[volt-nuts] HP 3456A AC cal
Richard Moore
richiem5683 at gmail.com
Sat Sep 13 23:04:00 EDT 2014
Thanks guys for the comments -- I dug into the manual, and all of the AC
gains are set by laser-trimmed resistors in ?think-film? packs which HP
called "fineline" resistors -- the AC tweaks appear to adjust the
low-end of range linearity by adjusting amp offsets. The RMS converter
is a log amp, an averager, and an anti-log amp. This circuit actually
works very well. I'm going to open the 3456 up and check test points 4
and 8 and see where the gain differences actually are.
I may have to actually measure the resistances of the packaged resistors
and see how they compare to one another -- I may be able to get better
full-scale accuracy by adjusting just one resistor... Those fineline
resistors are supposed to be 0.01% or better -- which means that HP
never expected the 3456 to be really good at AC. Yet it is a very
excellent 6-1/2 digit meter for DC and ohms, holding very close readings
when compared to my 3458A, so the AC performance is a little disappointing.
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