Sensoray 7429 - Manual (Page 26)

Set Filter Time Constant
This command establishes a software single-pole low-pass filter for the specified chan-
nel. The second command byte specifies a filter factor F which may have any value
from 0 to 255, inclusive. This table shows some sample F values and the corresponding
filter percentages.
Filter FactorFilter Percentage
0 0.0
32 12.5
64 25.0
128 50.0
192 75.0
255 99.6
All filter factors default to zero after a reset. This effectively disables the software fil-
ters, thereby maximizing the frequency response of all channels. A non-zero factor in
electrically noisy environments may help to reduce measurement noise.
Some applications require knowledge of the filter time constant. This function de-
scribes the relationship between time constant and filter factor:
t = b(F(-N,45 •
lnB(F(F,256))))
Where: t is the time constant in seconds.
F is the filter factor in the range 0 to 255.
N is the number of active channels.
COMMAND: (96 + CHAN),(FILTER FACTOR)
RESPONSE: NONE
Example: The sensor on channel 4 is to have a 12.5% low-pass filter. A filter factor
of 32 yields a 12.5% filter, so the command string is: (96 + 4),(32). Here is a sam-
ple QuickBasic implementation:
CALL SendByte (96 + 4)'opcode + channel
CALL SendByte (32)'filter factor
Assume that all sixteen channels are active. Using the above relationship between
time constant and filter factor, it can be shown that the channel 4 software filter has
a time constant of approximately 171 milliseconds.