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SVM2608 Programming 43
Resistance Measurement – Offset Method
The value returned by the resistance measurement offset commands (0x0022 through 0x002D) is
calculated using two current values. A voltage is measured when a current (I) is applied to the
circuit (V
ON
) as well as when the current is not applied (V
OFF
). The offset resistance value is then
calculated using the following formula:
I
VV
OFFON
−
=tMeasuremen ResistanceOffset
This might be useful when trying to measure a resistance in the presence of a voltage.
Resistance Measurement – Dynamic Method
The dynamic measurement resistance commands (0x002E – 0x0039) are used to determine a
resistance value for a given current while taking the non-linear nature of the current versus voltage
curve produced by diodes. The measurement is performed using two currents. The initial current,
I
1
, produces the initial voltage, V
1
. A second, lower current, I
2
, creates a second voltage, V
2
. The
dynamic resistance measurement value is calculated using the following formula:
21
21
tMeasuremen Resistance Dynamic
II
VV
−
−
=
When dynamic resistance is measured in the lowest current range (highest resistance range), no
“lower” current value exists. In this instance, current is turned off for the second measurement (I
2
= 0). In effect, this measurement is the same as an offset resistance measurement.
Self Test Command
The Perform Self Test (0x0011) command instructs the processor to perform an internal test using
the on-board reference voltage and on-board reference resistors. The purpose of this test is to
verify the functionality and accuracy of the system. A failure is indicated if the measurement is not
within 0.8% of the correct value.
This command can be sent to each channel independently to perform a self test on that channel,
the result of the self test is placed at the base offset for each channel (i.e. 0x000000 for Channel 0,
0x200000 for Channel 1, etc.). Each one of the 32 bits indicates the FAILURE (bit = 1) or
SUCCESS (bit = 0) of a test as follows:
For Channels 0 – 3:
Bit 0 - Measures +0.945 V on the 1 V scale
Bit 1 - Measures -0.945 V on the 1 V scale
Bit 2 - Measures +0.945 V on the 2 V scale
Bit 3 - Measures -0.945 V on the 2 V scale
Bit 4 - Measures +0.945 V on the 5 V scale
Bit 5 - Measures -0.945 V on the 5 V scale
Bit 6 - Measures +9.45 V on the 10 V scale
Bit 7 - Measures -9.45 V on the 10 V scale
Bit 8 - Measures +9.45 V on the 20 V scale
Bit 9 - Measures -9.45 V on the 20 V scale
Bit 10 - Measures +9.45 V on the 50 V scale
Bit 11 - Measures -9.45 V on the 50 V scale
Bit 12 - Measures 128 Ω on the 100 Ω scale
Bit 13 - Measures 128 Ω on the 1 kΩ scale