Author Topic: HP 3478A behavior on low ranges  (Read 65 times)

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Online radar_macgyverTopic starter

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HP 3478A behavior on low ranges
« on: Today at 04:57:07 am »
I have an HP 3478A that shows a slowly increasing count on lower DC volts ranges (anything less than 30V). Eventually, it goes overrange, and starts faintly beeping and zeroes itself, then repeats the cycle. Any resistance applied between the probes (I tried several 1M resistors in series) will bring the voltage back to zero, as will putting it into the 30 or 300V range. I suspect the latter is due to a voltage divider. Is this normal behavior?

The instrument has not been used in many years (possibly decades), I cleaned it out with compressed air but am afraid to use any solvents after seeing the weird hybrid inside. I also need to replace its cal battery... it measures about 1.2V, and it somehow is still calibrated!

Fun fact: it was actually a prototype unit (the son of a former colleague used to work at HP and was apparently on the design team). Some folks from Keysight visited our lab and I showed it to them, they remarked that the missing front badge and no serial number was a good sign it was indeed a prototype.
 

Online bdunham7

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Re: HP 3478A behavior on low ranges
« Reply #1 on: Today at 05:03:34 am »
Completely normal due to the small pias current (tens of picoamperes) charging up the input capacitance.  The resistance across the input is effectively nearly infinite.  Once you add 10M in parallel, the tens of picoamperes of bias current only result in hundreds of microvolts.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Online alm

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Re: HP 3478A behavior on low ranges
« Reply #2 on: Today at 05:04:10 am »
Yes, this is normal. Happens to pretty much every meter that is high impedance (GOhms) on the lower DCV ranges. This won't happen if it's actually measuring a signal with a finite impedance. Fix is to set it to a higher range where the input impedance is only 10 MOhm.


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