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David,
Arduino CPU pin impedance when configured as an analogue input is specified as 100MegOhms.? The charging source/sink is capable of 20mA and the measurement is a voltage ratio, essentially independent of the supply.? Quite interesting to do the sums for the sampling.? I agree that the thread is not easy to follow if you are in a hurry. It was not the absolute accuracy but the stability and repeatability that surprised me, having assembled? the basic circuit on a prototyping board out of curiosity.? I realised it would be useful to me as a stand alone device for measuring/sorting SMD capacitors in the 2-1000pf range, and I would commend it on that basis. PeterS???????? G8EZE -----Original Message-----
From: Dr. David Kirkby, Kirkby Microwave Ltd <drkirkby@...> To: [email protected] Sent: Sat, 3 Sep 2022 13:12 Subject: Re: [Test Equipment Design & Construction] A stable and repeatable low value capacitance meter On Sat, 3 Sep 2022 at 09:29, swallowp via <swallowp=[email protected]> wrote:
A while ago I came across a really simple capacitance meter at ?? I might have misunderstood it. The adverts on the page, PDF and circuit in separate files didn¡¯t make for easy reading.?
For small capacitors, unless the DC voltage is very high little energy will be stored in the reference capacitor in a reasonable time. That would suggest to me that a very high impedance voltage measurement would be needed to measure the voltage capacitor due to the short T=Cref Rload time constant.?
I have probably misunderstood the design.??? ?
I recall a thread on the Keysight forum where a guy had bought a Keysight handheld LCR meter. He measured a capacitor, then compared the valve to that measured by a Fluke and other handheld multimeters. All the multimeters gave approximately the same value, but the LCR meter something significantly different. I suggested that this may be due to the different methods of measurement - the LCR meter using AC and (probably) a 4 terminal connection, and the DMMs charging/discharging a capacitor and measuring the time constant. I don¡¯t know if that was the reason or not, but I do see significant differences between my handheld Tektronix DMM which hasn¡¯t been calibrated for decades, and an HP 4284A Precision LCR meter which has been calibrated by Keysight - not some Mickey Mouse lab.?
Peter S ??? ??? G8EZE Dave
Dr. David Kirkby,
Kirkby Microwave Ltd, drkirkby@... Telephone 01621-680100./ +44 1621 680100 Registered in England & Wales, company number 08914892. Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Chelmsford, Essex, CM3 6DT, United Kingdom |
