3457A Switching Noise on Measurement bus.

[TaylorD93]

Hi folks,

Just wondering if im being an idiot with my DMM. I was gifted a HP 3457A by my mentor when he retired. Its not been calibrated since about 2006.

I have been repairing a WEIR 4000T Linear PSU, and i had the DMM connected to its output terminals, while also having my Scope connected measuring output Ripple.

While the DMM was connected, i was detecting some odd switching noise on the PSU output on the scope, I thought it may have been noise coupled on the DMM leads as its a HiZ input, however disconnecting the DMM leads at the DMM (still connected at the PSU end) the switching noise disappeared. This leads me to believe its coupled in by the DMM.

Surely a DMM of this quality shouldnt be passing switching noise onto its measurement terminals? Am i missing something?

[Kleinstein]

In the AZ mode the DMM will cause some switching spike at the input. This can be quite noticable to a sensitive source - though a linear power supply of the usual power level should be OK with this.

The injected current can come from the switching of the input in the auto zero cycle. In the 3457 there are quite some series resistors for protection, so the spikes should not be that large. The 3457 DMM also has an extra precharge phase to reduce the switching spikes, but this may still be not perfect and there are some reports on defects in the precharge circuit. Such a defect would cause higher than normal switching spikes and likely also more input bias current. In more normal use such a defect may be essentially unnoticed.

[KT88]

There is a chance that the noise is coupled magnetically into the test leads. Have you tried to to move them around or make a poor mans twisted pair out of it?

[TaylorD93]

No not yet, I will try that.

I will also adjust the NPLC and see if that changes the frequency of the noise, if it does then its highly likely it is what Kleinstein suggested

thanks