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485 - under pulsed beam

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VBougeat
Posts: 2
Joined: February 3rd, 2016, 8:16 am
Country: France

485 - under pulsed beam

Post by VBougeat » February 3rd, 2016, 8:58 am

Hi,

I'd like to use an old 485 - to measure current under a pulsed radiation beam. Theoretically, my signal is made of 2 simultaneous components :
- one is square, last 5 us every 5 ms, and instantaneous current is 30 nA.
- the other is triangular, last 100 us every 5 ms, and maximum current is 3 nA.

I know that 485 ammeter is a DC device, but I'm doing preliminary measurements and I don't have other low level instrument. I want to measure the average current of my signal. Can you confirm me this is what I will have with the 485 -?

Thank you
V. Bougeat

Dale C
Keithley Applications
Keithley Applications
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Joined: June 10th, 2010, 6:22 am
Country: United States
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Re: 485 - under pulsed beam

Post by Dale C » February 3rd, 2016, 9:58 am

The analog rise time on the most sensitive three ranges on he 458 is about 60msec 10% to 90 of change.
The A/D converter is three readings per second.
There is no way the A/D could keep up with your signal.
Even the analog output does not have the bandwidth to help with this application.
The signals would not even resister...

VBougeat
Posts: 2
Joined: February 3rd, 2016, 8:16 am
Country: France

Re: 485 - under pulsed beam

Post by VBougeat » February 4th, 2016, 8:05 am

I don't understand what "The signals would not even resister..." means ?

I know that I won't be ablde to see the pattern of my signal. But what I'm interrested in is its mean value. Because of the long rise time, I thought the voltage output of the Current-to-Voltage Converter would be constant and equal to the mean value of the current input. And so the A/D converter frequency doesn't matter. Isn't it the case ?

And if my signal is made only of the second component (triangular, last 100 us every 5 ms, and maximum current is 3 nA), would the 485 behave differently ?

Dale C
Keithley Applications
Keithley Applications
Posts: 2849
Joined: June 10th, 2010, 6:22 am
Country: United States
Contact:

Re: 485 - under pulsed beam

Post by Dale C » February 4th, 2016, 10:21 am

That is the case. The A/D converter does not matter in this case.
Just look at the analog output.
But even the analog output wouldn't show the pulse. It is too fast. The analog circuitry could not keep up with it( current to voltage converter).
You might just try it.
Input a similar pulse and monitor the analog output with a scope. Maybe it will react enough for you to see what you need.
But from the specs I have my doubts it would catch the pulse or any reasonable facsimile of it.

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