True, if the zero error is more than a few microvolts. If it is then there
is likely something broken, but assuming you have a normal working HP5465A,
then
As far as zeroing the meter with its leads shorted, using "Math Null" (aka
Tare) is the only way to zero the 5465A's small offset that is left over
after "auto zero".
When using the Null command with the 5465A's  slow update rate, high
resolution, and filter off, you can measure voltages down to 0.1 uv if
you're real careful.
I find the digital "Null" is also very useful when measuring changes in the
differences between two similar but not exactly the same values or measuring
changes that occurred in a single device.
When desired the digital zero offset being used can be read out at any time
using "Recall", "Z" or any  new value can be stored using "Value_desired",
"Store", "Z".
I find when making meaningful Sub PPM voltage measurements, ANY Zero offsets
in the meter and leads have to be taken into account, and using the digital
null mode helps a lot.

ws

[volt-nuts] HP 3456A Setup
Charles P. Steinmetz charles_steinmetz at lavabit.com

Note that step 9a turns on what other manufacturers call a "relative"
or "tare" function -- it subtracts a reading taken immediately after
the function is activated from all subsequent readings (untill Math
Null is deactivated).  This is handy for cancelling the lead
resistance for resistance measurements, for example.  But it is
probably not what you want for normal DC measurements.  Instead,
leave Null off and just let Autozero zero out the residual for each
measurement.  If the meter has a zero error, you need to correct it
-- not just subtract the zero error you got a while ago from the
current reading.

Regarding the correct connection of the guard terminal, HP
Application Note AN-123 provides a thorough discussion of what to do
and why.  Connecting it to the Low input (as by pushing the Guard
button in) is often suitable, but not always.

Best regards,

Charles

woops, corrected "3456A"

True, if the zero error is more than a few microvolts. If it is, then there
is likely something broken, but assuming you have a normal working HP5456A,
then
As far as zeroing the meter with its leads shorted, using "Math Null" (aka
Tare) is the only way to zero the 5456A's small offset that is left over
after "auto zero".
When using the Null command with the 5456A's  slow update rate, high
resolution, and filter off, you can measure voltages down to 0.1 uv if
you're real careful.
I find the digital "Null" is also very useful when measuring changes in the
differences between two similar but not exactly the same values or measuring
changes that occurred in a single device.
When desired the digital zero offset being used can be read out at any time
using "Recall", "Z" or any  new value can be stored using "Value_desired",
"Store", "Z".
I find when making meaningful Sub PPM voltage measurements, ANY Zero offsets
in the meter and leads have to be taken into account, and using the digital
null mode helps a lot.

ws

My point was that doing this most likely does nothing to reduce
systematic errors -- rather, it just adjusts readings by the
particular random noise sample it takes when you activate the Null function.

Certainly, you can READ a 3456A to 0.1 uV on its lowest range, but so
what?  There is a big difference between resolution and
accuracy.  You have to be very careful that those glowing red digits
don't lull you into thinking you have measured something with an
accuracy of 0.1 uV just because the last digit reads tenths of a
uV.  In fact, the ACCURACY (uncertainty) of the reading is much, much
poorer than 0.1 uV -- in this case, well over +/- 2 uV.

If you look at the 3456A specs, you will see that the best specified
accuracy on the lowest range is +/- 22 ppm +/- 24 counts -- and
that's only for 24 hours.  (At 90 days it is +/- 34 ppm +/- 24
counts.)  On the lowest range, 24 counts is 2.4 uV.  So even without
any scale errors, you can only expect useful accuracy of +/- 2.4
uV.  The scale error can add another 2 or 3 uV to this, depending on
the voltage being measured.  So, you can read a 3456A down to 0.1 uV,
but you can't say that you MEASURED a voltage down to 0.1 uV with it.

We need to be very careful not to get sucked into the resolution
trap.  It is accuracy that volt nuts are interested in.

Best regards,

Charles