Section 7
7-3
7.1.3. Nanovolt Detector Calibration.
7.1.3.1. Introduction.
There is a 5 range, 6 ½ digit, nanovolt detector connected between the Rx - P
2
(Low) and Rs - P
2
(High) terminals, and we allow the user to calibrate the gain and
offset of each range for use as long-scale digital voltmeter, DVM, as an
auxiliary/debug function. The gain and offset coefficients are used only when this
DVM auxiliary function is selected. The 6675A does not require or have
calibration coefficients for resistance ratio measurements. The accuracy of the
resistance ratio measurement reported by the 6675A depends on the turns setting of
the current comparator and on the linearity of the nanovoltmeter (not the absolute
nanovolt detector reading). The gain and offset coefficients for each range will
change slowly with time ( 50 ppm/year) but this will not be noticed in a resistor
ratio measurement.
7.1.3.2. Nanovoltmeter Calibration.
You should always calibrate the voltmeter option with the
<Main_SoftCal_Voltmeter_Volts Set> routines whereas the
<Main_SoftCal_Voltmeter_Coeff Set> routines were for manually adjusting the
coefficients and this is not a recommended practice.
The numbers displayed in the menu
<Main_SoftCal_Voltmeter_Volts Set>
and menu <Main_SoftCal_Voltmeter_Coeff Set>
for zero volts and high volts are raw A/D converter counts with no offset or gain
corrections added in. The span of the counts is approximately ±950,000 and a
reading of ±200 counts for zero volts is sufficient.
The voltage displayed in voltmeter mode is given by the formula:
The suggested Reference voltage is 75% of range for all ranges and you should see
about 500,000 counts for it.
DisplayedVoltage=
(Inputcount - Zerocount)
(Referencecount - Zerocount)
xReferenceVoltage
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