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428-PROG, "OVERLOAD" only on Gain 10E9 and 10E10

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0lolol0
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Country: Indonesia

428-PROG, "OVERLOAD" only on Gain 10E9 and 10E10

Post by 0lolol0 » November 29th, 2012, 7:38 am

Dear Keithley Engineers

Hello

In order to make sure our 428, that has not been used for a long while, can still work well, I carried out "Performance Verification" process according to Instruction Manual (Document No.: 428-901-01 Rev. A).

I used potentiostat as a current source in place of calibrator, such as Model 263, and Keithley 2010 as a DMM. Basically, our potentiostat can provide constant current from 0 to +/- 10 mA, the minimum resolution is 3 pA.

Actually, our 428 passed the verification procedures of X10 Gain Accuracy, Voltage Bias Accuracy Check, and Current Suppression Accuracy Check.

But, in the Gain Accuracy Verification Procedure, I always encountered "overload" troubles in the of case of 10E9 and 10E10 Gain(V/A), no matter how small current to apply. Interestingly, the corresponding DMM readings of Gain 10E3 to 10E8 are all within the permissible ranges.

What's the problem do you think? is it because of aging Gain resistor of 10E9 and 10E10? or some other reasons?

If it's due to gain resistor, is it possible to replace them by myself or Keithley Korea Service Center?

Thanks in advance.

By the way, I was told by Keithley Korea Service Center that the calibration service of 428-PROG Current Amplifier is not available in Korea. It sounds a little bit surprising and disappointing.

0lolol0
Posts: 6
Joined: November 29th, 2012, 5:22 am
Country: Indonesia

Re: 428-PROG, "OVERLOAD" only on Gain 10E9 and 10E10

Post by 0lolol0 » November 29th, 2012, 8:48 am

I'd like to add some new info on "overload" issue

Even 428 displayed as "Overload" when Gain 10E9 or 10E10 was on, but DMM seemed to be able to get valid readings in the permissible ranges.

Isn't it strange?

Could I just ignore the "overload" message? Is it possible to damage 428?

RaymondR
Keithley Applications
Keithley Applications
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Re: 428-PROG, "OVERLOAD" only on Gain 10E9 and 10E10

Post by RaymondR » November 29th, 2012, 10:43 am

Hi 0lolol0,

What potentiostat are you using? It could be possible that there's a substantial offset current that is overloading the 428.

How are you generating your current? There may be loading effects if you're just using a resistor in series with your voltage source. The input resistance on those gains can be as high as 100 kOhm.

0lolol0
Posts: 6
Joined: November 29th, 2012, 5:22 am
Country: Indonesia

Re: 428-PROG, "OVERLOAD" only on Gain 10E9 and 10E10

Post by 0lolol0 » November 30th, 2012, 8:58 am

RaymondR wrote:Hi 0lolol0,

What potentiostat are you using? It could be possible that there's a substantial offset current that is overloading the 428.

How are you generating your current? There may be loading effects if you're just using a resistor in series with your voltage source. The input resistance on those gains can be as high as 100 kOhm.
Thanks for reply.

First, I used Bio-Logic VMP Multipotentiostat as a current source, I found "overload" message when 10E9 or 10E10 Gain was tested.

Then, I changed to use a more advanced potentiostat---Gamry Femtostat, good news is that only 10E10 brings "overload" message.

Here, I choose chronopotentiometry(CP) mode of potentiostat to generate stable and constant current between working electrodes and counter electrode/reference electrode (In two-electrode system, working electrode is the first electrode, reference connected with counter electrode as the second one), and they are directly connected with HI and LO of 428 input, respectively, without any resistor in series in between. Is it the right connection?

From current-time profile, the current value is almost the same as setting, it seems no offset current to be worried about. I even tried to apply 19pA instead of 190pA upon 10E10 Gain, but still "overload"

RaymondR
Keithley Applications
Keithley Applications
Posts: 204
Joined: May 18th, 2012, 12:12 pm
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Re: 428-PROG, "OVERLOAD" only on Gain 10E9 and 10E10

Post by RaymondR » November 30th, 2012, 9:29 am

Would you be able to connect a 1 GOhm resistor in series with a 1 V source and the 428? This will feed a 1 nA current into the 428. Basically, you want to see if a well controlled current will cause an overload or not. If it does then the instrument needs servicing, if not then we'll take a look at how you have your system wired up.

The 1 nA should read 1 V on 10^9 gain and 10 V on 10^10 gain. I'm not sure if you'll be able to find a 1 GOhm resistor, but if you can be sure to handle the resistor very carefully. The oils on your hand can reduce the resistance dramatically.

You could also find a 100 MOhm resistor and source 0.1 V and so on. The voltage will be harder to control at less than 1 or 10 mV for most power supplies, however, unless you have a specialized low voltage source.

Let me know what you think.

0lolol0
Posts: 6
Joined: November 29th, 2012, 5:22 am
Country: Indonesia

Re: 428-PROG, "OVERLOAD" only on Gain 10E9 and 10E10

Post by 0lolol0 » December 3rd, 2012, 8:45 am

Thanks for your reply.

I've carried out the test following your advice. (Please see attached photos)

I found a 10 Mega-ohm resistance, and connected it in series with Aligent Waveform generator (as a voltage source) and 428-PROG. When 5 mV is applied, about -4.7 V is successfully obtained by Keithley 2010 under a Gain of 1E10 V/A.

This time, I better understand and realize the importance of Shielding on the small signal measurement, as "overload" always comes out without Faraday cage in 1E10 V/A Gain no matter how low the voltage is applied.

But in the case of 1E11 (1E10 + x10 ) Gain, "overload" still always came out with 0.5 mV voltage and 10 Mega-ohm resistance, even aided by Faraday cage. I think it might be due to the following reasons, the relative unstable voltage output(0.5 mV), failed to use low noise coax cable (like 4801), etc. How do you think this issue?

So far, shall we come to the conclusion that 428-PROG is operating properly?

Actually, after 428-PROG performance test, I'd like to set up an electric detection system to measure ion channel currents through bilayer lipid membranes. As there's not much related documents(428-PROG for ion channel measurement) available -, could you please give me any comment or suggestion on this issue. Many thanks in advance. < I already PM my email address to you for your convenience>
Attachments
Inside Faraday Cage.jpg
Inside Faraday Cage
Inside Faraday Cage.jpg (177.35 KiB) Viewed 21728 times
Connections.jpg
Connections
Connections.jpg (329.75 KiB) Viewed 21728 times

RaymondR
Keithley Applications
Keithley Applications
Posts: 204
Joined: May 18th, 2012, 12:12 pm
Country: United States
Location: Cleveland, Ohio
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Re: 428-PROG, "OVERLOAD" only on Gain 10E9 and 10E10

Post by RaymondR » December 4th, 2012, 9:07 am

Hi 0lolol0,

If you have a scope available then you can check the quality of the 0.5 mV signal. If there is significant spiking and/or oscillation in your source then that might be what is overloading the 428. Try connecting a 100 - 1000 nF capacitor in parallel with the voltage source (across your generator red and generator black in your photos). Make sure that the input to the 428 is only from the branch of the resistor, though.

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