Amps: What is the Output Resistance for the Current Source of a SourceMeter?

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?

Answer:

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 Resistor

a) 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 =	2.50E+06
CurrentSource / MeasureRange	CurrentSense Resistor(Ohms)	Current SourceOutput Resistance(calculated)	Voltage SourceOutput Resistance(calculated)	Current SourceOutput Resistance(Specified)
1.00000 µA	2,000,000	5.00E+12	8.00E-01	1.00E+12
10.0000 µA	200,000	5.00E+11	8.00E-02	2.00E+11
100.000 µA	20,000	5.00E+10	8.00E-03	2.00E+10
1.00000 mA	2,000	5.00E+09	8.00E-04	2.00E+09
10.0000 mA	200	5.00E+08	8.00E-05	2.00E+08
100.000 mA	20	5.00E+07	8.00E-06	2.00E+07
1.00000 A	0.2	5.00E+05	8.00E-08	2.00E+05

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.