Possible autoranging DMM scheme? - Page 1
Forum section: electronics / projects

author: Analog Kid
author_rank: Super Contributor
timestamp: November 16, 2024, 11:27:03 pm
content: OK, here's a crazy li'l idea: let's say a guy wanted to build his own autoranging DMM. (Not necessarily a high-precision one, but good enough.) Is it possible that this scheme might work?The comparators are connected to different voltage references corresponding to the lower limit of the various ranges, with the higher-voltage ones first. Whichever comparator "trips" will activate the corresponding voltage-divider resistance in the DMM circuit. (Could also light LEDs to show the range.)Problems?Loading of the input by all those comparatorsComplexity? Maybe not too bad: each ref. could be just a single chip plus a few partsThis is a one-stage-beyond-daydreaming idea, so feel free to tear it apart if it's not feasible (but maybe don't be too hard on me).Ackshooly, instead of a mux, could just use analog switches to switch each R in and out of circuit.

author: TimFox
author_rank: Super Contributor
timestamp: November 17, 2024, 12:04:44 am
content: Since the highest full-scale will be more than 200 V, obviously each comparator will need a voltage divider at its input to keep the input voltage reasonable at high input voltage.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 17, 2024, 12:36:42 am
content: So, there's a glimmer of a sliver of a chance this might work?My scheme as presented above obviously glosses over many details like this.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 17, 2024, 12:39:42 am
content: Quote from: Analog Kid on November 16, 2024, 11:27:03 pmSimpler for fast auto-ranging would be to pick the highest voltage range first, then use that ADC result to choose the next range.Most ADC chips have some trade off of bits-vs-sample time, so that first check can be faster.External voltage pre-auto-range checks like you draw could speed that up a few milliseconds by skipping that first ADC check, but at the cost of a lot of parts.If your ADC only had a single very slow speed, maybe it is worth it ?

author: ejeffrey
author_rank: Super Contributor
timestamp: November 17, 2024, 12:46:49 am
content: The basic idea can work. As tim said, the main issue is protecting the inputs over the voltage range. At a minimum probably an attenuator, probably with a clamp diode.There are a few issues with this, a big one is that if each comparator has its own 10 megaohm voltage divider the total input resistance might be only 2 MOhm.For this reason it's better to put the autorange circuit after the primary input attenuator. A parallel approach like this is potentially very fast, but I think it's more common to sense whether the amplifier output exceeds certain threshold and move the range up or down one step.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 17, 2024, 12:55:47 am
content: Updated probationary schematic:Quote from: ejeffrey on November 17, 2024, 12:46:49 amWould that be a passive voltage divider or an active component, like a buffer?

author: bdunham7
author_rank: Super Contributor
timestamp: November 17, 2024, 01:47:19 am
content: There have been meters made that work something like this. You don't need multiple references, you can just use the same one (probably the same as the ADC) and connect it to multiple comparators with different voltage dividers setting the transistion levels. I think you'd want to this circuit to have its own unchanging high-impedance input divider and buffer, at least for a simple design.

author: Kim Christensen
author_rank: Super Contributor
timestamp: November 17, 2024, 01:58:12 am
content: You'd also want a bit of hysteresis between range changes.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 17, 2024, 02:11:13 am
content: One other wrinkle I probly should have mentioned: the plan might be to use this with a prepackaged DMMlike this one. So I wouldn't have any access to any of the internals, like the ADC. I'd just be using the external connections to the range-selection resistors. Would that work?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 17, 2024, 02:15:40 am
content: Quote from: Analog Kid on November 17, 2024, 12:55:47 amYou can use a single divider, shared, to keep things simple and high impedance.If you choose low offset parts, you can even manage extremes like a 2kV range down to 200mV range.If we assume that 2kV scaled to 2V at ADC in, then 200mV will be 200uV, which is easily checked by modern low offset & low bias opamps, used as comparators.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 17, 2024, 02:18:05 am
content: Quote from: Analog Kid on November 17, 2024, 02:11:13 amHow does the meter know which range you have externally chosen, to know where to put the decimal point ?What real world problem are you hoping to solver here exactly ?

author: Analog Kid
author_rank: Super Contributor
timestamp: November 17, 2024, 02:33:01 am
content: Quote from: PCB.Wiz on November 17, 2024, 02:18:05 amAha, excellent question. Looking at the datasheet (such as it is) for the li'l DMM module, it shows wiring options for setting the decimal point depending on the range, so in addition to setting the range resistor you'd need to short or open two points on the board using more analog switches.The problem? Wellll, I'd like to use one of these modules to display a range of voltages without having to manually switch the range.So here's the latest iteration:I'm guessing the input buffer should just be a voltage follower, for maximum input impedance and for gain ≈ 1?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 17, 2024, 02:56:12 am
content: Quote from: Analog Kid on November 17, 2024, 02:33:01 amIf you can find a magic 200V opamp ?Usually voltmeters have a high impedance lowest range, and then switch in dividers for higher ranges. (ie that buffer is only on lowest range)You might choose to have 10M on all ranges for simplicity.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 17, 2024, 03:28:31 am
content: OK, another go-round:With the input divider, the voltage to the comparators will range from 20mV to 20V (only 3 ranges: 200mV, 20V & 200V).Will a voltage-follower buffer be able to handle that range?Will a comparator work reliably down to 20mV?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 17, 2024, 03:51:46 am
content: Quote from: Analog Kid on November 17, 2024, 03:28:31 amUse an opamp as a comparator.OpAmps are quite cheap down to < 50uV offsets, but low offset comparators are less common.

author: TimFox
author_rank: Super Contributor
timestamp: November 17, 2024, 03:56:25 am
content: Even better is a good op amp with two Shottky diodes feedback network ahead of a fast comparator.(This works better with an inverting input.)

author: bdunham7
author_rank: Super Contributor
timestamp: November 17, 2024, 06:42:45 am
content: Quote from: Analog Kid on November 17, 2024, 03:28:31 amThe input buffer doesn't have to handle 200V since the threshold for the highest range is 20V. You can just use an input resistor and a protection circuit (zener diode, diode to a supply voltage, some other setup) to limit the voltage to the buffer to 25-30V or so. Or you can use the x10 divider and the same idea at a lower voltage.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 17, 2024, 07:16:40 am
content: Yes, 20V is the threshold of the highest range, but if the input is above that, let's say at or near 200V, that voltage will still appear at the input to the buffer, even when it triggers a switch to the highest range. So what am I missing here? Maybe I'm misreading your post?

author: Kim Christensen
author_rank: Super Contributor
timestamp: November 17, 2024, 04:33:38 pm
content: Quote from: Analog Kid on November 17, 2024, 02:11:13 amI wonder if that panel meter uses the ICL7106 chip. If so, you might find this AP note of interest:Building a Battery Operated Auto Ranging DVM with the ICL7106

author: bdunham7
author_rank: Super Contributor
timestamp: November 17, 2024, 05:54:56 pm
content: Quote from: Analog Kid on November 17, 2024, 07:16:40 amThe second sentence explains it? Draw that out and see if it makes sense.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 17, 2024, 09:35:52 pm
content: I did, and it still doesn't make sense to me.You'll have to pardon me here: I'm really not trying to be obtuse. It's just that sometimes I have blind spots and can't see what's right in front of my nose.So let's say a guy would add this input protection to the front end of this scheme:I can't see how that would work properly; it would limit the input voltage to approximately ±Vcc, right? Which is fine so far as input protection goes. But the meter (the DMM module tapped off the input) wouldn't be seeing the correct voltage anymore, and neither would the autoranging circuit. Anything above ±Vccwould be clamped.So how exactly would that suggestion of yours work? Help a guy understand.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 18, 2024, 09:48:15 am
content: You can use a single voltage divider chain with 3 or so (N + 1) resistors in series , and connect all comparators to the same divider.But normally you have a microcontroller in the thing. This means you can simply take a measurement, and then have your uC change the range depending on what the measurement is.

author: bdunham7
author_rank: Super Contributor
timestamp: November 18, 2024, 02:43:34 pm
content: Quote from: Analog Kid on November 17, 2024, 09:35:52 pmAdd a resistor between the DMM_IN tap and the junction between the two diodes.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 18, 2024, 08:33:01 pm
content: Quote from: bdunham7 on November 18, 2024, 02:43:34 pmOK, like so?I still don't see how that solves the problem:The buffer will still potentially see high voltagesThe buffer will never see actual input voltages beyond ±VCC(and neither will the DMM)The input to the DMM module will now be offset by the RSo what am I missing here?Or did you mean this instead?

author: Kim Christensen
author_rank: Super Contributor
timestamp: November 18, 2024, 09:13:47 pm
content: I think he means something like this:

author: unseenninja
author_rank: Regular Contributor
timestamp: November 18, 2024, 09:32:29 pm
content: Quote from: Analog Kid on November 16, 2024, 11:27:03 pmI'd suggest that you look at the circuit diagrams which are available for a large number of multimeters and learn from the people who designed them.There's no need to reinvent the wheel when there are already many examples of how to build a wheel.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 18, 2024, 09:45:04 pm
content: Quote from: unseenninja on November 18, 2024, 09:32:29 pmWell, I did look online before posting this. Found a lot of circuits, yes, but none involving autorangingexceptArduino-based ones, which is not my preference here at all. So thanks for your suggestion but no thanks.And some of us just want to see if we can inventa different type of wheel. Guess we're just peculiar that way.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 18, 2024, 10:23:29 pm
content: Quote from: Kim Christensen on November 18, 2024, 09:13:47 pmThanks (and thanks for redrawing my drawing), but I still don't get it.Seems to me that that scheme will still suffer from the problems I listed above.And are those switches the analog switches driven by my comparators?Can someone please explain how this might work, and try to be a bit more verbose than just "put a resistor there"? Please keep in mind that I am not an electronics expert; I'm still learning.TIA.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 18, 2024, 10:38:03 pm
content: The switches would be the one controlled from the comparator or DMM moule output. One would likely not want a direct control, but some added hysteresis or clocking to avoid jumping between ranges like crazy. If not in sync with the DMM part, there will be invalid results when the range is changed during a conversion. With the the conparator(s) after the switched divider would give signals to change the range up, down or keep it. So this would want some kind of clocking and logic (e.g. an up-down counter).

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 18, 2024, 10:47:27 pm
content: Quote from: Analog Kid on November 18, 2024, 09:45:04 pmThe circuit in #24 is widely available, and a solid starting point.Do you need this to work with both polarities ? The module you linked to looks to have auto-sign support.What input impedance do you need ? Your link give a higher impedance at 200mV - that is harder to meet with auto-range schemes, but 10Meg for example is easy.If we do unipolar and > 10Meg load by the DMM:If you want 200V / 20V / 200mV ranges, that can be done with a level sense path, and an ADC path. (200mV DMM in) This keeps range switches as low voltage analog.The ADC path is easy, you need Divide by 1 (no shunt resistor) or /100 or /1000The decision comparator path just needs to check which range to select, and it is ok if this clamps to Vcc. (200V thru 10Meg is 30uA)One solution would be 5V supply/ref and 20M+5M, for /5 on the sense, with two compare set points of 4V and 40mV.< 40mV means 200mV is OK and DMM can have gain = 1 (no shunts on)Above 40mV turns on the /100 shunt, for 20V range.Above 4V turns on another shunt in parallel, for /1000.If you use 20M, that is 202k and 20.02k (or 22.447k if you switch in parallel)On the high ranges the source see those 20M in parallel for a 10M netIf you are OK with 5Meg impedance, those can all halve, and at 10Meg, better than 1% parts are more available eg 10M, 0.1%, ±25ppm/°C is available.In Logic terms the compare outputs are200mV 0b0020V 0b01200V 0b11You need to drive the decimal points so a decoded analog switch like HC4052 series can do decimal points and range.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 18, 2024, 11:23:41 pm
content: Thanks; you gave me a lot to chew on there.Yes, I'm using a DMM module ("panel meter") that has those 3 ranges (200mV/20V/200V), and a stated input impedance of >100MΩ.Yes, it has automatic polarity indication. I'd be OK with a reduced input impedance if need be, say down to 2MΩ.Still quite puzzled by a couple things here:Wouldn't I want as straight a path to the DMM module as possible? It seems to me that this intervening circuitry will no longer deliver the input voltage to the module, and it won't read accurately. Or by "level sense path" do you mean the path to the DMM? in which case I cannot clamp the input voltage, can I?When you say "ADC path" you're referring to the branch going to the DMM module, correct? Because I have no access to the internals of that module, save the range divider resistors and the decimal point (whch as you point out can be driven by my bilateral analog switches).Are you suggesting that I sense the input voltage by sensingcurrentinstead?I think I'm starting to get some of this. We're talking about something like this, correct?Forget the input protection circuit: this won't ever be exposed to dangerous input (and indeed will probably never even need the 200V range). So now I just need to "tune" the downstream comparators (op amps, actually) to sense the various thresholds with a reference and voltage dividers.Now I just wrote "forget the input protection circuit", but perhaps I was a bit hasty there:So what if this sees a voltage near 200 volts: in order not to fry the DMM module the range-selection circuitry would have to respond faster than the DMM module to substitute the proper divider resistor in the DMM module. So maybe we do need input protection after all.But I still can't wrap my head around that: wouldn't that mean that the DMM module will never see a voltage above V+? How would that work?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 18, 2024, 11:43:11 pm
content: Quote from: Analog Kid on November 18, 2024, 11:23:41 pmYou need to be careful with labels.What you call DMM input, cannot be connected straight to 200V, so this net is called DMM divider.That divider (1:1 or 1/100 or 1/1000) and clamp diode combination (as in #24) then feeds the true DMM input. The DMM always runs on 200mV gain

author: Analog Kid
author_rank: Super Contributor
timestamp: November 19, 2024, 12:05:11 am
content: Quote from: PCB.Wiz on November 18, 2024, 11:43:11 pmIndeed we do.QuoteThe input to my DMMmodulecanbe connected directly to 200V, provided the correct divider resistors are installed ("RA" = 10KΩ, "RB" = 9.99MΩ). So I don't see how I could possibly get the correct reading if the input is clamped to V+; the DMM module "wants" to see the full 200V.The module even has dividers listed for 500V, though I'd never need to use these for my application.QuoteExcept a I noted above it doesn't always get 200mV (max.) on its inputs.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 19, 2024, 12:12:05 am
content: Quote from: Analog Kid on November 19, 2024, 12:05:11 amYou do not seem to be following your own posts.You have already claimed to not want to manually set the range, which means you must have a single fixed hard wired choice, and auto-range outside the box.That range must be 200mV, unless you wish to install a 200V opamp

author: Analog Kid
author_rank: Super Contributor
timestamp: November 19, 2024, 12:24:53 am
content: OK, misunderstanding, probably my bad for not explaining properly.First off, 'twould probably help to have some reference to the DMM module itself:So here's the deal: I want this to operate as an autoranging DMM, meaning it will automagically sense the input voltage and switch ranges accordingly.I'm basing this whole scheme on my belief--which may be wrong--that I can accomplish this by having the autoranging circuitchange the divider resistors used by the DMM module("RA" & "RB") as noted above. The analog switches driven by the comparators would switch these resistors in and out of circuit (as well as switch in the appropriate decimal point, so I'd need a few more analog switches).So backing up: is this possible? I thought I had the stamp of approval on this, but apparently not. So can this scheme work?If there's no possibility of this working I don't want to waste a lot more time--mine and yours--on this.I guess the alternative would be to leave the module set for 200mV, install the input protection diodes and scale the input accordingly? Which I guess is what you've been trying to describe. Would I get a correct display by just changing the decimal point for different ranges?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 19, 2024, 12:39:04 am
content: Quote from: Analog Kid on November 19, 2024, 12:24:53 amNo, you cannot practically, do a word for word clone of the physical swapping of the 6 resistors that the DMM text represents.ie it's easy with a soldering iron, but less practical with electronics.Thus the easy way to to throw away the words and RA RB references and do all of that workexternally(with module set for 200mV) using the circuit already given, of just switching shunts to ground.As a first step, you would remove RA and check what errors you see with a 5M or 10M series external feed resistor. (shunt resistors open)

author: Analog Kid
author_rank: Super Contributor
timestamp: November 19, 2024, 01:28:30 am
content: OK, maybe now we're getting somewhere.So, another iteration of symbols on a 2D representation of a circuit:Is this what you're getting at here?Because if it is, I have a major problem with how recursive this looks. I mean, if the input voltage is X, and the autoranging stuff picks range Y and enables that shunt resistor, then the voltage changes, is no longer X: wouldn't this create some kind of race condition, or at least a totally jumbled result?Obviously I'm still missing something. Hey, I've got the time: I'll keep plugging away until the solution reveals itself. Or not.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 19, 2024, 01:33:49 am
content: If you get too much error with the no shunts series dropper case, (200mV scale) your options are to lower the resistance (which then needs more current at 20V, 200V) or to switch in a lower series R just for 200mV range using an 350-400V OptoMOS relay or latching relay. Most OptoMOS parts spec 3-5mA drive, some spec under 1mA but you pay for that saving.If tests show 1M is ok, that's 200uA at 200V, or 200k is 1mA at 200V etc. ie you want the highest tolerable resistor that works ok, too keep momentary clamping current lowest.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 19, 2024, 01:39:39 am
content: Quote from: Analog Kid on November 19, 2024, 01:28:30 amDid you even look at the circuit in #24That has aseries resistoryou have totally missed and the shunts work to ground, then the DMM is fed from the top of the shunts.The range input comes straight from the DC in, and has its own clamp system. It is not affected by the shunts, (unless you have a high source impedance on your DUT )You only needtwoshunts andtwocomparators to get your 3 ranges - see my binary table above.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 19, 2024, 02:07:00 am
content: Sorry; keep your hat on.So another go-round:I don't understand what you meant by "The range input comes straight from the DC in, and has its own clamp system". Do you mean the diodes go on the output end of the buffer? If so, then what protects the DMM from overvoltage: the shunt resistors?Could I ask you a flavor: could you maybe sketch up your suggestion as a schematic? Hey, I don't care if you write it on a soggy cocktail napkin with a crayon.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 19, 2024, 03:18:17 am
content: Here is something rough.You can see the DMM pin ramp twice, as it range changes 20V to 200V. 200mV is compressed to the origin and is best checked with an OP test.If you do hit finite DMM Zin, the optoMOS was bothering me, but if this is unipolar, you can use a suitable voltage rated low Crs MOSFET, with the right gate voltages.This uses NMOS parts as analog switches.A 4052 part can also decode the decimal points drive and can drive a PNP for GS.Just scope check the DP nodes, to see if they are DC or direct segment connections, which may have strange voltages.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 19, 2024, 03:31:56 am
content: Wow, thanks.It's going to take some time for me to absorb all that, so no more comments for now.I do like how you use MOSFETs instead of analog switches. Much more to my discrete liking.

author: bdunham7
author_rank: Super Contributor
timestamp: November 19, 2024, 03:50:59 am
content: Quote from: Analog Kid on November 18, 2024, 08:33:01 pmNo, like this:

author: Analog Kid
author_rank: Super Contributor
timestamp: November 19, 2024, 10:29:07 pm
content: Quote from: PCB.Wiz on November 19, 2024, 03:18:17 amYikes; I think I actually understand most of what you drew there. Let's see how close I got: you can grade my homework here:D1 and D4 clamp the input voltage to the comparator leg.D2 & D3 clamp the input to the DMM leg.R12 & R13 are a voltage divider for the comparator leg.R1 isolates the comparator leg from the DMM leg.The comparators each drive a MOSFET that switches a shunt to ground into the DMM leg (after R1).Q1 is driven by either comparator going high (meaning a switch to that range), and it drives MOSFET M3 to shunt a lower resistance (200K) across the 20 meg isolation resistor for those ranges.R8 is the transistor's collector resistor.So how'd I do so far?Questions:o Shouldn't the 2 top clamp diodes (D1 & D3) be connected to V+? They're connected together, but not to any voltage source that I can see.o What's to prevent both comparators from going high (say at a higher input voltage) and putting both shunts across the input?o What's the purpose of the zener (D5)?One thing I still don't understand--and this is just my natural-born density at work--is how you can insert a very high resistance (R1, 20 meg) in the path to the DMM and still get accurate measurements from it. I have this thing about series resistors: I always imagine them dropping the voltage drastically. If someone can explain this, I'd be grateful.The (simulated) voltages look great, plenty accurate enough for my needs.Only one other quibble: I think earlier someone said it would be possible to use a single voltage reference instead of two as you show (4V & 40mV); I assume this would be done by use of a voltage divider on one or the other inputs to the comparators (opamps), yes?You can grade me now, Prof.Again, thanks for putting this together.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 19, 2024, 11:03:18 pm
content: Quote from: Analog Kid on November 19, 2024, 10:29:07 pmUsing a Zener means any clamp current does not feed into the supply and potentially increase it.With large resistances and some known residual load of eg 1mA, you could hope it was all ok, but if you pick a low current opamp, and test this with no additional load,you need to be careful with blindly 'connecting to V+' when hoping to clamp 200V.Quote from: Analog Kid on November 19, 2024, 10:29:07 pmNothing at all, in fact, that is exactly how it works. On the top range,both shunts are on, as per the earlier table.Quote from: Analog Kid on November 19, 2024, 10:29:07 pmIt depends on the meter impedance, which is why the suggestion tomeasure itwas made.The added parts M3,R4 etc cut that series resistance, to reduce that effect.The data you linked to says just > 100 MΩ. With no explicit pulldown on the low ranges, these CMOS meters can typically be 1GΩ.Most panel meters would be fine with even 1MΩ, and lower resistances allow things to be simpler. You may even eliminate M3,R4 etcThe DMM has a trim pot, so you can also adjust that to correct for the final few % drop.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 20, 2024, 10:58:56 pm
content: Quote from: PCB.Wiz on November 19, 2024, 12:39:04 amOK, I'm ready to test this thing. So I want to understand what you're suggesting here: you're saying to remove "RA", which is one of the divider resistors in the module. There are 2, "RA" and "RB". The default (for the 200mV range) is RA = 10MΩ and RB = 0Ω.Should I remove both of these to do this test? I have no idea how these are connected internally, or whether it'll work it all with both of them removed.I'm thinking what a guy would want to do is to first test the meter as-is with a known voltage, maybe just set up a battery and voltage divider to get, say, 100mV. Then make the alterations you're suggesting and test again with the same "voltage reference". Yes?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 20, 2024, 11:28:28 pm
content: Quote from: Analog Kid on November 20, 2024, 10:58:56 pmThe wording suggests those are a simple divider, so you keep RB=0 and remove RA = 10MΩ to get the highest impedance ( > 100MOhms )Yes, check it with 100mV and check again with 200k-10MΩ series resistance to see what effect that has.You would pick the highest series resistance that is comfortably still adjustable.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 21, 2024, 01:26:04 am
content: Well, success so far. Removed the 10MΩ resistor in the module, tried both 4.7MΩ and 10MΩ series resistors: got exactly the same reading as before. So that's good.Another question: I still need to set the decimal point for those other 2 ranges (20V & 200V), so I'll need two more MOSFETS to short 2 sets of contacts. Will it affect the existing shunt MOSFETs (M1 & M2 in your schematic) if I attach the 2 new gates to their gates?Ah, but there's a problem here: according to the datasheet, for the 20V range you're supposed to short P2, and P1 for the 200V range. But since the higher range will turn on both MOSFETS it'll shortbothof these, which I'm guessing probably isn't good. So dang, looks like I need some kind of logic there, a NAND gate or some such ... aaaargh.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 21, 2024, 02:04:22 am
content: Quote from: Analog Kid on November 21, 2024, 01:26:04 amYes, confirms maybe 1G Ohm ballpark input, as expected.Quote from: Analog Kid on November 21, 2024, 01:26:04 amI would first scope check those decimal points, to see if they are DC logic signals & what swing.It's just possible they might have LCD backplane - segment connections.A HC4052 or HEF4052 depending on what voltages you find, can decode the 2 binary to one of 4 switches.Addit : someone mentioned maybe a ICL7106 is used here, and that does get a little strange around decimal points.Segment drive is ~ 5.5V derived from an offset negative rail, (zener to V+) so they connect an external CD4030 from V+ to that offset rail and select DP manually.The logic interface for that kludge, is weird, best handled with a higher voltage CMOS HEF4052/CD4052

author: Analog Kid
author_rank: Super Contributor
timestamp: November 21, 2024, 03:26:20 am
content: Welll, I really don't want to mess with the innards of that module, so I think I'll skip 'scoping those connections for now.Here's the truth table I'm dealing with here:Looks like the 4052 could work to decode that; the last 2 rows are the outputs I need (P1 & P2) depending on the range selected (Range1: 20V and Range2: 200V). I'm assuming I'd use Range1 and Range2 as the address to the chip (0/1/3), right? Pretty ugly but that's the way the cookie crumbles, I guess.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 21, 2024, 03:49:50 am
content: Quote from: Analog Kid on November 21, 2024, 03:26:20 amThat means the 200mV and 200V to display identical DP's ? Less ideal on Autoranging part.- maybe you need a HV LED for 200V range as well ?Unless they have .1999 display ability ?

author: Analog Kid
author_rank: Super Contributor
timestamp: November 21, 2024, 04:12:46 am
content: Quote from: PCB.Wiz on November 21, 2024, 03:49:50 amGood idea.