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| Why is the Hi-Z (>10GΩ) impedance mode on DC only available for low voltages? |
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| 6SN7WGTB:
Did search and although plenty of threads explain meter impedance effects, which I understand, none appear to address: Taking an Agilent bench DMM as an example, why is the Hi-Z (>10GΩ) impedance mode on DC only available for low voltages? I suppose what puzzles me is that the current drawn from the CUT goes up as the voltage goes up, so why can't the Hi-Z function work at higher voltages than the often-cited 10V? |
| BillyO:
Check this thread: https://www.eevblog.com/forum/metrology/how-a-precision-dmm-have-a-high-input-impedance-in-the-gohm-rang/ |
| TimFox:
The higher voltage ranges on DVMs use a resistive voltage divider to reduce the high input voltage down to a voltage that is within the capability of the Hi-Z mode, which is usually lower voltage than the battery or power supply in the voltmeter ("naked" op amp input). 10 Megohm is a typical value for the input resistance of such networks. Fancier equipment, such as the Keithley electrometers, use more complicated electronics to "float" the amplifier guts up to a common-mode voltage of a few hundred volts. |
| David Hess:
TimFox said everything I was going to say. On a multimeter, high input impedance is only available as long as the input voltage is within the input range of the high impedance input buffer, which is typically +/-10 volts maximum, but some extend to +/-15 or +/-20 volts. Above this, a resistive input divider is needed which limits the input resistance. Electrometers which support a high input resistance at high voltages bootstrap their high impedance input buffer so that the supply voltage of the high impedance input buffer follows the input voltage, and then the high impedance input buffer drives the high voltage resistive divider. |
| Someone:
Further to the posts above on floating/bootstrapping the measurement, SMUs can do this too without needing a dedicated for purpose electrometer instrument. Guaranteed input impedances achievable in the gigaohm range and up (limited by leakage current). Actually making use of such high input impedances is challenging! small currents and capacitances really get in the way. |
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