Question :
Amps: What is the Output Resistance for the Current Source of a SourceMeter?
Answer :
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 Gainb) 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.
FAQ ID 69601
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