qt320 Quantum Research Group, qt320 Datasheet - Page 5

• Manufacturer Part Number: qt320
• Description: 2-channel Progammable Advanced Sensor Ic
• Manufacturer: Quantum Research Group

strength, even if the fiber density is too low to make the
plastic electrically conductive.
1.4.2 D
In some cases the circuit may be too sensitive, even with high
signal threshold values. In this case gain can be lowered by
making the electrode smaller, using sparse mesh with a high
space-to-conductor ratio (Figure 1-3), and most importantly by
decreasing Cs. Adding Cx capacitance will also decrease
sensitivity.
It is also possible to reduce sensitivity by making a capacitive
divider with Cx by adding a low-value capacitor in series with
the electrode wire.
1.4.3 H
Hysteresis is required to prevent chattering of the output lines
with weak, noisy, or slow-moving signals.
The hysteresis can be set independently per channel.
Hysteresis is a reference-based number; thus, a threshold of
10 with a hysteresis of 2 will yield 2 counts of hysteresis
(20%); the channel will become active when the signal equals
or exceeds a count of 10, and go inactive when the count falls
to 7 or lower.
Hysteresis can also be set to zero (0), in which case the
sensor will go inactive when the count falls to 9 or lower in the
above example.
Threshold levels of under 4 counts are hard to deal with as
the hysteresis level is difficult to set properly.
1.4.4 C
Channel 1 has less internal Cx than Channel 2, which makes
it more sensitive than Channel 2 given equal Cx loads and Cs
lQ
Figure 1-7 Burst lengths without Csx installed
(observed using a 750K resistor in series with probe)
ECREASING
YSTERESIS
HANNEL
B
ALANCE
S
ENSITIVITY
5
capacitors. This can be useful in some designs where one
more sensitive channel is desired, but if equal sensitivity is
required a few basic rules should be followed:
Since the actual burst length is proportional to sensitivity, you
can use an oscilloscope to balance the two channels with
more accuracy than by empirical methods (See Figures 1-7
and 1-8). Connect one scope probe to Channel 1 and the
other to Channel 2, via large resistors (750K ohms) to avoid
disturbing the measurement too much, or, use a low-C FET
probe. The Csx balance capacitor should be adjusted so that
the burst lengths of Channels 1 and 2 look nearly the same.
With some diligence the PCB can also be designed to include
some ground plane nearer to Channel 1 traces to induce
about 5pF of Csx load without requiring an actual discrete
capacitor.
1. Use a symmetrical PCB layout for both channels: Place
2. Connect a small capacitor (~5pF) between S1a or S1b
3. Adjust Cs and/or the internal threshold of the two channels
the IC half way between the two electrodes to match Cx
loading. Avoid routing ground plane (or other traces) close
to either sense line or the electrodes; allow 4-5 mm
clearance from any ground or other signal line to the
electrodes or their wiring. Where ground plane is required
(for example, under and around the QT320 itself) the
sense wires should have minimized adjacency to ground.
(either Channel 1 pin) and circuit ground (Csx in Figure
1-6), this will increase the load capacitance of Channel 1,
thus balancing the sensitivity of the two channels (see
Figures 1-7, 1-8).
until the sensitivities of the two channels are
indistinguishable from each other.
(observed using a 750K resistor in series with probe)
Figure 1-8 Burst lengths with Csx installed
QT320/R1.03 08/02