[time-nuts] OT: AC voltage standard

Bill Hawkins bill at iaxs.net
Tue Nov 6 19:28:18 EST 2007

Nice catch, John.

A solution provider would do well to understand the problem before
presenting the solution. The first paragraph of this morning's 5:52
AM (UTC-6) posting outlined the problem. A 1% solution will satisfy
a 6 bit calibration.

The reading is likely to be Average, not RMS, so the standard can be
a 50 +/- 0.5% chopped standard DC voltage. The RMS value of a square
wave is the same as the Average value, but for a sine wave the
Average is 0.9 times RMS. For an RMS display of an Average-reading
device, multiply the answer by the inverse of 0.9. Do the same with
evenly chopped DC to get the predicted reading of the AC digital
display. This is why true RMS meters are required if the shape is
not a sine wave. True RMS meters either heat something or do the

There are ways to use the standard cell, but they are overkill for
1% accuracy. The old thermocouple millivoltmeters used a standard
cell and galvanometer to set the current in a set of precision
resistors. The galvanometer was then used to balance the resistor
set to the unknown millivoltage.

If the scope has chopped sweep and you have suitable 0.2% resistors
to divide the source of chopped DC down to the standard cell volts,
then you don't need a galvanometer. Stabilize the cell, put both
scope probes on the resistors and crank up the vertical gain. Remove
most of the DC offset with vertical position, and trim one of the
channel gains to get a single trace line. Now briefly touch the
standard cell with one of the probes and note the difference.

Most standard cells outside of a controlled standards lab are not
accurate to 6 figures, and will not be permanently affected by a
scope probe for the accuracy desired here. 100 megohm impedance
is required for 6 figures, so a megohm probably drops that to 4
figures. Standard cells are calibrated by comparing them, not by
reading across each of them.

You know how you short the terminals of sensitive analog meters
for shipment, so the needle won't bang around? I got a standard
cell from the bay with a short between the terminals. It was not

Bill Hawkins

-----Original Message-----
From: Neon John
Subject: Re: [time-nuts] OT: AC voltage standard

None of that is particularly relevant here because he needs a simple
circuit to check the accuracy of a 6 bit ADC in a scope.  The RMS value
doesn't matter, as the output is a simple square wave that swings
between 0 volts and precisely the value of the DC source.  Neither does
the frequency.

>>> Not quite. The Average/RMS AC coupled value of a square wave
    varies with the duty cycle. That's why PWM works.

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