Hello,
for 10:1 resistance ratios, for example to check my Hammon precision divider, (250kOhm:25kOhm), I also use the 3458A, similar to acbern.
I use a 5442A as an ultra stable 10V reference, and measure input and output voltages (10V and 1V) simply on the fixed 10V range, using the 3458As 0.02ppm linearity.
This 10:1 transfer should be better (about 0.2.. 0.3ppm) than the specified 0.05 ppm of value +0.05 ppm of range (= 0.55ppm total), as latter includes linearity AND other instabilities of the LTZ and reference divider circuitry, and is barn door wide specified.
Last weekend, I also experimented on the Ratio mode, in comparison.
It can also deliver superb results for DCV : DCV, but you have to be careful, as its behaviour is not fully explained in the manual.
First, the Sense (-) and Input (-) must not differ greater than 0.25 V.
Second, The Sense reference input accepts +/-12V DC only, and always autoranges!
That means, it will change range from 12V to 1.2V , if the 1V output of my divider would be applied to the Sense jacks, and the 10V to the Input jacks.
That's no big deal, if the instrument has been properly autocaled just before, otherwise you will have inconsistent results due to range change errors.
But you have no control over offsets on the Sense input!
It's better for the ratio mode, if Sense and Input use the same, fixed 10V DCV range.
Therefore, for Ratio, always apply the higher voltage, i.e. 10V to the Sense input, and the lower voltage, i.e. 1V, to the Input jacks, but chose manual 10V DC for the Input first!
In advance, you may measure the offset of the 10:1 divider output first , and let the instrument calculate the Null, and then enter ratio mode.
The manual does not cover this special setting, but I strongly assume, that the instrument will really behave like this:
Sense = 10V DC range, Input = 10V DC range (manual, fixed)
Ratio Result = (Input - Offset) / Sense
As Sense is 10V, its offset plays no role here.
It is also recommended, to use statistics on the ratio function, to improve result and to control the stability.
In comparison, I did the following measurements.
The Hammon divider was not calibrated before the measurements:
1. Consecutive measurement on fixed 10V DC range, with averaging and statistics.:
Offset = 0.000 000 0V
U1 = 1.000 000 68V, sigma = 133nV, NSamp= 10 (one additional digit by statistics function!)
U2 = 10.000 011 2V, sigma = 155nV, NSamp = 10
Ratio = U1 / U2 = 0.099999956
2. Ratio function, Offset used on the divider output, ranges and voltages as above
Ratio mode reading of 3458A = 99.9999573E-03, sigma= 5E-09, N=16
So I conclude, that both methods may deliver identical results up to 8 digits (1e-8!!), but I feel much more comfortable, NOT using the Ratio function, as I can better overview, what I'm doing.
I would also avoid usage of alternative front / rear jacks for ultra precise ratio measurements, as this does introduce hard to control thermal voltages... The Jack-to-cable voltages are not included in the offset compensation/calibration !
And also, the front/rear switch is not very reliable any more, after these years..
Frank