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Model 2790 SourceMeter
mX+b rel
The mX+b function can be used to manually establish a rel value. To do this, set the scale
factor (m) to 1 and set the offset (b) to the rel value. Each subsequent reading will be the
difference between the actual input and the rel value (offset).
m/X + b (reciprocal)
This math operation lets you manipulate normal display readings (X) mathematically
according to the following calculation.
R = m/X + b
where:
Typically, this math operation is used with m = 1 and b = 0 (which are the default con-
stants). With these values, R is simply the reciprocal of the normal display reading:
R = 1/X
Example — Assume the normal displayed reading is 2.5Ω. The reciprocal of resistance is
conductance. When m/X + b is enabled (assuming m = 1 and b = 0), the following conduc-
tance reading will be displayed:
0.4 R
klqbp The m and b constants for m/X + b are coupled to the m and b constants for
The Model 2790 goes into an over-range condition whenever the input signal exceeds 120%
of the selected measurement range. Typically, the Model 2790 displays the "OVRFLW"
message when this over-range condition occurs. An exception is the m/X + b calculation. If
the input signal for m/X + b exceeds 120% of the selected range, the "UDRFLW" message
will instead be displayed.
As with the "OVRFLW" message, the decimal point is displayed along with the
"UDRFLW" message to indicate the selected range. For example, the message
"U.DRFLW V" indicates that the Model 2790 is on the 1V range and the input signal is
>1.2V for the m/X + b calculation to cause the over-range condition.
Keep in mind that the SxVohms calculation is actually a shortcut for the m/X + b math
function. Therefore, if the input voltage exceeds the selected range, the "UDRFLW"
message will be displayed.
®
Switch System Reference Manual
X is the normal display reading.
m and b are the user-entered constants for scale factor and offset.
R is the displayed result.
mX+b. If you change any of the constants for m/X + b, they will also change for
mX+b.
When using Rel, the rel'ed reading of the input signal is used by the m/X + b
calculation.
5-9
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