Precision DIY Voltmeter - Page 1
Forum section: electronics / projects

author: Eleclu
author_rank: Contributor
timestamp: October 24, 2023, 01:18:54 pm
content: Hello,I´m working on a digital voltmeter for quite some time now. A project from Scullcom Hobby Electronics enspired me. For example I also use the LTC2400 as an ADC. But my design is different in many ways. The analog part is completly isolated (power and data signals) from the digital part. The whole meter can be powered from an ordinary USB port.The reference voltage is generated by a LM399. The goal was to use mainly resistor networks.The meter has three measurement ranges:1. 0.0V to 1.0V (K1 and K2 are energized)2. 1.0V to 10.0V (no relay ist energized)3. 10.0V to 100.0V (K1, K3 and K4 are energized)The 10V range is working absolutly fine. I get good accuracy in comparison with my Solartron 7065 and Datron 1065. The noise is also very low (10µV digit is stable).The 1V range is also not bad but with more noise. I would like to reduce this some more, but i ran out of ideas.The 100V range is partly unusable. The 10mV digit isn´t stable at all. I think here are major design flaws in the voltage devider and its surrounding electronics.Maybe somebody is kind enough to point them out?Thank you for your help. I really appreciate it.Best wishesEleclu

author: dobsonr741
author_rank: Frequent Contributor
timestamp: October 24, 2023, 02:13:03 pm
content: Eleclu, the schematic is giving me a lot of headache, as it did not follow EE conventions. Many critical analog parts represented by footprint boxes, not the standard symbols. The “flow” is from right to left, the opposite what’s expected. Lines are drawn across the components.I could decipher your schematics, by looking up data sheets and investing some time, but it’s not what I expect when reviewing a design. So I suggest to redraw.

author: coromonadalix
author_rank: Super Contributor
timestamp: October 24, 2023, 03:47:10 pm
content: not being rude, check for the known leaked and officials meters schematics of the 34401a 34410a k2000 keithley 195 196 tektronix dm 5120 .... you'll learn how they made their inputs and voltage dividers thru precision Caddock resistors, and precision references voltages

author: Eleclu
author_rank: Contributor
timestamp: October 24, 2023, 03:57:27 pm
content: I´m sorry for that. The schematic is made in KiCAD and this program hasn´t the proper symbols. I made a simple schematic with your specification and simplified it a lot. So I removed the digital stuff and the reference circuit. I hope this will help.@ coromonadalixThank you for this tip. I already studied some schematics of commercial voltmeters and other diy projects. But I didn´t look for the hp meters.The conclusion of all my studies is, some meters achieve better results with less effort. I would like to understand why

author: coromonadalix
author_rank: Super Contributor
timestamp: October 24, 2023, 04:10:09 pm
content: k4 is not good you need a constant voltage divider impedance, normally any known meter as 10 meg ohms inputyou have divide by 1 10 100 1000 ....thats why i talked about known dmm inputs dividersi understand its not an ultra precise dmm but you need some basics on the inputshttps://www.instructables.com/Digital-multimeter-shield-for-Arduino/https://electronoobs.com/eng_arduino_tut112.phpcheck this projecthttps://digilent.com/reference/add-ons/dmm-shield/start,its a good start, sure it use dedicated dmm chipset but it shows some input ideasif you google arduino dmm projects youll get some good pointers, even ohms diode measurements, you'll learn a lot

author: Eleclu
author_rank: Contributor
timestamp: October 24, 2023, 04:16:22 pm
content: I did test with K4 and without K4 -> no change at all.K4 would only matter in the 1V range. But I see my worst problems in the 100V range.Believe me I tried your approach in the first iterations. Also some multimeters like Datron 1065 and some Flukes have the switch arrangement like my schematic to accomplish an high impedance measurement in low voltages.Thank you I will look it up

author: Kleinstein
author_rank: Super Contributor
timestamp: October 24, 2023, 04:57:39 pm
content: K 4 is not that bad: it allow to choose between 10 M (the divider) and high input impedance. There are other more problematic point:The schottly diodes at the protection have quite some leakage current. The more obvious choice are low leakage diodes like BAV199 and than a 2nd resistor after the first clamp. In addition the OP-amps to drive the voltage clamps is bad, as they would not work as intended when the supply is off. One may want a clamp path to the supply and through zener diodes.The 10 K series resistor is also on the low side. So it is protection to maybe some 100 V, hardly more. R13 also needs to withstand high voltage peaks, e.g. from ESD.The clamping at the ADC input with only the zener is not perfect: it still allows some -700 mV and at the upper end the clamping is soft. There may also be a problem when the 5/6 V supply comes up slow.So there should be at least additional series resistance for the ADC input as a 2nd protection. Alternatively I would power the buffer amplifier from the same supply as the ADC. The buffer may be OK with the divider without extra protection.The LTC2057 input amplifier may not be happy with a high source impedance and may want additional capacitance (or series RC) to ground - this may need some try and error.The LTC2057 may not be the best choice, though still not super bad.The commercial meters (maybe not so much at the 8 digit level) are designed with cost and performance in mind with quite some expertise in the design. E.g. some of the resistors may use simpler resistor pairs (e.g. DIV23 or similar) that don't look that special, but can still be good enough. They still need quite some effort - some may not be visble with custom resistor networks or even custom chips with more functions (e.g. HP34401, HP3457).The shown priciple is not that bad and cheaper parts could be used at some points. E.g. the LTC2400 is no longer such a great choice (though the old Hameg 8012 meter used it). There are more suitable ADCs (e.g. LTC2410, ADS1219, MCP3561 as a cheap one), that allow positive and negative voltages. Much of the switching (e.g. gain and choice of different input paths) can be done with CMOS switches. Still a SD ADC chip is not such a bad choice for the low end 6 digit range. The old meters used quite some effort for a multi slope or similar ADC, that start with a +-10 V range. The SD ADC chips are usually only +-2.5 V or +-5 V at most and thus need a somewhat different design.Much simpler are the DMMs that use a speciallized ADC chip set, but these are usually only lower resolution, like 4 digits and less.

author: Eleclu
author_rank: Contributor
timestamp: October 24, 2023, 05:46:57 pm
content: Thank you Kleinstein for your answerDiodes with less leakage are always great. Could this leakage be the source of some of the noise? The signals in the 100V range after the resistive divider are very low.My selected diode has ca. the same leakage specs as the BAV199. So I think a swap wouldn´t improve anything. Or do I overlook something?I designed the series resistance 10k to have less noise. The noise gets worse, if I increase this resistance. So I reduced it as much as possible. Your are right, the max. voltage this circuit can withstand, are ca. 200V. The reed relays can´t withstand over 200V, too.But your second resistor sounds interesting. I saw this an some voltmeter projects.The protection for the ADC isn´t good at all. I also want to eliminate the series resistance as an error source. If I can solve this stability issue, I will change this in the next revision.The first thing I want to try out is compensation of the op amp. Maybe someone can tell me a good starting point? This would be great.Also which op amp would be better suited for an application in high impedance?

author: dobsonr741
author_rank: Frequent Contributor
timestamp: October 24, 2023, 06:01:13 pm
content: My biggest concern you direct a bipolar +/- input to a single ended ADC. While a Zener intended to protect the DC, you might as well frying the ADC as pointed out, beside it will not work for negative Vin. Stressing the input of the ADC can create leakages beyond the data sheet specs. There are other reasons for the inaccuracy you experience, so I would remove the input protection diodes temporarily to see if they are the source of the leakage.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 24, 2023, 07:01:14 pm
content: Sorry my fault with the BAS416. I somehow got a different diode (some typo with the number ?) that was a schotty type. The low leakage BAS416 is perfectly fine. The higher forward voltage however needs additional resistance (e.g. 1 K) at the OP-amp input to limit the current to the chip internal diodes. The OP-amps are also still not good, though at least the relay turn off when there is no power and thus only a problem during transientsOne may bodge in 2 additional diodes to the supplies.What exactly goes wrong with the 100 V range is hard to tell. It could be a layout thing, some supply ripply, oscillation or hum that causes saturation at times (especially to the negative side).My first try would be some 100 pF + 100 ohm in serie to ground at the amplifier input. So I would not yet give up on the LTC2057. The noise impedance = noise voltage divided by noise current is still in the 70 K range and thus not that far off the 100 K from the divider.AZ OP-amps for a higher impedance source are tricky. The types for a +-15 V supply tend to be rather low voltage noise and high current noise. The LTC2057 is one of the lower current noise ones among the high supply voltage types. The current noise specs are anyway not that reliable. Some are OK, but some have are way to optimistic. E.g. the MCP6V51 would be very good from it specs (5 fA current noise), but the actual current noise I measured was slightly worse than the LTC2057.The specs are obviously false, like calculated min value from the input bias current. Microchip in addition uses a strange definition (average with sign) of the input bias that makes them look too good.Input bias and maybe also the noise can change with the impedance at the inputs.For really high impedance the LTC1250 or ICL7650 and likely AD8638 may work, but are only up to some 16 V supply. I got reasonable good results with an AD8628 and MCP6V76/MCP6V66, but these are only for a low 5.x V supply and would thus need more circuit. For the 2nd buffer a MCP6V76 or similar 5 V supply amplifier should work OK to limit the signal to the ADC. The divider provides enough protection (current limit) for the OP input.

author: Eleclu
author_rank: Contributor
timestamp: October 24, 2023, 08:54:00 pm
content: Thank you dobsonr741 and Kleinstein for your great advices.I will try the suggested compensation and measure the power supply for ripple and instability. I already did this a long time ago, but maybe I overlooked something.To make this a good and reliable meter i will definitly have to do some work on the protection circuits. I didn´t consider the case when there is no voltage. So the second series resistance is important.For now I will leave the LTC2057 because my pcb uses them already and there are not the worst choice.Thank you Kleinstein for your op amp suggestions.I will post the results after I made some tests:)

author: Eleclu
author_rank: Contributor
timestamp: November 06, 2023, 02:31:19 pm
content: This weekend I had time to play around with some compensation methods. I got great stability improvements with 100nF on the non-inverting input of the op-amp and 100nF between the inverting input and output. I will refine this values a bit more and maybe I can increase the stabilityWhen I finalized the compensation, I want to make a proper adc protection. But for that I will need a new pcb.