34410A voltage noise spikes depending on front or rear input terminals

[jfedison741]

I have been measuring some MAX6126 voltage references using an Agilent 34410A in DC voltage mode and the following two plots show the measured output voltage read over several hours. The first plot shows results measured through the front terminals where intermittent spikes (about 0.5 to 1 ppm from baseline) can be seen. In the second plot, measuing the same DUT now through the rear terminals, the spikes don't appear. My guess is that it could have to do with the vacuum fluorescent display coupling EMI to the DUT when the front terminals are used.

Would be glad to know if anyone else has observed something similar.

Best regards,
Jeff

[garrettm]

Send "disp off" to the unit via serial/ethernet or GPIB, it will turn the display off. This way you can know for certain that it is the display adding the observed noise.

I do this when logging with my Tektronix DMM4050 and it does reduce the measurement noise seen in my test setup, so it might help you too.

[HighVoltage]

It seems you are in Hi-Z mode, which can pick up anything out of the air on the input cables.

Just simply placing the cables from front to rear can have the a different picture because the wires are in a different location.

Make sure to use twisted cables and put them through some closed cores for noise reduction.

[MegaVolt]

Try turning off auto zero.

[Andreas]

Would be glad to know if anyone else has observed something similar.

Hello,

Looks like a EMI source which is closer to the front terminal than to the back terminal.
Usually this has nothing to do with the VFD more with a WIFI, LED Lamp or mobile phone.
Did you place the 3 100nF capacitors according to the data sheet directly at the voltage reference pins?

with best regards

Andreas

[Kleinstein]

Just in case it is a contact problem with the front to rear switch one could check the 2 Wire ohms zero point. Just operating switch a few times can improve things.

My main suspect would still be EMI of some kind.

When measuring a low impedance source it does not matter whether the DMM input is in high Z mode. The source would make the input low impedance. Magnetic interference is usually not stopped to easy from a little lower impedance input.

[tggzzz]

The first plot shows results measured through the front terminals where intermittent spikes (about 0.5 to 1 ppm from baseline) can be seen.

It looks like there is a "long" and regular interval between the spikes, which doesn't feel like it is cused by a display.

What's the interval between the spikes? That might give a clue as to the source.

[TimFox]

Do you have an LED bench light near the voltmeter?

[jfedison741]

Thanks everyone for the helpful comments. I have also been measuring the AD587 on a Geller SVR (without any of the trim circuit). Attached shows two plots, the first without twisting the leads and with Cout=0 (like a stock SVR), the second with twisted lead connections, shorter leads to the two 9V batteries and also added Cout=0.1uF (X7R) to the SVR. Room lights were off during these tests and no cellphones were nearby. In the second plot I am not getting the periodic voltage spikes so this appears to indicate either (or both) of the mods (added Cout and twisted+shortened leads) fixed the problem. Measuring at the ppm level can have it's challenges!

Best regards,
Jeff