Specifications

605xA and 606xB: Electronic Load Constant Resistance
Resolution
Question: Why are Agilent Electronic Loads constant resistance resolution speced in ohms on the low
resistance range, but in mSiemens on the two higher ranges?
Answer: In general, Agilent Electronic Loads are not a conventional "resistor". The loads consist of IC's, capacitors,
resistors, FETs, etc. They were designed with two major circuits, a cv and cc circuit. These circuits are used to
simulate resistance on the two upper ranges.
First, it is necessary to understand why there is a difference in the way in which the ranges are specified (mohms or
mS). The constant resistance (CR) mode in the load actually operates using either the constant current (CC) or
constant voltage (CV) circuits inside the load. The lowest CR range uses the CV regulating circuits, while the two
higher ranges use the CC regulating circuits. It is because of these differences in the circuits used to regulate the load
input that the specifications need to be different.
When the CV circuits are used, the load can be viewed as many resistors, all the same value (the resolution), in series
to produce the desired resistance. Then, changing the resistance is like changing the number of discrete resistors in
series. Therefore, the resolution is the value of one of these series resistors, and putting resistors in series changes the
resistance measured in ohms. For the 60501B, the "discrete resistor" or resolution that can be programmed is 0.54
mohms in the 2 ohm range.
When the CC circuits are used, the load can be viewed as many resistors, all the same value (the resolution), in
parallel to produce the desired resistance. Then, changing the resistance is like changing the number of discrete
resistors in parallel. Therefore, the resolution is the value of one of these parallel resistors, and putting resistors in
parallel changes the conductance measured in siemens. For the 60501B, the "discrete resistor" or resolution that can
be programmed is 0.14 mS (=7.14 kohms).
For example, in the 2 kohm range, you can program the load input from 2 ohms to 2 kohms (0.5 S to 0.5 mS) with a
resolution of 0.14 mS. This would be the equivalent of starting with about 3568 7.143 kohm resistors in parallel with
each other, and in parallel with a 2 kohm resistor, and removing one at a time until you had only the 2 kohm resistor
left.
Note that the resolution of the conductance is constant at 0.14 mS, however, the resolution of the total parallel
resistance is not constant. It depends on how many resistors you have in parallel.
If you have two 7.143 kohm resistors in parallel and remove one, the resolution looks like 3571.5 ohms. If you have
3568 7.143 kohm resistors in parallel and remove one, the resolution looks like (7143/3567) - (7143/3568) = 0.561
mohms. But the conductance resolution is constant at 0.14 mS.