qt1100a Quantum Research Group, qt1100a Datasheet - Page 13

• Manufacturer Part Number: qt1100a
• Description: 10 Key Qtouch? Sensor Ic
• Manufacturer: Quantum Research Group

2.8 ESD Protection
Normally, only a series resistor is required for ESD
suppression. A 10K to 22K Rsns resistor in series with each
sense trace to each key is normally sufficient. The dielectric
panel (glass or plastic) usually provides a high degree of
isolation to prevent ESD discharge from reaching the circuit.
The Rsns resistors should be placed close to and wired
tightly to the chip, not the keys.
If the Cx load is high, Rsns can prevent total charge and
transfer and as a result gain can deteriorate. If a reduction in
Rsns increases gain noticeably, the lower value should be
used. Conversely, increasing the Rsns can result in added
ESD and EMC benefits provided that the increase in
resistance does not decrease sensitivity.
2.9 Noise Issues
2.9.1 LED Traces and Other Switching Signals
Digital switching signals near the SNS lines will induce
transients into the acquired signals, deteriorating the SNR
performance of the device. Such signals should be routed
away from the SNS lines, or the design should be such that
these lines are not switched during the course of signal
acquisition (bursts).
LED terminals which are multiplexed or switched into a
floating state and which are within or physically very near a
key structure (even if on another nearby PCB) should be
bypassed to either Vss or Vdd with at least a 10nF capacitor
of any type, to suppress capacitive coupling effects which
can induce false signal shifts. LED terminals which are
constantly connected to Vss or Vdd do not need bypassing.
2.9.2 External Fields
External AC fields (EMI) due to RF transmitters or electrical
noise sources can cause false detections or unexplained
shifts in sensitivity.
The influence of external fields on the sensor is reduced by
means of the Rsns series resistors. The Cs capacitors and
the Rsns resistors form a natural low-pass filter for incoming
RF signals; the roll-off frequency of this network is defined
by -
If for example Cs = 4.7nF, and Rsns = 10K, the EMI rolloff
frequency is ~3.4kHz, which is much lower than most noise
sources (except for mains frequencies i.e. 50/60Hz).
Rsns and Cs must both be placed very close to the body of
the IC so that the lead lengths between them and the IC do
not form an unfiltered antenna at very high frequencies.
PCB layout, grounding, and the structure of the input
circuitry have a great bearing on the success of a design to
withstand electromagnetic fields and be relatively noise-free.
These design rules should be adhered to for best ESD and
EMC results:
LQ
1. Use only SMT components.
2. Keep all Cs, Rs, Rsns, and the Vdd/Vss bypass
3. Place the QT1100A as close to the keys themselves as
4. Do not place electrodes or associated wiring near
capacitor components wired tightly to the IC.
possible.
other signals, or near a ground plane. If a ground plane
is unavoidable, keep the SNS tracks very thin (e.g.
F
R
=
2✜R
SNS
1
C
S
13
2.10 Start-up Time
After a reset or power-up event, the device requires 400ms
to read the EEPROM, if one is connected, initialize the
device, and start acquiring signals. After this time, the part
will calibrate all keys. The calibration time depends on the
burst spacing but is about 450ms for a burst spacing of 3ms.
This time is proportional to the burst spacing (Section 4.14).
The burst spacing governs the time from the start of one key
acquisition cycle to the next, and can be set via serial Setups
or via the external EEPROM. Thus, the total start-up time
after a reset is about 850ms if the burst spacing is set to
3ms.
The device will communicate immediately after the Setup
block is loaded (from EEPROM. if any, or from defaults).
2.11 Operating Parameter Setups
The device features a Setups block area in internal RAM that
holds numerous configuration parameters determin ing how
the part will operate. Each key can be configured individually
for a wide variety of parameters as discussed in Section 4. In
addition, the device can be configured for the AKS ™ function
which treats participating keys as a group in which only the
key with the strongest signal will generate a response.
Standalone (with EEPROM) Setups: In standalone mode
with EEPROM, device setups are configured using an
external 93LC46A byte-mode EEPROM (see Table 1.2, page
5). This part can be programmed separately using a
commercially obtainable programm ing device then inserted
into the circuit, or, it can be programmed using a QT1100A in
serial mode via a PC interface with the 93LC46A in a socket
so that it can be transferred to the target PCB.
The EEPROM contents and default values are detailed in
Table 4-1, page 31. The last EEPROM entry should be a
CRC check byte. If the CRC byte is set to 0xD6, the CRC will
be ignored.
In standalone mode the EEPROM must have the first byte in
location 0 set to the value 0xD6 for the EEPROM to be read.
The rest of the Setup table must follow, starting at location 1
in the EEPROM.
5. Do use a ground plane under and around the chip
6. To prevent cross interference, do not place an
7. Keep the electrodes (and wiring) away from other
8. If there are switched LEDs or related wiring near an
9. Use a voltage regulator just for the QT1100A to
0.15mm) and relieve the ground plane widely around
them (e.g. 5mm clear space on all sides).
itself, back to the regulator and power connector (but
not beyond the Rs/Cs/Rsns networks).
electrode or SNS traces of one QT1100A near the
electrode or the SNS traces of another QT1100A or
similar device, unless they are synchronized with a
Sync signal in a way that adjacent traces and keys do
not have acquisition bursts on them at the same time.
traces carrying AC or switched signals.
electrode or SNS traces (e.g. for backlighting of a key),
bypass the switched traces to ground.
eliminate noise coupling from other switching sources
via Vdd. Make sure the regulator’s transient load
stability provides for a stable, settled voltage just
before each burst commences.
Copyright © 2003-2005 QRG Ltd
QT1100A-ISG R3.02/1105