3300/25 Dual Accelerometer Input Monitor Operation Manual
70
19.11 Non Standard Low Pass Filter Options
Use this procedure only if the desired low pass corner frequency is not listed in the table
in the previous section. The procedure consists of these two parts:
• Set the anti-aliasing filter (
f
AA
).
• Set the low-pass corner frequency (f
LP
).
Since the low-pass filter used on the filter board is a switched capacitor type, an anti-
aliasing filter is required to band-limit the input signal. Configure the anti-aliasing filter to
have a corner frequency,
f
AA
, five times that of the selected low-pass filter corner
frequency, f
LP
.
19.11.1 Setting the Anti-Aliasing Filter
1. Select the low-pass corner frequency you require, f
LP,
in the range 24 Hz to 22.372
kHz.
NOTE: the -3dB frequency is 1.06 x
f
LP
.
2. Calculate the necessary anti-aliasing corner frequency,
f
AA
, based on your
required low-pass corner frequency, f
LP
.
LPAA
f×5=f
If f
AA
≤ 68.57 kHZ, go to step 3.
If f
AA
> 68.57 kHZ, go to step 5 and use a value of M=255 and K=268.9. This will set the
anti-alias corner frequency to its highest value.
3. Select K according to this table:
4. Calculate the multiplier value, M, by using this formula:
K
f
=M
AA
where
f
AA
is the anti-aliasing corner frequency (in Hz) from step 2
K is the constant selected in step 3
M is the multiplier value. (M should be in the range 6 to 255)
Anti-Aliasing Corner Frequency, f
AA
,
Range
Use this value for K
120 Hz to 4.81 kHz 19.188
>4.81 kHz to 68.57 kHz 268.9
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