Figure 1-9 Bursts when SC > 0
1.5 TIMING
The QT320 runs two sensing bursts, one per channel, each
acquisition cycle (Figure 1-9). The bursts are successive in
time, with Channel 2 firing first.
The basic QT320 timing parameters are:
Ti
Tbs
Tbd1
Tbd2
Tbd
Tmod
Tdet
Basic timing interval
Burst spacing
Burst duration, Channel 1
Burst duration, Channel 2
Burst duration, Ch1 + Ch2
Max On-Duration
Detection response time
(1.5.1)
(1.5.1)
(1.5.2)
(1.5.2)
(1.5.2)
(1.5.3)
(1.5.4)
Figure 1-11 Burst detail
1.5.1 B
URST
S
PACING
: T
I
, SC, T
BS
Between acquisition bursts, the device can go into a low
power sleep mode. The percentage of time spent in sleep
depends on the burst spacing and the combined burst lengths
of both channels; if the burst lengths occupy all of the sleep
interval, no time will be spent in sleep mode and the part will
operate at maximum power drain.
The burst spacing is a multiple of the basic timing interval Ti;
Ti in turn depends heavily on Vdd (see Section 2.1 and Figure
5.7). The parameter ‘Sleep Cycles’ or SC is the user-defined
Setup value which controls how many Ti intervals there are
from the start of a burst on Channel 2 until the start of the
next such burst. The resulting timing is Tbs:
Tbs = SC x Ti
where SC > 0.
All the basic timing parameters of the QT320 such as
recalibration delay etc. are dependent on Tbs.
If SC = 0, the device never sleeps between bursts (Figure
1-10). This mode is fast but consumes maximum power; it is
also unregulated in timing from burst to burst, depending on
the combined burst lengths of both channels.
Conversely if SC >> 0, the device will spend most of its time
in sleep mode and will consume very little power, but it will be
slower to respond.
By selecting a supply voltage and a value for SC, it is possible
to fine-tune the circuit for the desired speed / power tradeoff.
1.5.2 B
URST
D
URATIONS
: T
BD
1, T
BD
2, T
BD
The two burst durations depend entirely on the values of Cs
and Cx for the coresponding sensing channel, and to a lesser
extend, Vdd. The bursts are composed of hundreds of
charge-transfer cycles (Figure 1-11) operating at about
500kHz. Channel 2 always fires first (Tbd2) followed by
Channel 1 (Tbd1); the sum total of the time required by both
channels is parameter Tbd.
Figure 1-10 Bursts when SC = 0
(750K resistor in series with scope probe)
lQ
6
QT320/R1.03 08/02
QUANTUM [ QUANTUM RESEARCH GROUP ]
