resistor at 0.1 Hz bandwidth).In the third image, I set the range to 10 nA/V (from 100fA/V). The output noise should mostly be voltage noise, particularly noise from the op amp's offset voltage drifting. (The current op amp has +/- 50mV offset and can drift by over +/- 100mV over a 50 degree temperature range). It seems like the op amp drift contributes a few mV of noise.The last image is just to show that my scope contributes basically no noise.

author: EC8010
author_rank: Frequent Contributor
timestamp: September 04, 2025, 08:55:42 am
content: Looks good; no mains hum. The way to prove mains hum (or its absence) is to trigger from line (which you've done), then invoke averaging. Because you're triggered from line, any mains hum is coherent and remains when averaging is invoked, but noise reduces.But what are those repetitive spikes in the first trace? Something from the oscilloscope's display refresh? Nothing to worry about, I'm sure.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: September 05, 2025, 12:50:56 am
content: Quote from: EC8010 on September 04, 2025, 08:55:42 amI'm pretty sure it's switching supply noise that gets aliased down to a low frequency.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: September 07, 2025, 03:33:09 am
content: It seems like the white cylindrical resistors have extremely high voltage coefficient.I thought I could bypass the 100x amplifier formed by the 741, and get a higher range of inputs while maintaining 1fA resolution. Unfortunately, it turns out that the white resistors have extremely high voltage coefficient. If I put +/- 10mV into the 50G resistor, I get +15 and -25 mV (which is what you would expect with a small offset). However, putting in +/- 5V into the 50G resistor results in only -8.15 and 8.22V out. It looks like 18% of the resistance is gone, even at under 10V across the resistor.Perhaps I need to also replace those white resistors with the glass type ones. I think those have about 100 ppm/V so they should only lose 0.1% at 10V.

author: EC8010
author_rank: Frequent Contributor
timestamp: September 07, 2025, 08:35:19 am
content: Sadly, you have hit another low current problem, all the practical very high value resistors have a significant voltage coefficient, although I've not seen one as large as that. Some years ago, I used a Keithley 6517B electrometer to measure voltage coefficient of a 10M metal film resistor and the result was larger than expected. So I repeated with a 10M carbon resistor, which was perfection. Then I realised that the 6517B's own resistors had similar voltage coefficient to the 10M carbon, so they cancelled, and that's why the metal film looked poor. According to a resistor manufacturer I spoke to, there is only one practical material for really large values. Rule of thumb appears to be that you want a physically large resistive element, so the glass encapsulated resistors having two small resistors inside (one's a trimmer) areprobablynot very good. They are certainly very microphonic, and microphonydoesgo down with physical size. I have an electrometer shelf screwed in the corner of two outside walls to deal with the microphony problem.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: September 07, 2025, 11:51:15 pm
content: I switched out the 10G ohm with two normal 5G ohm resistors. The resistors still measured at 10 gigaohm with 200V applied across it, so the voltage coefficient should be pretty good. (I measured 19.98 nA).The original white resistor seems to have an extremely high voltage coefficient.Here's what I got:100mV 10.29pA1V 104.4 pA10V 1.258 nA100V 27.56 nAWith the 50G resistor, I got100mV 2.05 pA1V 20.86 pA10V 209.2 pA100V 2.153 nAIt looks like the 50G vacuum resistor still has some voltage coefficient but it's not as bad.

author: EC8010
author_rank: Frequent Contributor
timestamp: September 08, 2025, 07:55:05 am
content: Wow! That's enormous. I notice that those white resistors are very small. Have you tried giving the 10G resistor a really good scrub with flux cleaner (to soften any muck) and rinsed it away with copious isopropyl alcohol (to move it away)? It's just possible that it might be surface contamination.

author: w42i
author_rank: Contributor
timestamp: September 08, 2025, 11:15:03 am
content: I quickly put your numbers into a spreadsheet. The difference looks even more dramatic when represented graphically.I've always wondered how stable and repeatable the voltage coefficient (actually the complete R(V) function, since it could be more complicated than a simple linear voltage coefficient) is and how much it is affected by temperature. If it stays the same, it is somewhat annoying but could easily be calibrated out.Does anyone have more information about this?

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: October 08, 2025, 04:22:23 am
content: Here are three Chinese resistors.The one on the left is a 1T resistor I bought from Taobao. It cost $20 and the seller clams it is 1.004T. For some reason, I am having trouble measuring this resistor because it keeps on causing my TIA to oscillate. (If I use a test voltage of 10V or 100V, I need to set the TIA to use 10G ohm + 15pF as the feedback, which leads to everything oscillating at 0.1 Hz). I bought two resistors. One of them seems to measure a few percent low with a test voltage of 100mV, and the other seems to be 50% low (which probably means I need to clean the part more).The 100G is 3% too low with a 100V test voltage. I had to bake the parts because initially one was 37G and the other was 88G. After blasting the resistors with a heat gun, both of them went to 97G (with a test voltage of 100V). I wonder how it might be possible to seal these resistors to prevent moisture from getting in again.The 500G is about 5-6% high with a test voltage of 100V. Both the 100G and 500G resistors don't cause oscillations for some reason. I think it might be because the leads are smaller so there is less capacitance to ground?The 100G cost me 18 bucks for 2, and the 500G also cost me 18 bucks (but only for 1). The 100G and 500G seem to have a much lower voltage coefficient compared to the other resistors I have, because they are physically larger.

author: EC8010
author_rank: Frequent Contributor
timestamp: October 08, 2025, 08:08:25 am
content: I have also had trouble with 1T resistors causing low frequency oscillation. Do the sums and you find that a fraction of a pF of capacitance across the resistor gives a time constant equal to the oscillation frequency.I had low frequency oscillation using a 100G resistor on an ADA4530-1. The problem turned out to be poor earth bonding to the chassis. The original PCB had isolated mounting holes, but the second PCB allowed the mounting bolts to connect chassis to the ground plane, and it all behaved.You might want to experiment with self-adhesive copper foil. One possibility is to try wrapping a narrow circumferential guard band around the resistor half-way along and bonding it to 0V to reduce surface leakage currents and perhaps capacitance.You are correct; physically larger resistors have lower voltage coefficient. They are also less microphonic.That is worrying that you had to bake the resistors. Some PCB conformal coating might do the trick without reducing the resistance. If it does, conductive paint as a guard band (and connected to 0V)mightsolve the leakage problem. Are you certain it was moisture absorbed into the resistor and not surface contamination? It's not unusual for these resistors to have been handled inappropriately (bare fingers), so your heat gun might just have evaporated contamination. Flux cleaner is a very effective solvent, followed by isopropyl alcohol to rinse the contamination away. Handle by the leads only.