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Keithley 428 Current Amplifier Overload

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slyarbro
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Keithley 428 Current Amplifier Overload

Post by slyarbro » March 12th, 2013, 9:12 am

I see that other people are having this problem as well but I haven't seen a clear response on an answer so I figure I'll try again.

So I'm trying to perform the performance checks according to the user's manual with some equipment substitutions. So I set up a circuit with a 9 V battery and added gigohm resistors in series with it until I got roughly 1.9 nA (which I verified with a HP 3458 multimeter). However, when I attach the setup to the current amplifier, I get an overload and I get a voltage output that is out of the accuracy specification.

So could there be some internal noise that I'm not seeing that is causing this overload? Maybe by using regular BNC cables instead of low noise BNC cables?

Dale C
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Re: Keithley 428 Current Amplifier Overload

Post by Dale C » March 12th, 2013, 10:45 am

Must be the noise.
I connect up a 1.9nA DC signal to the unit with a gain of 1e9 and there is no overload.
At this level I would recommend low noise coax cable, like the Model 4801.
It is important to keep the shield around the sensitive(high impedance) portion of the circuit.

slyarbro
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Joined: February 21st, 2013, 2:53 pm
Country: United States

Re: Keithley 428 Current Amplifier Overload

Post by slyarbro » April 23rd, 2013, 4:56 pm

I recently bought the low noise cable you suggested, but it still overloads despite anything I apply it to. I just connected the 428 output to a banana cable through a BNC to banana cable connector and I still get the overload error. So even a just short (connecting a banana cable to itself) causes an overload error on just 10^8 gain. I connect the low noise BNC cable connected to a banana cable to a 3458A DCI measurement and I read out 23 pA. Is there more information to this overload error?

Dale C
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Re: Keithley 428 Current Amplifier Overload

Post by Dale C » April 25th, 2013, 1:43 pm

Do you have a current source that can be used to test this?
Such as Model 6221? then you could input 1uA at 1e6 gain and see if this works.

KeepItSimpleStupid
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Re: Keithley 428 Current Amplifier Overload

Post by KeepItSimpleStupid » March 30th, 2014, 8:58 pm

If you look at the specs of the HP, it varies, The only real way it's going to be close to valid is if the HP mete is keptt in circuit.

The HO voltmeter has an inout Z of maybe 10 gigohms depending on the range of voltage measured. The ammeter is shunt ammeter and therefore introduces 545K of resistance, so if you measured 1.9 nA you could be measureing your gig ohm resistance + 545K. Naturally, when you take th meter out of the circuit, the current will be higher.

The same problem exists with the voltmeter, If you measure across the 1.9 G-ohms, you have 10 G || 1.9 that your mearuring.

With thise values of resistors, fixturing becomes important.

Even the 428 has 1K of resistance in series with it's input.

Without knowing your actual setup, there can be many problems. But you really have to keep all of the instruments connected and I can't tell if you did that from your post.

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