Product specifications

125
Power Products 10 Most frequently asked questions about using
Applications Information DC power products
(Continued)
More detailed specifications at www.agilent.com/find/power
9
Why are Agilent’s electronic loads
constant resistance resolution speced in
ohms on the low resistance range, but in
mSiemens on the two higher ranges?
In general, Agilent’s 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 speci-
fied (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 regulat-
ing 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 mea-
sured in ohms. For the N3302A, 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, chang-
ing 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 resis-
tors 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.