10GOhm input voltmeter gain problem - Page 1
Forum section: electronics / general

author: PartialDischarge
author_rank: Super Contributor
timestamp: February 18, 2023, 08:40:25 am
content: Built this circuit to measure 1..1000V for sources that have very high impedances, attached circuit.The gain is a bit less than it should be, any ideas why? I measured with inputs from 10V to 700V . The LMP7721 is running at +-2.5V so it's always seeing a common mode at its input but that should not degrade the performance for this.All 1G resistors, 10M and the input pin are soldered on the air1G resistors are 1% CRHV1206AF1G00FKE1, 10M is 0.1% MCA1206MD1005BP500. The tolerance of the resistors should not explain this according to my calculations, the 10G string should be more than 4% off for this to happen.

author: PartialDischarge
author_rank: Super Contributor
timestamp: February 18, 2023, 09:08:35 am
content: I’m thinking that maybe the soldering has changed the value of the 1G resistors by this 4%, is this possible?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 18, 2023, 09:22:46 am
content: Unless one was excessibe and maybe getting flux on the resistor body itself the 1G resistors should not change that much. Especially the resistance going up is not that likely.The part I would suspect is leakage current, maybe from half way down the resistor string to ground.

author: xavier60
author_rank: Super Contributor
timestamp: February 18, 2023, 09:53:21 am
content: I know little about high impedance voltage measurement, I do suspect that there should be guard tracks. Can we see the board?If it's even constructed on a PCB.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: February 18, 2023, 10:47:50 am
content: 1. show some photos.2. + input of the opamp is not blocked to GND by the NP0 capacitor so the noise from DCDC converter can cause unwanted effects3. the resistors you are using have specified 25ppm/V voltage coefficient (the resistor tends to decrease its resistance with increasing voltage across)4. if there are no better ideas change the circuit configuration to be inverting amplifier topology. This will remove the common mode problems.

author: iMo
author_rank: Super Contributor
timestamp: February 18, 2023, 11:13:17 am
content: The gain of your amplifier is set to ~9.7 (when dividing the Vout/Vout_ideal numbers in your table), linear over the entire range..

author: PartialDischarge
author_rank: Super Contributor
timestamp: February 18, 2023, 11:14:31 am
content: This is meant to be an add-on for any multimeter with a 9V battery attached.Noise from the DC/DC should not be a problem, I'm dropping from 5V to 2.5V with an RC which is the best way to reduce noise.Didn't want to add capacitance to the + node (yet) just to make sure leakage from that cap is a potential problem.The voltage coeff of the resistors is not it, that would show up as non-linearity but output is very linear, just out of gain.Photos attached, not clearly seen but all 1G resistors and 10M and input pin are "on the air"For now I added 400k to the 10MOhm and problem is solved, +-1% accuracy or better.

author: PartialDischarge
author_rank: Super Contributor
timestamp: February 18, 2023, 11:17:23 am
content: Quote from: imo on February 18, 2023, 11:13:17 amExactly, it is very linear, so adding 400k to the 10Meg (for now) solves the problem.Also correcting the ouput reading by 1.1% for each 100MOhm of added source impedance gives a very accurate measurement overall.

author: iMo
author_rank: Super Contributor
timestamp: February 18, 2023, 11:21:47 am
content: .. it should be "~0.97", but it does not matter, the key message is it is linear..

author: PartialDischarge
author_rank: Super Contributor
timestamp: February 19, 2023, 08:19:39 am
content: As a side note, I think this is an interesting project, for many applications measuring voltages up to 1000V with only the 10MOhms of DMMs is too low and not much is available commercially.

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: February 19, 2023, 10:27:43 am
content: Quote from: PartialDischarge on February 19, 2023, 08:19:39 amabsolutely a valuable project. more so, as 1 KV (or even 200 V) ranges on DMMs might not have their 10 Mohms input resistances as tightly specified as one may need. especially, when using a high voltage probe (say, FLUKE 80k-40 or similar), a not so well defined input resistance of the DMM spoils the accuracy of the whole measurement. the reason for the induced inaccuracy is not so much the DMM's input divider itself, it is the leakyness of the DMM's internal high voltage switches. even a"simple" HP3468A can run circles around a K2002 in such special use cases.

author: PartialDischarge
author_rank: Super Contributor
timestamp: February 19, 2023, 10:41:57 am
content: Quote from: Le_Bassiste on February 19, 2023, 10:27:43 amYes. Before building this I measured the 10Mohm resistor across some multimeters I have, and determined it cannot be relied upon for this, that's why it has to be its own divider+opamp. It is relatively simple and works great.

author: jonpaul
author_rank: Super Contributor
timestamp: February 19, 2023, 11:52:33 am
content: In decades of experience with HV measurement, we found the most accurate resistance hi-Meg R were from Victoreen in Ohio, or Craddock. These include precision hand trimmed film, in sealed glass ampoules, marked with the values.m100M,... 1000M..to 20T Ohm.We use the Genrad Megohmmeeters to check up to 1000V, Genrad 1863, etc.Such resistance Must be mounted on glass or Téflon insulators,Consider the voltage coefficient and aging of any hi Meg RWe wash resistance and insulation with 99% isopropyl alcohol before measurement, wait hours and or air dry prior to the measurements with gentle heat.Seems like 1kV 10kV 1..3% accuracy of resistance or voltage measurement is doing well.Bon SoiréeJon