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I would like to buy a small digital volt-panel-meter. My eye fell on this series from Murata:

DMS-20LCD (https://www.murata-ps.com/data/meters/20lcd.pdf)

This meter can be powerd by either a 5V supply or 9V battery.

The datasheet limits the Common Mode Voltage Range to +/-2V on the second page, but no further explanation is given. Page 4 in the datasheet mentions the Common Mode Voltage again in the first application example:

enter image description here

When I see the picture, I would say:

  • Vcm = 2.5V

  • Vin- = 2.5V - 0.833V

  • Vin+ = 2.5V + 0.833V

Since Vcm = 2.5V, it is outside the +/-2V range. Why would Murata give an example which doesn't meet its own specs? I'm clearly missing something here.

K.Mulier
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1 Answers1

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I see your dilemma. Firstly, yes, you are quite right that strictly-speaking, the common mode voltage on the diagram is 2.5 volts but you could also make an argument that it is 1.667 volts with a signal of +1.667 volts on pin 11.

I think this is what they mean although by no means is this clear in their document. In other words, make sure pin 12 remains within +/-2 volts if you want accuracy.

Andy aka
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  • Thank you sir. Their document is so ... unclear :-(.
    So pin 12 should remain within +/- 2 volts. What are the limits on pin 11?     – K.Mulier Jan 19 '18 at 14:05
  • That is how I read the situation. If your meter has a +/- 200 mV range then pin 11 has to be within +/-200 mV of pin 12 or you will reach full-scale limitations. It won't damage the device but you run out of measurement capability. If your input can measure +/- 200 volts then pin 11 can be as high as 202 volts or as low as -202 volts at extremes of the limitation imposed on pin 12. – Andy aka Jan 19 '18 at 14:07
  • Oh, okay. Thank you sir. You seem to have lots of experience (almost 200k)! Which panel meter would you suggest? I need 6 small panel meters, preferably ones that I can solder on a board :-). They need to measure the voltages of Li-ion cells (6 serial connected cells). – K.Mulier Jan 19 '18 at 14:09
  • I'm building a test-setup, and need to keep an eye on all the individual cell voltages :-) – K.Mulier Jan 19 '18 at 14:10
  • I have NO experience of panel meters! I've never used one or chosen one. – Andy aka Jan 19 '18 at 14:10
  • Oh, okay. Sorry sir :-) – K.Mulier Jan 19 '18 at 14:11