Table 1-1 Pin Descriptions
1 OUT1 Detection output, Ch. 1
Sense Ch 2 pin B
Sense Ch 1 pin A
Negative supply (ground)
Sense Ch 1 pin B
Sense Ch 2 pin A
7 OUT2 Detection output, Ch. 2
which requires several consecutive confirmations of a
detection before an output is activated.
The two channels of sensing operate in a completely
independent fashion. A unique cloning process allows the
internal eeprom of the device to be programmed for each
channel, to permit unique combinations of sensing and
processing functions for each.
The two sensing channels operate in interleaved
time-sequence and thus cannot interfere with each other.
Alternate Pin Functions for Cloning
Serial clone data clock
Serial clone data out
Serial clone data in
1 - OVERVIEW
The QT320 is a 2 channel digital burst mode charge-transfer
(QT) sensor designed specifically for touch controls; it
includes all hardware and signal processing functions
necessary to provide stable sensing under a wide variety of
changing conditions. Only two low-cost, non-critical capacitors
are required for operation.
A unique aspect of the QT320 is the ability of the designer to
‘clone’ a wide range of user-defined setups into the part’s
eeprom during development and in production. Cloned setups
can dramatically alter the behavior of each channel,
independently. For production, the parts can be cloned
in-circuit or can be procured from Quantum pre-cloned.
Figure 1-1 shows the basic QT320 circuit using the device,
with a conventional output drive and power supply
1.1 BASIC OPERATION
The QT320 employs bursts of variable-length charge-transfer
cycles to acquire its signal. Burst mode permits power
consumption in the microamp range, dramatically reduces RF
emissions, lowers susceptibility to EMI, and yet permits
excellent response time. Internally the signals are digitally
processed to reject impulse noise using a 'consensus' filter
Figure 1-1 Basic QT320 circuit
1.2 ELECTRODE DRIVE
1.2.1 SWITCHING OPERATION
The IC implements two channels of direct-to-digital
capacitance acquisition using the charge-transfer method, in
a process that is better understood as a capacitance-
to-digital converter (CDC). The QT switches and charge
measurement functions are all internal to the IC (Figure 1-2).
The CDC treats sampling capacitor Cs as a floating store of
accumulated charge which is switched between the sense
pins; as a result, the sense electrode can be connected to
either pin with no performance difference. In both cases the
rule Cs >> Cx must be observed for proper operation. The
polarity of the charge build-up across Cs during a burst is the
same in either case. Typical values of Cs range from 2nF to
100nF for touch operation.
Larger values of Cx cause charge to be transferred into Cs
more rapidly, reducing available resolution and resulting in
lower gain. Conversely, larger values of Cs reduce the rise of
differential voltage across it, increasing available resolution
and raising gain. The value of Cs can thus be increased to
allow larger values of Cx to be tolerated (Figures 5-1 to 5-4).
As Cx increases, the length of the burst decreases resulting in
lower signal numbers.
Figure 1-2 Internal Switching
It is possible to connect separate Cx and Cx’ loads to Sa and
Sb simultaneously, although the result is no different than if
the loads were connected together at Sa (or Sb). It is
important to limit the amount of stray Cx capacitance on both
terminals, especially if the load Cx is already large. This can
be accomplished by minimising trace lengths and widths.