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Effective input resistance in current mode at high temp

Posted: January 30th, 2019, 5:43 pm
by Matthew V
Based on this forum and the low level measurement handbook, I read that the input resistance when operating within ±1°C of the calibration temperature is

input resistance < voltage burden/full-scale current

Unfortunately, the lab I am working in routinely becomes much hotter than the calibration temperature. Should I estimate the input resistance using the following equation?

input resistance < ((voltage burden @ Tcal) + (Temperature coefficient of input voltage burden)*((Current Temperature)-Tcal))/(full-scale current)

I am not certain on how to appropriately apply the temperature coefficient of input voltage burden.

I am using Keithley Model 6514 electrometers.

Re: Effective input resistance in current mode at high temp

Posted: February 1st, 2019, 5:46 am
by Dale C
The effect on input resistance is not so much the ambient temperature as the internal temperature on the unit(6514).
As long as the unit(6514) is warmed up, usually about one hour, the input resistance should not be affected much.
Here is the input resistance calculation of the Model 6487 in current mode.
Since both the 6487 and 6514 are feedback ammeters when measuring current the method of calculating the effective input resistance is the same.

What is the effective input impedance on the Model 6487?

This is not a shunt style ammeter. So it is effective input impedance.
Simply take the voltage burden divided by the max reading on the range.
2nA range = 200uV / 2nA = 100k ohms
20nA range = 200uV / 20nA = 10k ohms
200nA range = 200uV / 200nA = 1 kohms
2uA range = 200uV / 2uA = 100 ohms
20uA range = 200uV / 20uA = 10 ohms
200uA range = 200uV / 200uA = 1 ohm
2mA range = 200uV / 2mA = 0.1 ohms
20mA range = 1mV / 20mA = 0.05 ohms

Re: Effective input resistance in current mode at high temp

Posted: February 1st, 2019, 8:34 am
by Matthew V
Thanks for the response, Dale C.

I should have also mentioned earlier that in the lab the temperature does not remain stable. The temperature continues to rise in the lab with all the equipment on even after waiting 7+ hours. The electrometers were recalibrated within the last month in a 72°F environment. However, I don't know the internal temperature of the units during this calibration. I have looked through the manual, and I can't find anything on the unit having the capability to measure and display its internal temperature. Are the units designed to regulate their internal temperature at Tcal despite the environment? I don't see any fans on the unit that could be used to do so.

I guess my real question would be how to use the "temperature coefficient of input voltage burden" which is listed in the specifications for the 6514. I typically see the temperature of the measurement environment exceeding the calibration environment by more than 10°C, so I know that the internal temperature must be higher than the Tcal if the unit is unable to regulate its internal temperature. Is it appropriate to ignore the "temperature coefficient of input voltage burden" completely?

I am using this information to determine the upper and lower bound on my calculations. I would rather overestimate the effective input impedance than underestimate it.