[volt-nuts] 731A output impedance

WB6BNQ wb6bnq at cox.net
Tue Nov 27 01:10:21 UTC 2012

Hi Bob,

If all you intend to use the 10 volt output, perhaps it would be better to move the
sense string positive terminal to the output banana jack leaving the 1K resistor in the
circuit.  That is, take the top connection R8A and move it to the other side of R18,
preferably right at the banana jack.

You would still have to be mindful of the load current (i.e., keep it under 5 ma), but
that would correct for the load error at the terminals of the instrument.  The only
other thing to consider would be to possibly move the other end of the sense string
(bottom connection of R11) directly to the minus banana jack to make sure the sense
string is not part of the main current path.


Bob Smither wrote:

> The 731A is specified to have an output impedance of less than 1.1K in the 10V
> setting.  A meter with 10 M input Z therefore causes a .01% (100 ppm) reading error.
> I understand that when operated using a null meter the output impedance is not a
> problem.  I would like to use my 731A as a bench standard for my DMMs.
> The 10V output is taken through a 1K resistor (R18) from the output pin of the
> op-amp.  Clearly the op-amp output pin is much lower impedance.
> The op-amp is an LM301A and appears to be conservatively compensated.
> I am thinking of shorting R18.  I understand it provides some protection to the
> op-amp and may be a factor in the op-amp stability.
> Other than the above, can anyone see why this would be a bad idea?
> Thanks,
> --
> Bob Smither                                             smither at c-c-i.com
> Circuit Concepts, Inc.                                       281-331-2744
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