6517B difficulty up-ranging from 200pA to 2nA on auto-range

Hello,

I am using a Keithley 6517B electrometer to provide a bias voltage and to read current in a circuit. I am using SCPI commands to operate the device remotely. I would like to use auto-range, but have found that the instrument does not up-range from the 200 pA range to the 2 nA range when I use "READ?" to trigger a measurement. However, the instrument completes the correct transitions for other ranges when using "READ?".

I can get the 6517B to up-range from 200 pA range to the 2 nA range by re-issuing "CONF:CURR" (or "MEAS?", but "meas" works because it includes "conf"). This is after the instrument has been configured for a current measurement (i.e., querying "conf?" would return "curr:dc"), and it needs to be done every time I want to get out of the 200 pA range. I know that using "conf" performs a number of actions, and ideally I do not want to re-do all of these every single time I take a reading - there must be a more efficient way.

How can I configure my instrument to properly use auto-ranging for the current measurement? I've checked that auto-range is enabled, and that the upper range limit for auto-range is 20 mA.

Steps to repeat issue
---
I am using the GPIB interface with a GPIB-Ethernet adapter, and the readings I obtain are the same as those shown on the device's front panel.

1. Setup

netcat -v 192.168.0.98 1234
Connection to 192.168.0.98 1234 port [tcp/*] succeeded!

*IDN?
KEITHLEY INSTRUMENTS INC.,MODEL 6517B,4584330,A14/700x

*rst
conf:curr
form:elem read,unit,vso,stat
syst:zch off
sour:volt 25
outp on

conf?
"CURR:DC"

sens:curr:rang:auto?
1

sens:curr:rang:auto:llim?
2.0000000E-11

sens:curr:rang:auto:ulim?
2.0000000E-02

sens:curr:rang:upp?
2.0000000E-02

3. Measuring using "read?"  in the 200 pA range works as expected:

read?
+143.0885E-12NADC,+0025.000Vsrc
read?
+141.1126E-12NADC,+0025.000Vsrc

sens:curr:rang:upp?
2.0000000E-10

sour:volt 35

read?
+208.8394E-12NADC,+0035.000Vsrc
read?
+208.4471E-12NADC,+0035.000Vsrc

sens:curr:rang:upp?
2.0000000E-10
sens:curr:rang:auto?
1
sens:curr:rang:auto:ulim?
2.0000000E-02

meas?
+0.241637E-09NADC,+0035.000Vsrc

read?
+0.249885E-09NADC,+0035.000Vsrc

sens:curr:rang:upp?
2.0000000E-09

5. When increasing the current further, up-ranges occur as expected.

sour:volt 52
read?
+021.0036E-09NADC,+0052.000Vsrc

sens:curr:rang:upp?
2.0000000E-07

sour:volt 25
read?
+143.4189E-12NADC,+0025.000Vsrc

sens:curr:rang:upp?
2.0000000E-10

sour:volt 35
read?
+208.3761E-12NADC,+0035.000Vsrc

sour:volt 40
read?
+208.2913E-12NADC,+0040.000Vsrc
read?
+208.3752E-12NADC,+0040.000Vsrc

sens:curr:rang:upp?
2.0000000E-10

meas?
+0.333612E-09NADC,+0040.000Vsrc

sens:curr:rang:upp?
2.0000000E-09

sour:volt 35
read?
+209.2242E-12NADC,+0035.000Vsrc
read?
+209.3727E-12NADC,+0035.000Vsrc

ABOR
read?
+209.8183E-12NADC,+0035.000Vsrc
read?
+209.7916E-12NADC,+0035.000Vsrc

CONF:CURR
read?
+0.255592E-09NADC,+0035.000Vsrc

Hello Maia,

That is very odd behavior.

I see you have the latest firmware, A14.

What if you manually operate the instrument from a reboot/factory defaults?  Does the current range adjust appropriately then?

As you increase the source voltage a little bit, does the 200pA range ever report overflow?
If you have a volt meter or scope channel, what does the 2V output do as you approach full scale 200pA and push past?

Seems like the :MEAS:CURR? rather than a :READ? will be needed.

How is your remote connection to the instrument?  Looks like you have a LAN to something converter?  Using GPIB or RS-232 on the 6517B?

Hello Andrea,

I see the same thing when manually operating the instrument from the front panel, but only under certain conditions: it happens if I change the trigger mode from continuous to a manually triggered one-shot, or if I first issue an *RST command remotely and then return to local operation.

If use the instrument immediately after power-up, with no change to the trigger settings, my range changes work as expected - i.e., if I skip step 5 below or don't issue *RST, everything seems ok. But, I would really like something that works after an *RST, so that I can keep my remote settings consistent in case someone else uses the picometer and changes my settings.

I now have set up a test circuit with a 100 MOhm resistor. I can re-create the issue using exclusively local operation by:

1. Power-on the Keithely6517 by pressing the power button
2. Press "I" to select the current measurement function
3. Press "AUTO" to turn autorange on. The screen displays "Autorange: A"
4. Press "ZCHK" to turn zero-check off
5. Switch to one-shot mode with a manual trigger (this is to copy the behaviour on *RST, which I want for remote operation). Press "CONFIG", "TRIG", select BASIC, then select MODE and ONE-SHOT. Then, press "CONFIG", "TRIG", SOURCE and select MANUAL. This provides the same functionality I get after an *RST, where I only trigger a reading by pressing the TRIG key
6. Set the voltage to 0.01 V using "OPER" and the arrow keys to set a voltage output
7. Press "TRIG" to trigger a reading. The display shows 75 pA, roughly as expected (expected 100 pA, but given my low-quality/unshielded test circuit, 75 pA is reasonable). The 2V output at the back gives me -0.77 V on my multimeter.
8. Set the voltage to 0.02 V 
9. Trigger a reading. The display shows 176 pA, roughly as expected. The 2V output at the back gives me -1.8 V
10. Set the voltage to 0.03 V
11. Trigger a reading. The display shows 208 pA, but I would expect around 300 pA. The 2V output at the back gives me -2.1 V.
12. Press the up arrow key once, to uprange. The screen displays "Range: 2nA"
13. Trigger a reading. The display shows 0.280 nA, roughly as expected. The 2V output at the back gives me -0.27 V
14. Return to autorange
15. Set the voltage to 0.3 V
16. Trigger a reading. The display shows 3nA, as expected. This means the uprange from 2nA to 20nA happened automatically, which is good. The 2V output at the back gives me -0.29 V

Try again with more current. This immediately follows step 14 above (i.e., autorange is still on, same setup)

1. Set the voltage to 0.01 V
2. Trigger a reading. The display shows 75 pA, as expected. The 2V output at the back gives me -0.77 V
3. Set the voltage to 0.03 V
4. Trigger a reading. The display shows 209 pA, but I would expect around 300 pA. The 2V output at the back gives me -2.18 V
5. Set the voltage to 0.3 V
6. Trigger a reading. The display shows 208 pA, but I would expect around 3 nA. The 2V output at the back gives me -2.18 V
7. Press the up arrow key once, to uprange. The screen displays "Range: 2nA"
8. Trigger a reading. The screen displays "OutOfLimit" - at least in the 2nA range I have an indication that something is wrong! The 2V output at the back gives me -2.94 V
9. Uprange to the 20 nA range
10. Trigger a reading. It is 3 nA, as expected. The 2V output at the back gives me -0.293 V

My remote connection is with a GPIB-Ethernet adapter from Prologix. In the end, I have a C++ program that will send commands to the 6517, but used a terminal to send commands for my first post so that it is easier to see exactly what I am sending. Technically, I am sending these commands to the adapter, which then forwards them to the 6517's GPIB interface.

Thank you
 

Hi Maia,
I'm going to ask the factory to take a look at this.
Excellent detail here and steps to duplicate.  Thank you for taking the time to put it so clearly.

Andrea

Hello Maia,
Factory took a look using a 6517B at A14 firmware, same as you.
Not able to duplicate any issue related to auto range as the applied V to resistor changes.
Factor suggested that your specific unit may have a service issue (RMA).
Andrea