Author Topic: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617 (Read 99764 times)

MiDi · « **Reply #150 on:** February 08, 2020, 02:16:23 pm »

Looks good, was confused short moment

, because graph did not correspond to last post - but now recognized your edit.
You could measure effective Frontend offset voltage for V&A from Preamp output when in Zero Check.

_Wim_ · « **Reply #151 on:** February 08, 2020, 08:31:53 pm »

Yes, when making the plot I realized I made a stupid mistake before. Just finished exchanging Q308 by an LMC662AIN. Was quite a fiddly job, should have ordered a DIP part instead of SMD. Running with the case open at the moment to let traces of remaining IPA vaporize. Noise spikes at a first glance seem lower, but unit is still drifting due to the cleaning (I think).

Will test offset later on, still need to adjust the unit also. Looked into calibration, but those 100Gohms resistors cost quite a lot, even for "only" 1% accuracy and not the stated 0.08%

from the manual.

_Wim_ · « **Reply #152 on:** February 08, 2020, 09:05:55 pm »

BTW, my unit is boardrev K, latest datecode I can find is 86 week 29.

_Wim_ · « **Reply #153 on:** February 09, 2020, 08:02:15 am »

These are the logs after changing to a LM662AIN (with case still open). Results look very similar to the Q308, peaks a little lower. I measured the offset voltage when in zero check, and was able to adjust it varying between +16µV and -18µV on the 2V output.

Kleinstein · « **Reply #154 on:** February 09, 2020, 09:56:12 am »

Some of the spikes could be from radioactive decay (radon gas) or cosmic rays. These can produce ions in the air and that cause such spikes. Air is a good isolator, but the air gap should not be to large and DC electric fields can matter.

HighVoltage · « **Reply #155 on:** February 27, 2020, 07:38:54 pm »

Just bought one more Keithley 617, this was the cheapest one so far.
This time I watched out for the "screw in the back" and got a revison L in like new condition and never opened.
Revision number on input board: 617-162-02L

Date code are around 9323

Oddly, one electrolytic cap looks bulged, will check that one first.

_Wim_ · « **Reply #156 on:** February 28, 2020, 07:24:57 pm »

I wonder if others see similar behaviour:

when switching from resistance mode to current mode (after being in resistance mode for a longer period), it takes almost 1 hour for the current to settle to around 0fA (see attached plot, 1mV=1fA). When switching ranges in current mode (or switching to coulomb mode and back) I do not see this behaviour, it only happens after measuring resistances. Also briefly switching to resistance mode and back to current mode cause some offset, but this settles to zero within a couple of minutes. Seems like some sort of dielectric absorption happening, as it always nicely comes back to zero. No cable connected in current mode, for resistance a cable was connected.

Kleinstein · « **Reply #157 on:** February 28, 2020, 08:16:38 pm »

The description for the settling after coming from resistance measurements sound like dielectric absorption. This can also be a problem for a cable if there is no driven shield (triax) used.

_Wim_ · « **Reply #158 on:** February 28, 2020, 08:35:29 pm »

Quote from: Kleinstein on February 28, 2020, 08:16:38 pm

The description for the settling after coming from resistance measurements sound like dielectric absorption.

That is what I thought also, but I wonder if it is normal behaviour for this unit, or I still need to fix something. To repeat what I did:
1) measure some high value resistors using a triax cable with crocodile clips (or any other cable-test fixture)
2) disconnect the cable and put a shield cap on the input of the 617
3) switch to the lowest range of current mode and wait until it settles around zero

MiDi · « **Reply #159 on:** February 29, 2020, 09:36:10 am »

This may indicate bad reed relays.
I did some measurents for settling of old and new ones and the new ones settle a lot faster towards >100T $\$\Omega\$$ .

_Wim_ · « **Reply #160 on:** February 29, 2020, 10:02:42 am »

Quote from: MiDi on February 29, 2020, 09:36:10 am

This may indicate bad reed relays.
I did some measurents for settling of old and new ones and the new ones settle a lot faster towards >100T $\$\Omega\$$ .

That is also what I am afraid of. Did you replace also the smaller COTO/Wabash-relays (like K303)?

These would be my initial suspect, as I do not see the behaviour when switching between ranges within current and coulomb, but do see it when switching to resistance/voltage.

MiDi · « **Reply #161 on:** February 29, 2020, 06:51:02 pm »

Does this also apply when there is short circuit in resistance/voltage mode?
Open circuit in these modes lets the input settle to one of the HV-rails (~+-250V) and if this is applied longer, the relays could suffer from DA.

Just to be sure :

Quote

but do see it when switching to resistance/voltage

means when switching from?

Currently my repair is on hold, I want to check several things before putting the expensive and fragile reed relays at last in.
Have thought of replacing the small ones for resistance range switching, but their aging should not be that relevant.

PS: Would appreciate high res pictures of bottom EM-Board Rev K/L.

_Wim_ · « **Reply #162 on:** February 29, 2020, 06:57:26 pm »

Quote from: MiDi on February 29, 2020, 06:51:02 pm

Does this also apply when there is short circuit in resistance/voltage mode?
Open circuit in these modes lets the input settle to one of the HV-rails (~+-250V) and if this is applied longer, the relays could suffer from DA.

Good point, did not test that yet. Will try tomorrow

Quote from: MiDi on February 29, 2020, 06:51:02 pm

Just to be sure :
Quote
but do see it when switching to resistance/voltage
means when switching from?

Yes, when returning from resistance/voltage to current mode.

Quote from: MiDi on February 29, 2020, 06:51:02 pm

PS: Would appreciate high res pictures of bottom EM-Board Rev K/L.

I will try to take some tomorrow (rev K board in mine).

MiDi · « **Reply #163 on:** February 29, 2020, 09:01:04 pm »

I think the only relay that could be affected by DA this way is K307 for zero check.
It seems to be the most sensitive relay as it is always connected in parallel to measurement path.

_Wim_ · « **Reply #164 on:** February 29, 2020, 09:15:10 pm »

When I enable zero check in current mode the display immediately goes to zero+-1, no slow effect is seen.
The slow settling continuous in the background while the zero function is on, and it makes no difference switching to zero and back or not, the slow settling takes about an hour in the lowest current range.

From this I would think K307 cannot be the culprit, but I could be wrong. As is it not easy to find suitable replacements for the COTO relays, I have no easy way to test this.

Kleinstein · « **Reply #165 on:** February 29, 2020, 09:57:35 pm »

The slow response / settling looks like some DA. A tell tail is that a short time in resistance mode also results in faster recovery. The DA can be in a capacitor, but also one of the relays or possibly the board itself. The parasitic capacitance is relatively small, but the material in the capacitor is usually much better.

The effect of the resistance measurement mode with a high resistance is that the input voltage goes up and thus DA is activated.
Switching to a voltage reading may be another case that applies a voltage to some of the caps / relays.

While testing the relays is tricky, as a replacement is hard to find, one could check the capacitors. PS caps are still available, though not a regular type anymore.

MiDi · « **Reply #166 on:** February 29, 2020, 09:58:55 pm »

DA cannot be reset by anything (except by time), it is modeled by parallel path with resistor and capacitor in series (or multiples).
So I do not think that this excludes K307 as source, but it may not be the only possible source.
Only simple way to check that comes to my mind is to desolder/cut lead of K307 from input side.

_Wim_ · « **Reply #167 on:** March 01, 2020, 07:42:15 am »

Quote from: Kleinstein on February 29, 2020, 09:57:35 pm

While testing the relays is tricky, as a replacement is hard to find, one could check the capacitors. PS caps are still available, though not a regular type anymore.

PS caps (electrolytic) have already been replaced during the repair (as some low cost caps were installed by the previous owner). I did not touch any of the ceramic and foil caps so far as this seems risky to make things much worse.

_Wim_ · « **Reply #168 on:** March 01, 2020, 07:48:04 am »

Quote from: MiDi on February 29, 2020, 09:58:55 pm

DA cannot be reset by anything (except by time), it is modeled by parallel path with resistor and capacitor in series (or multiples).
So I do not think that this excludes K307 as source, but it may not be the only possible source.
Only simple way to check that comes to my mind is to desolder/cut lead of K307 from input side.

My thinking was that because the voltage over K307 changes between output voltage and ground when enabling/disabling the zero check, I should see a difference in slope.

But if we are indeed talking about DA charged to one of the high voltage rails, this little difference in voltage might not produce a noticeable effect in the slope.

I will do some further testing this afternoon hopefully.

_Wim_ · « **Reply #169 on:** March 01, 2020, 07:06:17 pm »

Quote from: MiDi on February 29, 2020, 06:51:02 pm

PS: Would appreciate high res pictures of bottom EM-Board Rev K/L.

Here you go

_Wim_ · « **Reply #170 on:** March 01, 2020, 07:18:03 pm »

Quote from: MiDi on February 29, 2020, 06:51:02 pm

Does this also apply when there is short circuit in resistance/voltage mode?
Open circuit in these modes lets the input settle to one of the HV-rails (~+-250V) and if this is applied longer, the relays could suffer from DA.

When measuring a short, the effect is similar but smaller. Measured current is also positive instead of negative. The settling time is directly related to the time the meter was is resistance mode (so it is for sure a DA effect):

20 sec in resistance mode with a short: 1min40 sec to settle to <+100fA (waiting to zero takes way too long...)
20 sec in resistance mode open circuit: 4min10sec to settle to >-100fA
60 sec in resistance mode open circuit: 9min15sec to settle to >-100fA

Would be great to see results from another unit to see what is "normal".

Noopy · « **Reply #171 on:** March 02, 2020, 01:19:22 pm »

Hi all!

MiDi sent me a leaky Q308 and I took some pictures of the die.
A very small thing...

There is some dirt caused by the opening of the can but on the right gate there is a spot that´s not "normal dirt":

With different illumination that thing at the side of the metal structure looks always different than "normal dirt".
Perhaps that´s a production problem leading to a leaky JFET?

Some more pictures here: https://richis-lab.de/K617.htm

HighVoltage · « **Reply #172 on:** March 02, 2020, 01:30:52 pm »

@Noopy,
Thanks for these nice insight pictures.
Do we actually know, who the manufacturer of Q308 is and is there a datasheet around ?

Noopy · « **Reply #173 on:** March 02, 2020, 01:35:45 pm »

Quote from: HighVoltage on March 02, 2020, 01:30:52 pm

@Noopy,
Thanks for these nice insight pictures.
Do we actually know, who the manufacturer of Q308 is and is there a datasheet around ?

I was happy to get such an special IC!

Unfortunately I also wasn´t able to identify the manufacturer. Couldn´t assign the "M" logo...

Gyro · « **Reply #174 on:** March 02, 2020, 01:53:56 pm »

Quote from: Noopy on March 02, 2020, 01:19:22 pm

With different illumination that thing at the side of the metal structure looks always different than "normal dirt".
Perhaps that´s a production problem leading to a leaky JFET?

Degraded performance caused by an ESD strike? $:-\$ It presumably tested fine at manufacture.

It would be interesting to compile a 'rogue's gallery' of such damage.

P.S.

Quote

Couldn´t assign the "M" logo...

Motorola maybe?


EEVblog Main Site	EEVblog on Youtube	EEVblog on Twitter	EEVblog on Facebook	EEVblog on Odysee

Author Topic: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617 (Read 99764 times)

Share me