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Fluke 185 Review

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ErnestB:
I know, long shot, but would somebody maybe know what value and type the 2 capacitors C99 and C100 are?
I suspect these throwing off my ACV reading. Thanx in advance.
Edit: I measure (in circuit) about 45pF each.

shakalnokturn:
I don't know if he values but the values you have measured seem consistent with placement in circuit and ceramic colour.

They're there for frequency compensation on the higher frequencies. Unless they've both gone resistive there's little chance they'll be causing measurement error at 100Hz.

I'd be more suspicious of the opto-isolated analog switches if anything.

ErnestB:
Hi, thanks for the answer and valuable insight. What I have found when doing some tests today is that the measured value strongly decay with frequency. I did the test on about 100, 1kHz, 10kHz en 20kHz. Fron 1kHz to 10kHz bigger jump down. Within this freq. range the decay was about 7.5% on both 0.5V and 5V range (0.4V and 4Vrms sinus used). First I was thinking that the main coupling capacitor ESR was high, but when I shorted that C I have not seen much difference. Than I was thinking about those capacitors parallel to every "range resistor" (1k, 10k, 100k and 1M). But because both 0.5V and 5V range behaving almost the same that was less obvious. So I have de-soldered those 2 AC compensating capacitors and have experimented with some regular through hole ceramic capacitors instead. Some values made it more freq. depended. At the end used 2x 10p and I have to say I do not have a very good capacitor meter for lower values, so the original one's could also be about 10p.
So maybe you know more about the capacitors used in this meter, as you said regarding the color? Are those MLCC? As I see the compensation is very critical. I could imagine that if the capacitor would have some ESR, also freq. dependable then it can throw off the whole AC measurement. Maybe for that 10k could use adjustable resistor to try to adjust the values...

helius:
The ceramic compensating capacitors are in a measurement circuit, so they are going to be C0G (NP0) caps. The color of this material is typically gray. When people talk about "MLCC" they are usually referring to barium titanate based dielectrics which are more beige or brown in color. Those Class II dielectrics are not very stable and not suitable imho. (See the recent thread on "MLCC X5R Capacitor Degradation")

ErnestB:
Thanks. These capacitors are not beige indeed, more gray. Indeed NPO capacitors would be necessary here. But I have to figure out the values, as I think those are responsible for the AC measurement being out of specs. Frequency response of this 4 component network have to be as flat as possible between 40Hz and 20KHz, and they probably calibrate the last part (offset) in the firmware. I can not do that, but I could place a small Pot. instead of that 10k resistor to tweak the last part.. But I suppose it will not be easy, as the input and output impedance plays the role...
Edit: hmm, and what am I compensating for anyway :-) so this is a very specific case high pass filter, that works in conjunction with the rest of the measuring circuit. And you have different ranges too....

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