28
The device and principle of operation L-502.
It can be argued that by setting optimal
n
su
/ n
av
settings for each channel, we are trying to
optimize the
timing of the signal conditioning
associated with the inter-channel passage and the
resolution of the ADC
.
It is important to note that in the L-502, the ADC averaging algorithm described here (by the
simple average method) is considered as an inseparable part of the analog-to-digital converter itself,
although physically the averaging procedure is performed by means of FPGA using 24-bit integer
arithmetic.
Such an averaging operation increases the real resolution of the ADC by suppressing the
random components of the signal of different nature, improves the signal/noise by suppressing
the high-frequency components of the spectrum above the Nyquist frequency of 0.5*f
ch
for a
given physical channel
associated with one (or more) logical channel. Note in passing that digital
filtering by the Blackfin processor (or high-level software) has a fundamentally different active
filtering area, because it is below the Nyquist frequency.
Once again, we emphasize that "default" settings of L-502 have
n
av
=1, and there is no
averaging process "by default".
3.3.7. Relative switching delays in ADC channels.
This information will be important only for that class of multi-channel data acquisition tasks
where the magnitude of the relative signal delay between the ADC channels is important for
measuring relative phase delays. For this class of problems, the theoretical calculated latency values
in the ADC channels are taken into account in the delay equalization algorithm based on one or
another method of signal interpolation.
For ADC mode without averaging (
n
av
=1), the relative switching delay between adjacent
ADC channels within one frame (in the order of polling the control table) is equal to
t
sw
, and
between the last channel of the previous frame and the first channel of the next one is
t
sw
+
t
d
.
If the averaging mode is used (
n
av
>1), where
n
av
are selected equal for all ADC channels, the
absolute delay for each channel will decrease by the same amount 0.5*
n
av
*
t
ref
.
Therefore, the
relative delay will remain equal to
t
sw
between neighboring channels of one frame and equal
t
sw
+
t
d
between the nearest channels of neighboring frames separated by interframe delay.
If the averaging mode is used (
n
av
>1), where
n
av
are assigned in the control table different for
i- and j-th logical ADC channel, the absolute signal delay on the i- channel will decrease by
0.5
n
av
(i)*
t
ref
,
and the relative delay j channel towards the previous i- (within one frame) becomes
t
sw
+ 0,5*
t
ref
(n
av
(i)-
n
av
(j)), or becomes equal to
t
sw
+
t
d
+ 0,5*
t
ref
(n
av
(i)-
n
av
(j)) between the
nearest i-th and next j-th channels of neighboring frames separated by interframe delay (if more
precisely, for the last channel in the frame always i =
n
k
, and for first, always j = 1).
