Specifications

Analog Input Modules
31007715 6/2008 137
Offset and
Maximum Count
You can apply an offset value to the low end of the operating current range and a
maximum count to the high end of the operating current range. Offset and maximum
count can be applied on each channel independently. This feature allows you to
calibrate the analog input channels to match your equipment.
These parameters are provided only for sensor compensation, not for scaling. The
module is able to measure over the physical range of 4 to 20 mA or 0 to 20 mA. An
offset adjustment will move the interpretation of the low end of the range, and a max
count adjustment will move the interpretation of only the high end of the range.
Offset
If you are using a signed input data format, an offset can be configured as a decimal
or hexadecimal value in the range -767 to +767 (0xFD01 to 0x02FF).
If you are using an unsigned input data format, an offset can be configured as a
decimal or hexadecimal value in the range 0 to 1535 (0x05FF).
In either case, the value represents a current offset in the range 4 mA ± 0.38 mA
(for the 4 to 20 mA operating range) and 0 mA ± 0.48mA (for the 0to20mA
operating range).
By default, the offset on both channels is 0 (indicating no offset applied).
Maximum Count
If you are using a signed input data format, a maximum count can be configured as
a positive decimal or hexadecimal value in the range 31 233 to 32 767
(0x7A01 to 0x7FFF).
If you are using an unsigned input data format, a maximum count can be configured
as a decimal or hexadecimal value in the range 62 465 to 65 535
(0xF401 to 0xFFFF).
In either case, the value represents a current in the range 20 ± 0.38 mA (for the
4 to 20 mA operating range) and 20 ± 0.48 mA (for the 0 to 20 mA operating range).
By default, the maximum count on both channels is 32 000 signed or 64 000
unsigned (indicating no gain applied).
In the 4 to 20 mA operating range, an ideal linear current representation (one
without offset or max count adjustments) is interpreted using the formula:
z V
n
= (I
ai
- 4 mA) x 2 000 for a signed input data range
z V
n
= (I
ai
- 4 mA) x 4 000 for an usigned input data range
where V
n
is the numerical count and I
ai
is the analog input current
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