I'm testing a new DMM I purchased, AM-240 by Amprobe. It claims *over*
100Mohm impedance in 400.0mV mode. So I charged a 4.7uF Mylar capacitor to 36mV DC, and then placed the AM-240 (while in 400mV DC mode) across the Mylar cap. After 25 minutes and 40 seconds it was 35..2mV. That comes to 14Gohms. So I thought it may be due to bias current or offset voltage, and reversed the DMM polarity. Same results. So then I charged the Mylar to 200mV. Same results.Then, I measured the parallel resistance of my 4.7uF Mylar cap by charging it to 184.8mV, disconnected the AM-240, and 1050 seconds later connected the AM-240 and measured 177.1mV. That comes to
5.25Gohms, which is what I would expect from this capacitor. Actually, for months I've been telling people my guesstimate for this cap is 5Gohms.Anyhow, what kind of circuit are they using in this AM-240? It appears as if it *resists* change! When it is disconnected it tends to somewhat maintain the DC voltage, regardless of polarity. IOW, lets say it's measuring the DC voltage on the Mylar cap, and it's 180mV. Then one of the leads is removed. The AM-240 DC voltage decreases a bit, not too much, but it slows down, and tends to hang around, say
160mV. If I reverse the polarity, to -180mV, the same thing happens except it hangs around at -160mV. If the AM-240 was measuring say 35mV, and then disconnected, it tends to hand around at oh 20mV to 30mV.Very interesting DMM. Not sure to like or dislike this.
Thanks for any info. Paul