© Quantum Research Group Ltd.
obtained; see Section 5.7. Conversely longer spacings permit TLC2272 type opamp is a good example of the type of device
which should be employed.
higher burst lengths but slow down response time.
Spacings from 250µs to 2ms are available.
Figure 3-1 Circuit: The first opamp is a charge integrator
whose output ranges between 0V to -2.5V. A JFET is used as
a reset switch for the integration capacitor C14. Since most
opamps are not fast enough to integrate the nanosecond
duration transient charge pulses coming from the Y lines and
the switched Cz capacitors, a large, non-critical capacitor C11
is used to temporarily store transient charge until the opamp
can assimilate it over the following microseconds.
3.9 PLD Circuit and Charge Sampler
The PLD should be a CMOS 22V10 type having no internal
pullup or bus-keeper resistors in order to limit leakage
current. ICT’s PEEL22CV10AZ is a good example device,
and code for this part can be found in Section 6.
The PLD performs two functions: Y line clamping and transfer
switch gating.
The second stage opamp must invert the first opamp output
in order to provide a positive-going signal to Ain of the
QT60xx5. This stage is also used to facilitate the introduction
of offset from the R2R network (Section 3.12).
The PLD clamps Y-lines to ground whenever key charge is
not being collected. The charge integrator should only receive
charge starting just before an X line goes high, to a point just
after the transition (the X-Y ‘dwell time’). It is an essential
function of the PLD to neutralize charge keys during the
negative transition of X lines; without this, charge-transfer
would cease to function after a single X pulse, and multiple
pulse bursts would be impossible.
The second stage must be clamped with a low-C diode as
shown (BAV-99 preferred) so that negative excursions of the
amplifier do not under-drive the Ain pin of the device. An
output resistor further limits possible Ain+ currents. Without
clamping there can be high currents taken from Ain which can
lead to device latchup, requiring power to be cycled to restore
operation.
The PLD also acts to generate a pulse that sets the dwell
time for the QS3251 8:1 charge sampler switch. A simple
PLD-based RC network controls the QS3251 gate pin ‘E’
starting from when line YG becomes active to a time after X7
or XS transition high. XS is the logical-OR of X0..X6; X8 and
XS are OR’d together in the PLD so that any single X line can
trigger the timing network.
Figure 3-2 Circuit: The first opamp is a positive-gain high
impedance configuration which amplifies the small voltage on
Cs (C7). The reset transistor is a small-signal N-fet. C7 also
receives charge cancellation capacitances C8 and C9. The
R2R DAC offset is injected into the summing junction of this
amplifier.
X-Y dwell time can be measured with an oscilloscope by
timing the interval from XS or X8 to 22V10 output F9. Dwell
times of 70ns - 90ns work very well to suppress the effects of
surface moisture films. Longer times are acceptable if such
moisture is not anticipated.
The second stage amplifier has a positive gain that provides
final amplification.
This design is simpler to implement but has lower gain than
the circuit of Figure 3-1.
R2 and/or C5 in Figure 3-1 should be adjusted to provide a
timing dwell delay from the rise of an X line to the rising edge
of Y-enable (QS3251) of around 75ns +/-20%. Shorter dwell
times will begin to cause the suppression of human touch
signals as well. If resistors and capacitors are used in line
with the X and Y matrix lines for EMC and ESD suppression
(Section 3.22), excessively short dwell times can seriously
deteriorate signal gain. The circuit should be evaluated for the
amount of signal loss by comparing delta signals due to touch
both with and without the EMC circuits.
3.11 Sample Capacitors
Charge sampler capacitor Cs (C14 in Figure 3-1, C7 in Figure
3-2) should be the values shown. They should be either NP0
or C0G ceramic or PPS film for thermal stability reasons. The
two Cz capacitors should be NP0 or C0G types only. The
transient charge absorber C11 can be a 10% X7R type.
More information on how the Cs and Cz capacitors function is
described in Section 1.2.
The values of capacitance should not be altered from the
reference schematics; value changes can cause acquisition
gaps to occur which can result in keys that cannot calibrate.
R2 and C5 can be eliminated to provide the full 167ns of
dwell time output by the QT60xx5. C5 should be replaced by
a connection to ground, and R2 should be open-circuited.
Source code for one type of recommended 22V10 can be
found in Section 6. The 22V10 should have conventional
CMOS I/O structures without ‘bus-keepers’ or pullup resistors
in order to work optimally.
3.12 R2R Resistor Ladder
The R2R ladder network (RN1 in Figure 3-1) should have a
value of 100K ohms and a precision of 7 or 8 bits. The R2R
connects to the summing junction of the first or second
opamp depending on the circuit; it is used to offset the analog
signal down with increasing binary input value. The R2R
value is determined for each key during calibration by an
algorithm that seeks to put the signal Ain+ at 2.5 volts. This
binary value only changes when a key is recalibrated or after
powerup during the normal startup calibration cycle; drift
compensation does not change R2R drive.
While the QS3251 is gated by the signal on its ‘E’ pin from
the PLD, the actual switch being controlled is determined by
the YS0, YS1, YS2 lines from the QT60xx5.
3.10 Opamps
The amplifier chain should be configured as shown in Figures
3-1 or 3-2. The opamps should have a GBW product of at
least 2MHz, have rail-rail CMOS outputs, and be able to
operate from split-rail supplies (split-rail capable only in the
case of Figure 3-1). To eliminate leakage current issues the
amplifier should be a JFET or CMOS input type only. TI’s
The R2R is driven by the matrix X lines; this is possible since
Ain+ is only read after the completion of each burst, therefore
this dual-use of X drive lines does not pose a conflict so long
as these lines are not heavily loaded.
lQ
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