Siemens SIMATIC ET 200AL, System Manual

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High-speed analog modules
8.2 ST modules vs HS modules
Analog value processing
122
Function Manual, 06/2014, A5E03461439-AC
Effect of load on the settling time
The settling times to the setpoint achieved with analog output modules depend on the
applied load, among other things. To take advantage of the faster conversion times, only
small loads (e.g., max. 47 nF) are permitted in HS modules as specified in the technical
specifications.
The type of load has an effect on the form of settling to the setpoint when using output
modules. Capacitive loads reduce the rise time due to the charging process and can cause
the output signal to overshoot. Inductive loads result in a delayed current increase due to
remagnetization and therefore also a shorter rise time. The figure below shows the output
signal with no/small
①
, medium
②
and high
③
capacitive load.
①
Reference curve of a signal with no/small load capacity
②
Signal with a load capacity of 47 nF
③
Signal with a load capacity of 100 nF
t
E
Settling time
t
1
Module ends conversion at the terminal of the analog output channel and outputs the analog
signal
t
2
Signal
①
has settled and the specified analog output value has been reached
(residual error 1 %)
t
3
Signal
②
has settled and the specified analog output value has been reached
(residual error 1 %)
t
4
Signal
③
has settled and the specified analog output value has been reached
(residual error 1 %)
Figure 8-7 Effect of different loads on the settling time