QT301-IS 데이터 시트보기 (PDF) - Quantum Research Group
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QT301-IS Datasheet PDF : 10 Pages
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2.2 CS / CX Dependency
The signal value is a direct function of Cs and Cx, where Cs
is the fixed sample capacitor, and Cx is the unknown
capacitance. These two values influence device sensitivity,
resolution and response time, making them very important
parameters.
Sensitivity and resolution are also a function of the size,
shape, and composition of the electrode, the composition
and thickness of any dielectric overlaying the electrode, the
composition and aspect of the object being sensed, and the
degree of mutual coupling between the electrode and the
object being sensed.
Figure 2-2 Acquisition Burst with Sync Signal
Sync Signal
Acquisition
Burst
2.3 Burst Length
The burst length is described by the following formula:
BL = k
ln(
Cs
Cs+Cx
)
Where ‘k’ is a constant, typical -0.51 (this may vary slightly
from device to device).
The response is thus a logarithmic curve; each doubling of
Cs increases the signal level and differential sensitivity by a
factor of two. Likewise, doubling Cx reduces the signal level
and differential sensitivity by a factor of two (Figures 6-1, 6-2,
page 8).
2.4 Sync Input
Bursts can be synchronized to external noise sources such
as mains frequency to suppress the effects of interference
coupled from such sources using a circuit such as that
shown in Figure 2-6. By synchronizing with noise sources,
the noise itself becomes highly correlated with the acquired
data, and AC alias components effectively disappear from
the signal. Sync works best on low frequency, highly
repeatable signals, such as mains frequency (50/60 Hz).
Figure 2-2 shows the effect of sync pulses on the burst rate.
A sync signal triggers a burst on the rising edge.
There is a Sync timeout of 100ms as shown in Figure 2-3. If
Sync pulses cease for >100ms, the Sync signal will be
treated as being lost and the device will start to acquire at its
own default rate again. When using the Sync feature it is
important that the Sync pulses are spaced less than 100ms
apart.
Figure 2-2 shows the acquisition burst in relation to Sync
pulses. If no rising edge is detected for 100ms, the QT301
will revert to the default timing shown in Figure 2-1. Figure
2-4 shows the sudden start of a train of Sync pulses and the
effect on the acquisition bursts.
Should the sync signal overclock the acquisition bursts
(Figure 2-5), the device will trigger on the next rising edge
after a delay of Tbd+2.5ms.
The 2.5ms is the minimum gap between bursts is to allow Cs
to properly discharge; Sync is not possible during this interval
nor is it possible to re-sync during a burst.
Figure 2-3 Acquisition Burst: Sync Lost
Sync Signal
Acquisition
Burst
100ms
Figure 2-4 Acquisition Burst: Sync Reacquired
Sync Signal
Figure 2-5 Sync Overclocked
Acquisition
Burst
Sync Signal
Acquisition
Burst
LQ
3
QT301 R1.04 21/09/03
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