Function description
Operating Manual PSSu E S 2AI I se(-T)
21417-EN-04
17
The module always uses two's complement representation for internal processing, irre-
spective of the configured data format. The values from 0000
H
to FFFF
H
form a number
circle in the two's complement representation. 8000
H
follows 7FFF
H
(= 32 767
D
) and is inter-
preted as the lowest negative number (= -32 768
D
). 32 767
D
is never exceeded; the value
never falls below -32 768
D
.
4.3.1.4
User scaling
User scaling is a second level of scaling. You can use this scaling to correct local influ-
ences. The function is the same as that of manufacturer scaling, but a different value is
used for amplification factor 1:
}
y = (a
2
/ 256
D
* x) + b
2
or
}
y = (a
2
/ 100
H
* x) + b
2
a
2
= Amplification factor * 256
D
User scaling is deactivated in the default setting. The default value for offset is 0. The de-
fault value for gain is 256
D
(100
H
). That corresponds to amplification factor 1.
Key
:
}
y
: Digital value after user scaling
}
x
: Digital value before user scaling
}
a
2
: Gain
}
b
2
: Offset
}
a
2
/ 1024
D
: Amplification factor
4.3.1.5
Example calculation
Task:
When there is 20 mA at the input, the PII should show a decimal value of 20 000. User
scaling (Gain a
2
) should be used in this case. Manufacturer scaling should not be changed.
All numbers are decimals.
Solution:
Manufacturer scaling
y = (a
1
/ 1024 * x) + b
1
and user scaling
y = (a
2
/ 256 * x) + b
2
act consecutively, giving:
y = (a
2
/ 256 * ((a
1
/ 1024 * x) + b
1
)) + b
2
or:
y = (a
1
* a
2
* x / 262144) + (a
2
* b
1
/ 256) + b
2
With default values for a
1
= 8194, b
1
= 0 and the default value b
2
= 0:
y = a
2
/ 32 * x
a
2
= y * 32 / x
Due to the hardware calibration the converter's input range is divided so that the 10 V end
point is assigned the value x = 4095. With the default values x = 4095 and y= 20 000 the
result is:
Gain a
2
= 156