16 Bit Multimeter "DIY" - Page 1
Forum section: electronics / general

author: Olivia
author_rank: Regular Contributor
timestamp: February 22, 2022, 07:10:58 pm
content: Since I couldn´t finde the specific topic here I´m going to ask bluntly:I am currently planning a 4 3/4 Digit multimeter (I know sounds kinda boring compared to others but its a start), and am on the search for a Reference Voltage source that can supply 5.0V, as thats the intended maximum input voltage for the intended ADC, a MAX 195BEPE.Everything from the Input to the ADC input has been planned and trimmmed for up to 5V input to the ADC.Thank you in AdvanceOle

author: Kleinstein
author_rank: Super Contributor
timestamp: February 22, 2022, 07:29:34 pm
content: There are plenty of 5 V reference - which are actually available is another point. The usualy sources often have a search function that includes the in stock point. For the 4.5 digit range this would likely be a better bandgap type refrence like the max6070, ref5050 or similar.The ADC is a bit unusual for a DMM, though still possibly. The mormal case would likely include quite some averaging / digital filtering to suppress mains hum.

author: Olivia
author_rank: Regular Contributor
timestamp: February 22, 2022, 07:54:36 pm
content: Well the MAX 195 is just one of the options. I took this one onto my priority list because it its socketed, so i can switch it out. Another spot on the list is the MCP 3427.

author: Gyro
author_rank: Super Contributor
timestamp: February 22, 2022, 08:23:10 pm
content: If you want the added stability of a buried Zener reference, and don't mind expending more supply current, then you could try an AD586.EDIT: (You didn't specify handheld or Bench DMM).

author: Olivia
author_rank: Regular Contributor
timestamp: February 22, 2022, 08:45:20 pm
content: Well my plans for the Multimeter are "household" applications i.e. being able to measure 0 to 50V with the 2^16 Steps that a 16Bit ADC can get you.I don´t know exactly how other DMM´s are constructed but my plan is to have a Input Amp which gives me a 0 to +5V signal (an a polarization signal) for the ADC.The ADC could then be either a 16bit or something else, depending how accurat i could get the entire ensemble to work.The Current measurement is planned to be done using a up to 500mV Drop (which compared to the other multimeters I have access to is quite fine) and a .5, 5 or 50 Ohm shunt.Comparing it to the Multimeter I´m using at work which gets up to 2,5V drop in the 500mA range, i find this pretty harmless.This is then amplified to the desired 0 to 5V for the ADC via the "depolarizer".Resistance is measured via a controlled current source (have to work on that one).The entire device isn´t meant for speed or accuracy, its meant to be functional.QuoteI am planning for it to be a stationary i.e. bench DMM

author: Gyro
author_rank: Super Contributor
timestamp: February 22, 2022, 09:10:02 pm
content: 500mV drop sounds rather high by modern standards, 100 - 200mV is a more typical burden for even modest handheld DMMs (0.1R lowest shunt). Imagine that you are trying to measure the current draw of a 5V (or even worse, 3V3) logic circuit.For a Bench DMM, it is often better to do away with the current ranges completely and use external 4 terminal (kelvin connected) shunt resistors instead, using the mV range.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 22, 2022, 09:13:39 pm
content: The max195 is quite fast and could allow to do digital RMS for AC measurments with a reasonable BW. It still needs a good (flat frequency response) front end for that. For more DC operation one would need to do averaging in SW and maybe a little AA filtering too if lowest noise is wanted. So it is a bit more to do for the µC, but still not a bad choice. Having a +-5 V range is also nice. With using a pseudo differential reading of 2 input signals (low side and high side) one could even get a +-10 V range without a divider.The max195 is however a pretty expensive chip. Replacing it is still not fun even if in a socket.The MCP3427 is not really that attractive. It has an integrated reference, but the integration time is not that suited for 50 Hz operation. One can get a bit around this by averaging 3 (or multiples of that) conversions in SW. Still the integration time is not very accurate and thus more mediocre 50 Hz suppresion is expected. That said I had an earlier nearly finish DMM project (only volt and amps) using the related MCP3421. It does work reasonable - though never got really finished and still mediocre shunts and divider.I would consider the ADS1219 or MCP3562 worth a look. They are 24 bit ADCs, but one is OK to through away a few bits. With a modern design gain is adjuted by a numerical factor and a little more resolution (even if not noise free) can help to avoid rounding errors.Usually it is better to let the ADC handle both polarities and not do a manual / separate change in the polarity. It is already a weak point in many dual slope ADCs to handle positive and negative signals separate.

author: Olivia
author_rank: Regular Contributor
timestamp: February 22, 2022, 09:57:41 pm
content: Well, considering that this is my first DMM i dont think I´ll integrate a AC measurement just yet.The voltage drop that i used as a measurement for he current measurement comes from a very very old multimeter that uses a 1kOhm shut for the µA range.Still this is in the quite early design phase, so I can still play around with the measurement ranges and specifications freely.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 22, 2022, 10:12:57 pm
content: The burden voltage drop at the shunts can be chosen later. It is a balance between the need to measure small votlages (good amplification) and the heat release. In some cases a lower burden can save on the power rating of the shunts. So some 200 mV for the shunt is usually a reasonable compromise. More drop can come from the fuse. The shunts many still need to withstand up to some 1.5 to 2 V worst case, as that is about the clamping voltage from 2 diodes.A point that can make quite some difference is the question of manual ranging and auto ranging / computer controlled range setting.For the design the voltage range of the ADC makes a difference. I would not care too much how this translates to ADC resolution, as the ADC LSB steps and the display steps don't need to be the same. There is possibly added resolution from oversampling and in some ADCs noise of more than 1 LSB. So the ADC nominal resolution does not directly related to the counts and ranges. Quite often the ADC could give more theoretical resolution and the number of digits is more set by the stability and noise level. With an output to the computer it does not really matter if there are more digits send than the noise or stability would for a display suggest.

author: Karel
author_rank: Super Contributor
timestamp: February 22, 2022, 11:15:42 pm
content: Some time ago I built a DVM based on & inspired by this video:I filled in the missing gaps and made some improvements but basically it's the same design andit works surprisingly well and it's stable.

author: mycroft
author_rank: Regular Contributor
timestamp: February 24, 2022, 01:23:17 pm
content: Could you share those improvements?Thanks.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 24, 2022, 02:48:07 pm
content: The old LTC2400 milivoltmeter in the video is a bit limited in that it is one polarity only. There were a few iterations for the input buffer and protection, some with not so ideal properties (leakage from Schottkey / zener diodes). The OP777 in the early version also has a bit drift and bias, that is not good. The later used AD8628 is a good choice with acceptable bias and low noise. One may also get away with lower bias and slightly high noise versions ifo ne does not need resolution down to the 100 nV range. The large cap at the OPs output deserves a few.A big limitation at least in the early version is that the buffer has no negative supply and thus does not work correct around zero - that LTC2400 should at least work a few mV to the negative side too and could do a proper zero adjust, but this does not help if the OP is at it's limit.

author: Olivia
author_rank: Regular Contributor
timestamp: February 26, 2022, 08:18:36 pm
content: If anyone is wondering, I currently am working on the exact circuit of the input board.Turns out, for the functions that I want i could get the three needed resistor networks in a DIP-16, if they are isolated enough.The current Boards/ Sections are:A0 Power Supply (concept)A1 ADC Module (concept)A2 Switching Logic driving (concept)A3 Input and Range switching (Currently WIP)A4 Resistance measurement Current Source (concept)

author: Kleinstein
author_rank: Super Contributor
timestamp: February 26, 2022, 08:45:59 pm
content: The reistors in the networks are usually reasonably good isolated, but the voltage may be limited. So directly at the input this may not be good enough. For later uses like a reference divider or gain the isolation should be OK. The demands can still be different depending on the part of the circuit and ranges (could be high if you want a very low current range (to the pA resolution). Some standard dividers are also available in a SO8 and SOT23 case.For a high voltage divider at the input there are special resistor networks available with a SIL pinout.In some areasy a chain of equal, but seprate resistors could be an alternative to a special network. With parts from the same roll, chances are the matching can be relatively good - no garanteed, but chances are good.

author: Olivia
author_rank: Regular Contributor
timestamp: March 17, 2022, 08:26:07 pm
content: So, while this post was quite silent for some time I have modified the input board massively.And not just that, Ive worked on the PSU and the Resistance Measurement Board.The boards/ sectors are:IA1: PSUIA2: ADC Module & Internal Protection SystemIA3: Input BoardIA4: Resistance Measurement SourceIA5: Voltage ReferenceIA6: Internal Calibration/ Sanity Check ModuleI think I`m on Revision 6(a) of the input board. Which is just a fancy voltage divider that divides everything down to +-5V (or 10V in some cases)Then there is the ReMeSo, ofr which I now have a good idea and a concept.The same goes for the PSU and I even have some Concepts on how to connect the ADC in an elegant way.

author: Karel
author_rank: Super Contributor
timestamp: April 08, 2022, 10:43:39 am
content: Quote from: mycroft on February 24, 2022, 01:23:17 pmSorry for the delay, here is a part of the schematic.The input opamp is still the same as in the original project (which I copied) but I would like to replace it with an OPA140 (but it's out of stock now).

author: Andreas
author_rank: Super Contributor
timestamp: April 08, 2022, 05:55:12 pm
content: Hello,according to data sheet the 1uF on the input of the ADC will give a relative large offset and full scale error.I am usually using 680-820 pF for C210 and 825R for R204 to get lowest noise and best INL. (together with LTC1050/LTC2057/ADA4522)with best regardsAndreas

author: Karel
author_rank: Super Contributor
timestamp: April 09, 2022, 06:39:47 am
content: Quote from: Andreas on April 08, 2022, 05:55:12 pmInteresting, I'll try that.

author: Olivia
author_rank: Regular Contributor
timestamp: April 24, 2022, 01:46:41 pm
content: I am currently working on the input portection and bootstrapping circuit for the input amplifiers.As I have heard, the Datron 1061 (iirc, its definitely a Datron DMM) can get a full 1kV applied to its input even when measuring on the lowest range.Some Multimeters use Zener Diodes for the input protection and I was wondering if one could use multiple 1N4007 as protetion diodes in combination witha set of resistors both in front and behind the diodes as well as a GDT between the first resistors.The primary OpAmps for the input are intended to be LMP7721 and the bootstrapping Amps are intended to be OPA106 (or other comparable sub pA 30V OpAmps)

author: Kleinstein
author_rank: Super Contributor
timestamp: April 24, 2022, 02:47:43 pm
content: The protection has 2 parts: a clampling part, that normally used 2 low leakage diodes and than something like 2 zener diodes or a TVS or similar. The slightly tricky part here is not keep the leakage low and make sure it also works when the supply is off. The 1N4007 is of limited use here as the leakage is more on the high side and the current is normally limited to a few mAs even with 1 kV at the input. This is kind of needed to avoid excessive heat.The 2nd part is the current limiting part - e.g. series resistors with high withstand voltage or PTCs or high voltage FETs (especially depletion ones) or a combination of those. A GDT is mainly used for short spikes of very high voltage, like ESD - especially in the combination with FETs. With resistors for the main protection one may get away without a GDT.

author: Olivia
author_rank: Regular Contributor
timestamp: April 25, 2022, 06:50:42 am
content: I know that the 1N4007 are happily leaking up to a few µA, thats why I chose them.The 4007 should easily conduct the few pA that I would need without any kind of voltage drop (except ohms-style)I do intend to place multiple 4007 in series, so that even the max. rated voltage (1kV) wont make them sweat.

author: Olivia
author_rank: Regular Contributor
timestamp: April 25, 2022, 06:56:58 am
content: I hope the schematic is at least somewhat understandable.The usual Voltage difference at the OpAmps (those are Precision Amps and their respecitve bootstrapping circuits) should not exceed about +-12VThe input current is as such:dU <30V: <2.5pAdU 30-1200V: <5µAdU >1200V: Whatever the GDT and the resistors in front of it decide to conduct.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 25, 2022, 07:30:58 am
content: The 1N4007 should not leak to the µA range, more like low nA at low voltage. The leakage may reach the µA range when at 1000 V, but this may as well also be only the test limit.The diodes as series element are not a good idea. With low current / low voltage (e.g. < 10 mV) they still act a little like a high value (e.g. 100 M range) resistor. So a little bias can cause quite some offset and they also add the noise like such a large resistor. In addition the diode action can cause rectification of superimosed AC and this may not cancel out perfectly for the back to back diodes, as parasitc capacitance and leakage plays a role. So even a very leaky diode that acts like a 1 M resistor would not be good.The easier way is a resistor (e.g. 100 K - 300 K range) or a chain of resistors in sereies so they can withstand a high voltage and dissipate enough power.With the usual DVM ciruits 1 input is connceted to ground or a low impedance connection. So the 2 input terminals are not symmetric, but one is low impedance and the other is high impedance. The low impedance link is needed for possible common mode current.

author: b_force
author_rank: Super Contributor
timestamp: April 25, 2022, 12:00:20 pm
content: When making these kind of things, I would start differently.1 - What is the expected accuracy?A lot of people don't realize that their fancy new multimeter has something like a 0.06%+5d (or something) accuracy on the voltage.Practically meaning that the last digit won't do much.2 - Are you able to calibrate eventually the DIY multimeter yourself?Resistor dividers, other range switches etc, it all adds up to the error.When you're able to calibrate a meter by another device, you have to put a lot less effort in the design itself.

author: Gyro
author_rank: Super Contributor
timestamp: April 25, 2022, 07:37:50 pm
content: Quote from: Ole on April 24, 2022, 01:46:41 pmThe Datrons do it the 'brute force way'. The actual clamps are back to back zeners in series with inverse parallel low leakage jfets. The heavy lifting is done by a series chain of four 22k 3W resistors. This chain can dissipate 12W continuously (as you say, 1000V protection on all ranges). EDIT: That's for the high impedance ranges from 10V down to 10mV, the higher ranges go through the normal 10Meg attenuator so are protected anyway.You can find all of the relevant service manuals on K04BB...http://www.ko4bb.com/getsimple/index.php?id=manuals&dir=06_Misc_Test_Equipment/Datron

author: Kleinstein
author_rank: Super Contributor
timestamp: April 25, 2022, 08:50:15 pm
content: A string of resistors is a quite common solution, e.g. found in the HP34401, HP3456, The SDM30xx and many other meters. The resistors are sometimes lower power and may not withstand the full 1000 V for an extended time. The main downside it the large size (to get rid if the heat) and when going beyond 7 digits the extra noise from the resistors. The next step up is to turn of the input path with overload for an extended time.The resistor chain is cheap and reliable, so not a bad solution up to some 100 nV resoluition. It gets even more practical if one limits the maxium voltage for an extend time to a lower value, like 500 V.

author: Olivia
author_rank: Regular Contributor
timestamp: April 26, 2022, 10:09:14 am
content: I do intend to utilize other means of protection as well, with the hardwired protection being a last line of defense.Additionally I will utilize a Guard Terminal (I do have to purchase some triax cable for it anyways so I could utilize it as my test lead as well)The plan, as of now, is to utilize a set of DC preamplifiers (and maybe later some AC ones too, but thats another story), one for Voltages and one for current.The preamps should utilize the minimum possible bias current and would consist of the actual signal OpAmps and the Bootstrapping circuit.

author: Karel
author_rank: Super Contributor
timestamp: May 23, 2022, 10:33:18 am
content: Quote from: Andreas on April 08, 2022, 05:55:12 pmI tried it and there's a big improvement of the INL(my "reference" is just a 34461A)Next step is to try the LTC1050 but I need to order it.Thanks!

author: Olivia
author_rank: Regular Contributor
timestamp: June 15, 2022, 09:26:03 pm
content: Hiho,I haven´t given anyone an update in quite some time.So here we go:IA1 Powersupply:-Inguard Transformator Output 1:Bipolar +/- 20VAC for +/-24V Rails with low Current and +/-15V Rails and +/-5V Rails.Supply for all OPAmps and analog switches as well as the Vref module-Inguard Transformator output 2:Bipolar +/- 15VAC for +/-12V Rails and 5V RailSupply for Relais and High Current Source and the Inguard ProcessorIA2 ADC-AD7177 based ADC Sector (very similar too the HPM7177)IA3 Analog Frontend-DCV LN Amp via OPAx140 with bootstrapping with near Zero Current Auto-Zero-DCV LI Amp via ADA4530 for very high impedance sources-DCI via 100mR and 10R Shunts and (this is for the ac part aswell) 5mR for very high currents-DCI Low Current via Inverting Amplifier feat. ADA4530 and 1k and 100k (may change R value)-Signal Amplifiers with x1, x10, x100, and x1000 amplification-Analog I/O (at the joint connecting the Amplifiers to the ADC Sector) for +/-10V differential.IA4 Voltage Reference and BufferVref Module in the HP3458 A9 style and a HPM7177-style divider with trim to get the Vref to 4.9xxV with the 5V rails being at 5.1xxVIA5 Resistance Current Source/ Variable Voltage SourceModule based on two Caddock 1776C6815 Decadal Divider Networks-Produces Constant Currents and Voltages in decadal steps.

author: free_electron
author_rank: Super Contributor
timestamp: June 15, 2022, 10:38:19 pm
content: before you start doing all that goodness : ask yourself if the chosen A/D is the right way to go ... there is a reaon all multimeters use a dual or multislope converter : they can be calibrated very simply and all errors are self erasing.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 16, 2022, 09:31:47 am
content: For the origial 16 bit aim a ready made SD ADC is not such a bad idea. It just is made to work with a 3 or 5 V reference and not with a 7 V reference.The main advantage of a multislope ADC is that they can easy work with a higher reference voltage like 7 V or 10 V and have an input range of some 10 V, which may be convenient to get high input impedance also for up to 10 V or even a little more. With an SD ADC this needs extra effort.For the beginning it is OK to start with a simpler/cheaper solution - the 1st version has a chance to fail or at least have quite some weak points. The learning part is not so much about using expensive parts, but to get reasonable performance even from cheap parts.The ohms part is a separate topic. A simple Ohms version should be OK with more normal resistors, at least for the start. It would be after one has a somewhat working solution to think about what parts need to be better and plan for a final, stable version.With the usually relatively low power (e.g. some 2 - 5 W) for a DMM it does not make much sense to use 2 transformers. This only adds to the coupling capacitance and leakage and the extra tranformer loss is likely more than heat from linear regulators droping a little more voltage. For a higher supply with low power one could consider a mains frequency charge pump to double the votlage.Expecially with an ADC like AD7177 one usually will not need a gain of 1000, except for a special nV input amplifier. One can even question the need for a gain of 100. Some DMMs (e.g. SDM3055) seem to have no analog gain before the ADC (in this case AD7190), at least not for the voltage ranges. The smaller input range of the ADC chips means less gain is needed.

author: iMo
author_rank: Super Contributor
timestamp: June 16, 2022, 11:27:21 am
content: Anecdotal note: I did many times use ADS1100 (2 diff inps, 6pin chip w/ I2C, 8smps/s @16bit, internal 2.048V ref w/ 5ppm/K) and it worked nice. Put a higher voltage floating opamp stage (discussed here several times) with a divider in front of it and you will get a nice voltmeter (as the first step in your effort).

author: macaba
author_rank: Regular Contributor
timestamp: June 16, 2022, 01:10:03 pm
content: Quote from: Ole on June 15, 2022, 09:26:03 pmAmong the bigger challenges of the HPM7177 were:1. Fully differential input amplifier CMRR [for both DC+AC] (solved with high performance resistors, CMRR trimpots, TEC temperature regulation and very careful amplifier selection/layout)[Note: ADC CMRR specification does not help here - front end CMRR error shows up as differential error on ADC inputs]2. SAR input charge injection (solved with high performance/speed amplifiers)With those 2 tricky design points in mind, you might be able to make things a lot easier with the AD4630-24 instead of AD7177. It has a similar noise specification, with the addition of a few very useful features...It's a differential ADC that also fully supports single ended input [see Figure 36 of datasheet] (with In- fixed at REF/2 or 0V, and In+ between 0V and REF) and has defined specifications in the datasheet to support this thus allowing single-ended input stage (with output shifted to fit the ADC CM range) which is easier to get right. Challenge 1 solved.It also has easy-drive inputs which use pre-charge tricks to significantly reduce input charge injection, making the front end design easier again by allowing a wider range of amplifier choices. Challenge 2 solved.As a side perk - the loss of dynamic range from not using differential input can be alleviated by running both ADC channels of the AD4630-24 in parallel from a single input.If this ADC was released at the design time of the HPM7177, it probably would have been the HPM4630!

author: Olivia
author_rank: Regular Contributor
timestamp: June 17, 2022, 09:43:18 pm
content: So concerning the Choice of ADC,my first choice was the AD7177 because the HPM7177 was based on it so i wouldnt have to do most of the hard work.I´ve looked around for some other comparable ADC´s and here is my choice so far:AD7177:DS, max 10kSPS, 32bit0.07µVrms noise @5SPS (24.6 bits)AD4630-24SAR, max 2MSPS, 24bit17.6 (105.7dB SNR) Noise Free Bits @2MSPSLTC2500-32SAR, max 1MSPS, 32bit17.5 (104dB SNR) Noise Free Bits @ 1MSPS148dB Dynamic Range @61SPSADS1263DS, max 38kSPS, 32bit+24bit21.9 ENOB @2.4kSPS (SINC4) ADC1I might have to rework the noise specifications and make a better comparison.

author: iMo
author_rank: Super Contributor
timestamp: June 18, 2022, 09:55:19 am
content: Quote from: 3roomlab on June 18, 2022, 08:54:55 amIn the meantime you may create the schematics, design a quality pcb and write the software..PS: I think that from 85% the shortage is caused by the "business practices of the distributors" thus many P1/P2/P3 customers order directly these times..

author: maat
author_rank: Regular Contributor
timestamp: June 18, 2022, 04:16:56 pm
content: Quote from: 3roomlab on June 18, 2022, 08:54:55 amThat is a MAYbe

author: Olivia
author_rank: Regular Contributor
timestamp: July 04, 2022, 12:15:06 pm
content: So, I´ve been brainstorming over the last few weeks and have found a slight problem,I need a set of analog switches with very low Ileak and preferably a low RonThese Switches would be used to connect the various functions to the amplifiers and the ADC(s)Therefore I could allow myself to use OP´s like the AD797 or LT6018 for minimized noise.My ideas so far:1. Utilize the MAX326/7 and use 1.5kR Ib canceling resistors2. Try to build a JFET multiplexer with a low resistance/ low leakage FET (e.g. 2N4858)

author: Kleinstein
author_rank: Super Contributor
timestamp: July 04, 2022, 12:26:31 pm
content: Getting good CMOS swtiches is currently a bit tricky. The lower R_on ones (e.g. DG411) have usually more leakage. So one kind of has to compromise. Quite often one does not need low R_on.The leakage current depends on the voltage (may be nonlinear and temperature dependent) and cancelation is usually hard.The AD797 is likely not the right choce in a DMM. One should choose OPs that don't need extra low On_resistance.Dending on the circuit one can tolerate some leakage / offest and just correct numerically.

author: Olivia
author_rank: Regular Contributor
timestamp: July 04, 2022, 01:16:23 pm
content: I just checked with Maximintegrated.com, Digikey has just shy of 300 327 in a DIP-Package in stock.Those switches would be used for most of the instument, i.e. Function connetcing, amplifier gain selection (that might be done via JFETs or Relais) and the input AZ-Cycling as well as the shunt selection on the sense side.But for the ADC driving the current plan is to use the schematic from the HPM7177 and maybe add AD797 instead or auxilliary to the AD8065.Then I´d be able to use JFET´s more easily, since my signal is not +/-15V max. and I can just use -5V Gate Voltage to lock the JFETs.

author: sahko123
author_rank: Frequent Contributor
timestamp: July 04, 2022, 04:54:38 pm
content: I believe adi may be phasing out the ad7177 be it for a successor to have fresher ppms or because of too low a demand to merit spinning up a few wafers so even if they make more to restock chances are it'll either be sooner than 2023 or never again.

author: iMo
author_rank: Super Contributor
timestamp: July 04, 2022, 05:47:30 pm
content: Look at Octoparthttps://octopart.com/search?q=ad7177&currency=USD&specs=0there is a seller with large stock (be careful, of course)..

author: Kleinstein
author_rank: Super Contributor
timestamp: July 04, 2022, 07:06:03 pm
content: The max327 are very low leakage, but also quite high in resistance. Except for very low bias inputs there is not much need for such low leakage. Another popular choice is DG211B. Ti also has a few new CMOS chips with low leakage (e.g. TMUX611x and a few more). Directly at the ADC input one may get away with lower voltage rated parts, that may be better available. There the high resistance of the max327 is likely a problem.JFET switching is possible, but it needs an extra negative voltage some 5 V below the range. This makes it a bit tricky for a +-12 V signal range. One also need an auxiliary signal to drive the gate when on. Driving from a buffer behind the switch can cause quite some switching spike. A positive point is that there is essentially no leakage when on. The availability may be a bit better than with special specific CMOS switches. The JFETs of choice are MMBF4117 , MMBF4393 and maybe MMBFJ202 or 2Sk208.

author: iMo
author_rank: Super Contributor
timestamp: July 04, 2022, 07:48:43 pm
content: FYI - femtoAmps leakage with 1N4148 protection diodes..Will work with bipolar the same way..PS: 52fA with 1N4007 ( ͡° ͜ʖ ͡°)

author: Olivia
author_rank: Regular Contributor
timestamp: July 04, 2022, 08:28:43 pm
content: QuoteIf I had gone for ultra low leakage with ADA4530 for example I would have chosen something similarBut for my current design iteration I´m going with DPAD1 low leakage diodes and 30k Resistance up front.The DPAD are connected to the power rails of the MAX327 (i.e. +/-15V)