Extra 10V DC range on HP3457A

[maxwell3e10]

The HP 3457A meter has many nice features but also a flaw in that its 10V DC range is very noisy because it's attenuated by a factor of 100 and then is measured on 0.3 V range.

So I thought I would setup my own 10 V range (actually 12 V full scale) using the LT5400-5 precision resistor chip. The chip has four 1 MOhm resistors, so the input impedance of a 1:4 divider is 4 MOhm, a little too low, but not too bad.

I have the 44492 scanner card, so I setup the chip inside the terminal block so that one scanner channel measures the raw voltage and another channel measures the divided voltage. I also added a 0.033uF capacitor across the 1 MOhm resistor.

Measuring the resistor ratio of the chip with the meter I got 3.9999 +/- 0.00001. Measuring the voltage ratio for a 3 V input voltage I got  a ratio of 3.99998 +/- 0.00002. So it roughly works at 20 ppm level, but should be characterized better. Also my meter may have been calibrated only once in its lifetime (cal. number =34), so that could be another limitation.

Measuring the input noise with shorted inputs I get  2 uV rms on 10 NPLC setting, compared with 12 uV rms when using the native 30V range. The noise is probably even lower, I need to setup the 7th digit readout.

Using the scale math of the meter with scale=0.25, one gets the 10 V display readout. One can also setup an offset number, so this new range can be calibrated just like a native range.

[e61_phil]

nice idea 

I sold my 3457A due to the high noise.

[maxwell3e10]

I played a bit more with characterizing the voltage divider based on LT5400-5 resistor network to get an extra 12V range on HP3457.

The chip that I got gives exactly 4:1 ratio as well as I can measure, within about +/-2 ppm.

The noise of the meter  on the 3V range with the divider connected is increased just by the Johnson noise of the divider.
10 PLC: short circuit: 280nV rms, divider shorted: 352 nV rms, difference: 210 nV rms
100 PLC: short circuit: 89 nV rms, divider shorted:109 nV rms, difference: 63 nV rms

The divider, when shorted, presents a 750 kOhm resistor, so the Johnson noise for 3 Hz BW (10 PLC) is 190 nV rms, for 0.3 Hz BW (100 PLC) is 60 nV rms. This matches very closely with the measured noise increase. So the current input noise of HP3457A is negligible.

Measuring the calibration of the 30V range on my meter relative to the 3 V range with a 2.5 V reference I found the 30 V range is higher by 2 ppm. After I correct for this, measuring a 10 V reference on the 30 V range and on the new 12 V range with the 4:1 divider connected to the 3 V range agrees within about 1 ppm, as shown in the plot.

The noise on the attached plot is not that good because I am using a cheap AD584 reference, which dominates the noise.

Next project in this metrology on smallest possible budget exercise: setup a battery voltage source.

[Kleinstein]

With no DC voltage it is normal to see only Johnson noise from a set of resistors. Excess noise only comes in when there is a bias (usually DC). However there could be current noise from the meters input amplifier (or AZ switches). So the result of no extra extra noise shows really low current noise for his 3457.

The is one downside of the LT5400 divider: it does not like higher voltage, e.g. from ESD.  AT least from what I read, the LT5400 might be ESD sensitive to a certain degree.