Siemens SIMATIC S7-300 Reference Manual page 290

Analog Modules
Default Settings
The default settings of the module are "voltage" for the measuring method " and
" 10 V" for the measuring range. You can use this combination of measuring
method and measuring range without parameterizing the SM 331; AI 8 x 16 bits in
STEP 7.
Measuring errors with common-mode voltages
The SM 331; AI 8 x 16 bits can make measurements in the presence of AC or DC
common mode voltage.
For AC common mode voltages at multiples of the filter frequency setting, the
suppression of interference is accomplished by the integration period of the A/D
converter and by the common mode suppression of the input amplifiers. For AC
common mode voltages < 35 V
negligible measurement error.
For DC common mode voltages, only the interference suppression of the input
amplifier stage is available to minimize the effect of the common mode voltage.
Therefore, some accuracy degradation occurs in proportion to the common mode
voltage. The worst case error occurs with 50 VDC between one channel and the
other seven channels. The calculated worst case error is 0.7 % at 0 to 60 °C, and
measured error is typically ≤ 0.1% at 25 °C.
Special characteristic of parameter assignment to upper and lower limit values
The parameterizable limit values (triggers for hardware interrupt) differ for the
SM 331; AI 8 x 16 bits from the range of values contained in Table 4-50.
The reason for this is that numerical methods in the module software for evaluating
the process variables prevent values up to 32511 from being reported in some
cases. The process input value at which a hardware interrupt for an underflow or
overflow occurs depends on the calibration factors for an individual channel and
can vary between the lower limits shown in the table below and the value 32511
(7EFF ).
H
4-84
, the suppression ratio of > 100 dB results in
RMS
Programmable Logic Controllers S7-300 Module Data
A5E00105505-03