Vishay G4-DT, Техническое руководство, Страница: 90 / 174

Technical Manual
6-8
Filter function
In the instrument the measured values are filtered according to the settings chosen by
the user during set-up. The filter settings can be selected from a wide range. This
means that the instrument can be configured from very stable indication to more fast
response but with less stable readings depending on the demands from the actual
application. Note that the same filter setting apply to all inputs (A – D) in a function
block.
The user must consider the effects of different settings on the application. Choosing a
lower update rate will generally speaking give a more stable reading and may be used
when an operator should make a visual reading of the measuring value. A faster
update rate is typically selected when the instrument is used for closed loop control or
data logging/recording of fast processes.
When using the measured value, as feedback in a closed loop system great care must
be taken when choosing the filter settings. The overall phase shift in the loop has a
crucial influence on the stability. The measuring filter and the analog output filter both
give a contribution to the total loop phase shift. If communication is used to acquire
measuring value from the instrument, the delays and update rate in the network or on
the serial communication line, must also be taken into account when designing a
closed loop system.
Filter characteristics.
The parameters ‘HSWF Update Rate’ / ‘WFIN Update Rate’ selects the update rate
and filter characteristics of the measuring value. HS WF2 and WF IN / WF IN2 modules
have different ranges of settings.
With the ‘WFIN Update Rate’ parameter set to ‘300 Hz ++’ the update rate will be
300 Hz (i.e. same as ‘300 Hz’) but the filter has been optimized for high bandwidth, and
less phase shift, rather than effective damping. This also applies to ‘HSWF Update
Rate’ parameter when set to ‘800 Hz ++’ (using 800 Hz update rate). In both these
cases the user must consider the risk of aliasing. Avoid using these high bandwidth
settings (800 Hz ++ / 300 Hz ++) if excessive noise exists.
The figures in the tables below include the influence of the analog output filtering set to
400 Hz filtering bandwidth. Setting the analog filter bandwidth to a lower value will
reduce the frequency values in the tables. See Analog output filter table below.
WF IN / WF IN2 module filter
Update rate
(fs) - 3 dB damping
frequency Damping at
frequencies
≥
fs / 2
-40
°
phase shift
frequency
300 Hz ++ 63 Hz
≥
20 dB
11 Hz
300 Hz 22 Hz
≥
90 dB
4.5 Hz
150 Hz 14 Hz
≥
90 dB
2.3 Hz
75 Hz 7.6 Hz
≥
90 dB
1.2 Hz
37 Hz 3.9 Hz
≥
90 dB
0.60
19 Hz 2 Hz
≥
90 dB
0.30
9.6 Hz 1 Hz
≥
90 dB
0.15 Hz
Note: fs is the update rate.