AADE L/C Meter drift..

On a sunny day (Mon, 28 Jan 2013 11:05:47 -0800) it happened Robert Baer wrote in :

Pretty good, mine drifts more than 3 pF in an hour. :-) Pressing zero does not thake that many calories.

Mine drifts a bit more then 3 pf in a hour. Still best bang for the buck I have found.. The repeatability is decent, in the short term, and that is what I need.

Steve

If I had sold it to you, I'd apologize profusely, refund your money immediately, and send you a prepaid shipping box with which to return it. Then I'd sign you up for my competition's mailing list. :-|

-- john

Usually it takes 2 zeroing attempts to get zero. The drift, while apparently seen by others, is irksome and begs for explanation as to why it exists. To paraphrase the saying, just because 20 million Frenchmen see this problem, does not make it right. Hell, even the first Fairchild uA709 op amps did not drift like that..

Apples and oranges. Do you realize how tiny 0.06pF is?

It's the capacitance of a fraction of a millimeter of twisted pair wire. It's the capacitance of less than one millimeter of (narrow!) PCB trace over a ground plane. It's the capacitance between the open contacts of a small low-capacitance reed relay. It's less than one tenth of the gate capacitance of the smallest, lowest capacitance discrete switching MOSFET I know of (SD214).

It's... well, I suppose you get the picture by now.

Jeroen Belleman

Yeah. You don't know enough MOSFETs :-)

You can buy a chip or beam lead schottky around 0.06 pF, packaged diodes around

Just moving your hand a little affects the AADE, so zeroing is tricky. It has a plastic case. The drift doesn't surprise me... it's a simple LC oscillator and frequency counter.

There's no range switching in the AADE front-end. Full scale is 1 uF, so 1 pF is

Incidentally, FR4 has a horrible capacitance TC, in the ballpark of 950 PPM/deg C, so that must hurt, too. He has a bunch of parts and traces on the critical nodes.

Jeroen Belleman schrieb:

Hello,

very well said.

there is another prefix for so small values, 0.06 pF is 60 fF, Femtofarad. The next one is aF or Attofarad. Both are so small that only capacitances within an IC require these units.

Bye

On a sunny day (Wed, 30 Jan 2013 13:27:17 +0100) it happened Uwe Hercksen wrote in :

Capacitance between earth and moon is 152 uF You need long test leads.

Please, enlighten me!

Jeroen Belleman

is

one

PPM/deg

Check, spacing of red/input terminal is extremely close to a switch (for starters). Concerning range switching, the MCU does that by changing frequency..which seems to put a hole in that analysis.

is

one

PPM/deg

The schematic is online on the AADE site. The uP measures the LC oscillator frequency but can't change it. There is a CALIB relay that adds some test capacitance, but it's additive, used at powerup. The "range switching" is software, downstream of the frequency measurement.

Sorry, I was just joking...

is

one

PPM/deg

Put a scope and Eput meter on it, found it is just an oscillator at least similar to -a- schematic; range switching is/in software for display.

Q: Obvious why frequency changes WRT capacitor, but why does the amplitude run from around 500mV pk-pk (if i remember right) at lo capacitance to around 8mV pk-pk (if i remember right)? "schematic" indicates i was looking at the top of the parallel LC..

pF is

one

PPM/deg

critical

Check.