EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: d-smes on November 19, 2015, 03:18:25 am
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I'm confused by the description of how the DCV Ratio works in the Keysight Truevolt Series Digital Multimeters, Operating and Service Guide. Quoting the manual: "The ratio is the voltage on the Input terminals divided by the reference voltage. The reference voltage is the difference of two separate measurements." The figure in the manual illustrating the connections shows DCV1 attached to the Input terminals and DCV2 attached to the Sense terminals. My interpretation, then, is DCV Ratio = DCV1 (Input) divided by reference V where Vref = DCV1 - DCV2 or DCV2 - DCV1 ("difference of two separate measurements" but undefined which way).
What doesn't make sense is that when the two quantities are equal, one would assume the DCV Ratio = 1. But from the above equations, the ratio goes to infinity (Vref = 0). Something's not right here...
The manual goes on to say: "These measurements are the DC voltages from the HI Sense terminal to the LO Input terminal and from the LO Sense terminal to the LO Input terminal." There appears to be a misprint; I suspect the red "LO" should be "HI". I don't quite know how to interpret this (doesn't appear to match the illustration). Regardless, it doesn't resolve my confusion with the first paragraph.
Can someone please explain how this measurement works? I don't have the meter yet, so I can't experiment... TIA.
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Well, everything is correct, no typo in the description, but you just have to read more carefully this following sentence, about which difference is calculated!
The problem about ratio , and also 4W Ohm measurement is, that Sense Low and Input Low jacks may be on different potentials.
A maximum difference of 2V is allowed.
The voltage across the Sense Terminals, High versus Low are NOT fed into a separate differential amplifier.
Instead, the potentials on High and Low Sense are measured separately, each relative to the Input Low jack, which also is the common analogue ground of the instrument.
The boundary conditions are also explained on page 257 of the user manual.
So in summary, for these modes, i.e. DCV ratio and 4W Ohm, the instrument therefore makes 3 or 2 consecutive voltage measurements on Sense High = U(SH), Sense Low = U(SL), Input High = U(IH), all related to Input Low.
For ratio, it then calculates U(IH) / (U(SH) - U(SL))
For 4W Ohm, it calculates (U(SH)-U(SL))/I(ref.)
All HP/agilent/Keysight bench DMMs work identical in this aspect, so for better understanding, you can grab the schematics of the 34401A, or better of the 34410/411A (which are more similar to the new TrueVolt instruments), and track these different signals into the input multiplexer.
Frank
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Hi!
Just in case someone does not know this already...
The schematic for the 34410/34411A has not been "officially" released, but a very kind helpful person at Keysight put it on their forum :)
Thanks!
I myself have a Agilent 34410A, and the schematics will come in handy if it fails some day!
BTW I did a small fan mod on it, since this small fan had the most irritating sound of all my test equipment :-[
Looking inside, there seems not to be items creating much heat, so I put a "noise killer" in series with the fan, and also put a switch on the backside so I can bypass the circuit and run on full speed.
I have talked to an accredited lab, and they were interested in doing an experiment, doing two calibrations, with fan on low and original speed. I suspect there will be a tiny difference, but will be interesting to see how much!
Full schematic:
http://www.keysight.com/owc_discussions/thread.jspa?threadID=36971&tstart=-1 (http://www.keysight.com/owc_discussions/thread.jspa?threadID=36971&tstart=-1)
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Thank you for the detailed explanation and schematic! Makes sense now.
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Unfortunately the forum link with the schematic is not working anymore...
Someone can kindly share a copy? :)
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Luckily I had a local copy! :)
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:-+ :-+ :-+