To assist with this problem, the QT510 waits 500µs after
coming out of sleep mode before acquiring to allow power to
fully stabilize. This delay is not present before an acquisition
burst if there is no preceding sleep state.
Use an oscilloscope to verify that Vdd has stabilized to within
5mV or better of final settled voltage before a burst begins.
2.6 ESD Protection
Since the electrode is always placed behind a dielectric
panel, the IC will be protected from direct static discharge.
However even with a panel transients can still flow into the
electrode via induction, or in extreme cases via dielectric
breakdown. Porous materials may allow a spark to tunnel
right through the material. Testing is required to reveal any
problems. The device has diode protection on its terminals
which will absorb and protect the device from most ESD
events; the usefulness of the internal clamping will depending
on the panel's dielectric properties and thickness.
One method to enhance ESD suppression is to insert
resistors Rs1, Rs2 and Rs3 in series with the element as
shown in Figure 1-1; these are typically 4.7K but can be as
high as 10K ohms.
Diodes or semiconductor transient protection devices or
MOV's on the electrode traces are not advised; these devices
have extremely large amounts of nonlinear parasitic
capacitance which will swamp the capacitance of the
electrode and cause false detections and other forms of
instability. Diodes also act as RF detectors and will cause
serious RF immunity problems.
See also next section.
2.5 PCB Layout and Mounting
The E510 PCB layout (Figure 1-3) should be followed if
possible. This is a 1-sided board; the blank side is simply
adhered to the inside of a 2mm thick (or less) control panel.
Thicker panels can be tolerated with additional position error
due to capacitive ‘hand shadow’ effects and will also have
poorer EMC performance.
This layout uses 18 copper pads connected with intervening
series resistors in a circle. The finger interpolates between
the copper pads (if the pads are narrow enough) to make a
smooth, 0..127 step output with no apparent stair-casing. The
lateral dimension along the centre of each electrode should
be no wider than the expected smallest diameter of finger
touch, to prevent stair-casing of the position response (if that
matters).
Other geometries are possible, for example triangles and
squares. The wheel can be made in various diameters up to
at least 80mm. The electrode width should be about 12mm
wide or more, as a rule.
The SMT components should be oriented perpendicular to
the direction of bending so that they do not fracture when the
PCB is flexed during bonding to the panel.
Additional ground area or a ground plane on the PCB will
compromise signal strength and is to be avoided. A single
sided PCB can be made of FR-2 or CEM-1 for low cost.
‘Handshadow’ effects:
With thicker and wider panels an
effect known as ‘handshadow’ can become noticeable. If the
capacitive coupling from finger to electrode element is weak,
for example due to a narrow electrode width or a thick, low
dielectric constant panel, the remaining portion of the human
hand can contribute a significant portion of the total
detectable capacitive load. This will induce an offset error,
which will depend on the proximity and orientation of the hand
to the remainder of the element. Thinner panels and those
with a smaller diameter will reduce this effect since the finger
contact surface will strongly domina te the total signal, and the
remaining handshadow capacitance will not contribute
significantly to create an error offset.
PCB Cleanliness:
All capacitive sensors should be treated
as highly sensitive circuits which can be influenced by stray
conductive leakage paths. QT devices have a basic
resolution in the femtofarad range; in this region, there is no
such thing as ‘no clean flux’. Flux absorbs moisture and
becomes conductive between solder joints, causing signal
drift and resultant false detections or temporary loss of
sensitivity. Conformal coatings will trap in existing amounts of
moisture which will then become highly temperature
sensitive.
The designer should specify ultrasonic cleaning as part of the
manufacturing process, and in extreme cases, the use of
conformal coatings after cleaning.
2.7 EMC and Related Noise Issues
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 can be reduced
by means of the Rs series resistors described in Section 2.6.
The Cs capacitor and the Rs resistors (Figure 1-1) form a
natural low-pass filter for incoming RF signals; the roll-off
frequency of this network is defined by -
F
R
=
1
2✜R
S
C
S
If for example Cs = 47nF, and Rs = 4.7K, the EMI rolloff
frequency is ~720 Hz, which is much lower than most noise
sources (except for mains frequencies i.e. 50 / 60 Hz). The
resistance from the sensing element itself is actually much
higher on average, since the element is typically 50K ~ 100K
ohms between connection points.
Rs 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:
1.
2.
3.
4.
Use only SMT components.
Keep all Cs, Rs, and the Vdd bypass cap close to the IC.
Do not place the electrode or its connecting trace near
other traces, or near a ground plane.
Do use a ground plane under and around the QT510
itself, back to the regulator and power connector (but not
beyond the Cs capacitor).
Do not place an electrode (or its wiring) of one QT510
device near the electrode or wiring of another device, to
5.
lQ
5
QT510 R6.04/0505
QUANTUM [ QUANTUM RESEARCH GROUP ]
