27
3.3: Operation principle
switching period. L-502 has, by default, that for
n
sw
> 1 all ADC readouts are flipped, except for
the last one, during the switching period - this creates the greatest time for setting the signal after
switching (due to "idle" ADC conversion cycles), therefore the least stringent requirements are
imposed to the impedance of the signal source. On
, with
n
sw
=3, such conditions are set "by
default" for the logical channel 1: the first two counts are always discarded, and the third one is
used. But the real tasks of using multichannel ADCs do not assume that the impedances of the
signal sources are the same, and for channels with connected low-impedance sources it would be
good not to discard at least some of the ADC samples, but to use them for averaging the data,
thereby increasing the enable when measuring this channel. This possibility is provided in L-502
due to the fact that in every cell of the control table, besides the physical channel number, there is
also the averaging factor
n
av
, by default,
n
av
= 1. Averaging factor
n
av
={1,2,…,128} means: "how
many counts of the ADC from the end of the switching cycle of this channel will be used to
averaging the data". Accordingly,
n
su
=
n
sw
- n
av
means "how many ADC counts from the
beginning of the switching cycle of the given channel will be discarded", or "how many periods
t
ref
will be used to set the signal at the ADC input after switching".
n
av
= 2 is installed in the control table,
which means that for
n
sw
=3 the result of the last two conversion periods in one switching phase will
be used for averaging, and one first period is added to the time of signal establishment after
switching. For logical channel 3, all three samples of the ADC are used for averaging, and therefore
the minimum time is assigned here to establish a signal after switching.
1
2
1
3
2
Канал 1
Канал 2
Канал 3
t
k
n
k
=3
n
d
=2
t
d
t
k
t
ch
n
sw
=3
Кадр
Межкадровая
задержка
Кадр
Межкадровая
задержка
n
av
=1
n
av
=2
n
av
=3
n
su
=2
n
su
=1
n
su
=0
Номер
логического
канала АЦП
Момент сэмплирования одиночного отсчёта данных АЦП
Моменты сэмплирования отсчётов данных АЦП и выдача усредненного отсчёта
Отброшенный отсчёт данных АЦП
t
ref
t
sw
3
1
Fig. 3-3. The principle of obtaining ADC data (in detail)
From
it also follows that the set non-zero interframe delay actually increases the
settling time for the first logical channel. This can be used, for example, by associating the first
logical channel with the physical channel to which the signal source is connected by the largest
impedance.
Channel 1
Channel 2
Channel 3
ADC logical
channel
number
Frame
Frame
Interframe
delay
Interframe
delay
Sampling time of a single sample ADC data
Sampling timing of ADC data and output of averaged readout
Discarded ADC data sample
