Siemens Simatic S7-200 System Manual page 386

S7-200 Data Sheets
Input Calibration
The module calibration is used to correct the gain error at full scale. Offset error is not
compensated. The calibration affects all three input channels, and there may be a difference
in the readings between channels after calibration.
To calibrate the module accurately, you must use a program designed to average the values
read from the module. Use the Analog Input Filtering wizard provided in STEP 7-Micro/WIN
to create this program (see Section 5.3). Use 64 or more samples to calculate the average
value.
To calibrate the input, use the following steps.
1. Turn off the power to the module. Select the desired input range.
2. Turn on the power to the CPU and module. Allow the module to stabilize for 15 minutes.
3. Using a transmitter, a voltage source, or a current source, apply a zero value signal to
one of the input terminals.
4. Read the value reported to the CPU by the appropriate input channel. The reading with a
zero value input indicates the magnitude of the offset error. This error cannot be
corrected by calibration.
5. Connect a full-scale value signal to one of the input terminals. Read the value reported
to the CPU.
6. Adjust the GAIN potentiometer until the reading is 32,000, or the desired digital data
value.
Data Word Format
Figure A-35 shows where the 12-bit data value is placed within the analog input word of the
CPU.
A variance in repeatability of only 0.45% of full scale can give a variance of 144 counts in
the value read from the analog input.
AIW XX
Figure A-35
Note
The 12 bits of the analog-to-digital converter (ADC) readings are left-justified in the data
word format. The MSB is the sign bit: zero indicates a positive data word value. The three
trailing zeros cause the data word to change by a count of eight for each one count
change in the ADC value.
A-62
MSB
15 14
0 Data value
Unipolar data
Data Word Format
LSB
3 2
12 Bits 0 0 0
S7-200 Programmable Controller System Manual
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