Product(s) :SourceMeter Instruments: 2400, 2410, 2420, 2425, 2430, 2440, 6430
Question : What is the output resistance for the current source for the SourceMeter Instruments?
The specified output impedance of the SourceMeter series in current mode is determined from the Load Regulation specification, which states "0.01% + 100 pA" which is the regulation from no load to full load, i.e. full volts( 200 V, most ranges) to no volts:
The output resistance is calculated as delta Vo / delta I
For the 100 uA range, for example, 0.01% of range + 100 pA equals 10.1 nA, so the Minimum Output Impedance is 200V / 10.1nA, or about 20 Gigohms.
The table below also lists the the Output Resistance for each range calculated from Engineering analysis. This represents the most likely value , but NOT the guaranteed value. The minimum specified value is also listed.
Engineering analysis for Output Resistance:
Current Source and Voltage Source Output Resistance both depend on the value of the Current Sense Resistora) Current Source Output Resistance = Current Sense Resistance × Amplifier Open Loop Gain
b) Voltage Source Output Resistance = Current Sense Resistance / Amplifier Open Loop Gain
Amplifier Open Loop Gain determined by:
(Op Amp Open Loop Gain = 200,000) × (Output Stage Gain = 5) × (Diff Amp Gain = 5) / Main Loop Closed at Gain = 2)
Approximate Amplifier Open Loop Gain =
CurrentSource / MeasureRange
Current SourceOutput Resistance(calculated)
Voltage SourceOutput Resistance(calculated)
Current SourceOutput Resistance(Specified)
Measured Output impedance
You can also MEASURE the output resistance of a given instrument in a very similar way.
For example we made the following measurement on the 100 microampere range. With the output set to 100 microamps and the compliance set to 105 volts and a load of 1 Megohm, the current read 99.997 uA, and the output Voltage read 100 Volts. When we shorted the 1 Mohm load the current increased to 99.998 uA.
The output resistance is then calculated as delta Vo / delta i, where delta Vo is 100 volts and delta i is 1 nA (i.e. 99.998-99.997 uA) . This gives an output resistance of 1E11 ohms, or 100 Gigohms for this range. But the calculation from the load regulation spec is the more conservative approach.