precise +-1500 volt gigaohm input voltmeter - Page 1
Forum section: electronics / general

author: alireza7
author_rank: Regular Contributor
timestamp: October 18, 2017, 11:22:42 am
content: hi i am making on a precise high voltage gigaohm input voltmeter.i found out this topic :https://www.eevblog.com/forum/metrology/how-a-precision-dmm-have-a-high-input-impedance-in-the-gohm-rang/and this schematic:https://www.eevblog.com/forum/metrology/how-a-precision-dmm-have-a-high-input-impedance-in-the-gohm-rang/?action=dlattach;attach=260049;imageit is my schematic but i didn't assemble varistors and gas tube:there is two problem in my circuit:1-i figure out with those series 1k resistor in the input the buffer do not follow the input2- when i bypassed these resistor the output seems to follow the input but it's not precise and it have drift and also it's drift is not linearwhat should i do to get the exact input at the output of buffer?also an other question: if i assemble varistors and gas tube ? do they draw leakage current and influence the input and decrease the input impedance?is there a better way for protection?

author: e61_phil
author_rank: Frequent Contributor
timestamp: October 18, 2017, 01:27:31 pm
content: Do you want to measure 1kV with an input impedance of ~1G?Why do you build such a complicated buffer and not simply a voltage divider?

author: alireza7
author_rank: Regular Contributor
timestamp: October 18, 2017, 03:32:56 pm
content: Quote from: e61_phil on October 18, 2017, 01:27:31 pmi want to measure +-1500v with an input impedance of 1gigaohm or higheryour answer is in this link please read this topic:https://www.eevblog.com/forum/metrology/how-a-precision-dmm-have-a-high-input-impedance-in-the-gohm-rang/

author: Alex Nikitin
author_rank: Super Contributor
timestamp: October 18, 2017, 03:47:20 pm
content: Quote from: alireza7 on October 18, 2017, 03:32:56 pmJust listen toe61_phil- use a 9.99Gohm HV resistor in series with a 10Mohm meter at 2V range. That's it - you have a high voltage 2kV scale meter with 10Gohm input resistance. Simple, safe and much cheaper.CheersAlex

author: HighVoltage
author_rank: Super Contributor
timestamp: October 18, 2017, 04:33:15 pm
content: Quote from: Alex Nikitin on October 18, 2017, 03:47:20 pmThat is really good advise.Most GOhm resistor are only good for a few hundred volts and when they break from over voltage, they can shorten out. This could potentially damage the volt meter.I would use 10x 1GOhm in series and then the 10 MOhm resistance of the meter.And with 10 pieces of GOhm resistors, you can match them until you have 9.99 GOhms

author: alireza7
author_rank: Regular Contributor
timestamp: October 18, 2017, 05:22:45 pm
content: Quote from: HighVoltage on October 18, 2017, 04:33:15 pmwould it be precise?what about low voltages in this configuration does it works good and precisely say for 1V input?can i get 6 digit precision with this configuration?

author: David Hess
author_rank: Super Contributor
timestamp: October 18, 2017, 06:37:05 pm
content: Test the various shunt input protection elements for leakage.Could circuit construction be contributing to leakage?The MOVs on the input do not strike me as something which should be used at a high impedance. Also, some gas tubes have surprisingly high leakage.I might add some capacitance to the non-inverting input to filter out RF. This might be tricky to do while maintaining low leakage and fast settling do to dielectric absorption but two capacitors could be used in series to bootstrap the one which matters.Alternatively, I think the value of the resistors in the 1 kilohm string should be a much larger value; why are they so low? If they were larger, then the input resistance combined with the transistor capacitance would provide some filtering.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: October 18, 2017, 07:32:08 pm
content: Quote from: alireza7 on October 18, 2017, 05:22:45 pmIt is easy to get 6 digits if your meter is capable of it. It is not easy to get 6 meaningful digits, especially at high voltages. What ever you do - with a HV buffer or without - you'll need to divide the voltage to a measurable level. You can just as well do it with a plain HV divider. 5G and 10G HV resistors (5kV to 15kV ) are available from Mouser for not much money (below $10), these would give you 100ppm/C or better temperature stability and 1% tolerance, for exampleOhmite Slim-MOX series. If you look harder and prepared to pay more money, you can get a ready made 1000:1 divider with 20-100G resistance and <10ppm/C ratio tempco.CheersAlex

author: Dave
author_rank: Super Contributor
timestamp: October 18, 2017, 07:41:03 pm
content: Your choice of opamp is terrible for this purpose, because OP07 has a BJT input stage and thus non-negligible input bias current. Replace it with something that has JFETs on the input.Varistors are not suitable for high input impedance, they will ruin everything.

author: e61_phil
author_rank: Frequent Contributor
timestamp: October 18, 2017, 08:13:40 pm
content: I think you should tell us more about the problem you wan't to solve. Measuring 1V on 1500V FS doesn't make sense for me.If you really need a very low input current (makes more sense to me talking about current instead of resistance) you have two options:1. reduce the voltage over your input impedance (will need high voltages in the measurement system)2. use very high input impedanceWhat stability and accuracy do you need? I build a +/-10kV measurement system with sub-ppm stability. But it has 100Meg input impedance due to the availibility of good resistors.

author: David Hess
author_rank: Super Contributor
timestamp: October 18, 2017, 08:40:21 pm
content: Quote from: Alex Nikitin on October 18, 2017, 07:32:08 pmWatch out for the voltage coefficient of resistance in high voltage resistors; it will quickly undo the expense of low tolerance, low temperature coefficient, and low tracking tolerance. When I looked into building my own high voltage probe, the voltage coefficient of resistance contributed the largest part of the error budget.Annoyingly the voltage coefficient of resistance is not always specified and I would assume that if it is not, that the resistor is not suitable for precision high voltage applications unless steps are taken to compensate for it like significant voltage derating or using the same resistor throughout the divider while maybe grading them against each other which does not require an absolute measurement. Dividers built on the same substrate should avoid this problem but I would not take it for granted.Quote from: Dave on October 18, 2017, 07:41:03 pmalireza7 did not specify a source resistance. If it is higher than about 50k then the OP-07 is the wrong choice if OP-07 precision is desired. A bipolar part like the LT1012 would be acceptable up to 200k or so. The old LM11 would be good to 1M. (1) Above that I agree, an FET input part should be used.Note that the error from bias current is separate from the error do to input resistance. The OP-07 and LT1012 have a high enough input resistance and bootstrapping just raises it.(1) These numbers are somewhat misleading in that operational amplifiers with *worse* precision allows for a higher error from input resistance. But the LT1012 really is that much better than the OP-07 because they are designed for different applications and have similar precision. In the old days, the 308 and then later the LM11 would have been used.