QT110A-ISG Atmel, QT110A-ISG Datasheet - Page 2

• Manufacturer Part Number: QT110A-ISG
• Description: IC SENSOR TOUCH/PROX 1CHAN 8SOIC
• Manufacturer: Atmel
• Series: QProx™r
• Type: Capacitiver

Specifications of QT110A-ISG

• Touch Panel Interface: 1, 2-Wire
• Number Of Inputs/keys: 1 Key
• Resolution (bits): 14 b
• Data Interface: Serial
• Voltage Reference: Internal
• Voltage - Supply: 2.5V, 3.3V, 5V
• Current - Supply: 9.5µA ~ 26µA
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Output Type: Logic
• Interface: 2-Wire
• Input Type: Logic
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Package / Case: -
• Other names: 427-1137-2

1 - OVERVIEW
The QT110A is intended to replace the QT110 as a lower cost
alternative. This device functions identically to the QT110,
except that it is more sensitive. To compensate for the
sensitivity increase, it is required to do either of these two
things:
Option 1 is very simple and guarantees that the sensitivity of
the QT110A is identical to the older device. Option 2 requires
some trial and error to test the sensitivity of the touch pad or
prox field, so that it is about the same as before. Cs changes
ranging from 10 to 60% may be required depending on the
circuit layout and electrode design.
All other aspects of this datasheet are identical to the QT110
datasheet except for this section, the specification changes on
pages 8 and 9, and the part marking.
1.1 BASIC OPERATION
The QT110A employs low duty cycle bursts of charge-transfer
cycles to acquire its signal. Burst mode permits power
consumption in the low microamp range, dramatically reduces
EMC problems, and yet permits excellent response time.
Internally the signals are digitally processed to reject impulse
noise, using a 'consensus' filter which requires four
consecutive confirmations of a detection before the output is
activated.
The QT switches and charge measurement hardware functions
are all internal to the QT110A (Figure 1-3). A single-slope
switched capacitor ADC includes both the required QT charge
and transfer switches in a configuration that provides direct
ADC conversion. Vdd is used as the charge reference voltage.
Larger values of Cx cause the charge transferred into Cs to
rise more rapidly, reducing available resolution; as a minimum
resolution is required for proper operation, this can result in
dramatically reduced apparent gain.
The IC is highly tolerant of changes in Cs since it computes the
signal threshold level ratiometrically. Cs is thus non-critical and
can be an X7R type. As Cs changes with temperature, the
internal drift compensation mechanism also adjusts for the drift
automatically.
Piezo sounder drive: The QT110A can drive a piezo sounder
after a detection for feedback. The piezo sounder replaces or
augments the Cs capacitor; this works since piezo sounders
are also capacitors, albeit with a large thermal drift coefficient.
If C
required. If C
ceramic capacitor in parallel. The QT110A drives a ~4kHz
signal across SNS1 and SNS2 to make the piezo (if installed)
sound a short tone for 75ms immediately after detection, to act
as an audible confirmation.
LQ
CMOS
LOGIC
piezo
1. Increase the Cx loading to ground on SNS2 by 10pF
2. Decrease the Cs value
3.5 - 5.5V
is in the proper range, no additional capacitor is
1K
piezo
Figure 1-2 2-wire operation, self-powered
is too small, it can simply be ‘topped up’ with a
Twisted
pair
n-ch Mosfet
1N4148
2
3
4
OUT
OPT1
OPT2
8
1
Vdd
Vss
SNS2
SNS1
GAIN
+
10µF
7
5
6
C
s
R
2
s
Option pins allow the selection or alteration of several other
special features and sensitivity.
1.2 ELECTRODE DRIVE
The internal ADC treats Cs as a floating transfer capacitor; as a
direct result, the sense electrode can in theory be connected to
either SNS1 or SNS2 with no performance difference.
However, the noise immunity of the device is improved by
connecting the electrode to SNS2, preferably via a series
resistor Re (Figure 1-1) to roll off higher harmonic frequencies,
both outbound and inbound.
In order to reduce power consumption and to assist in
discharging Cs between acquisition bursts, a 470K series
resistor Rs should be connected across Cs (Figure 1-1).
The rule Cs >> Cx must be observed for proper operation.
Normally Cx is on the order of 10pF or so, while Cs might be
10nF (10,000pF), or a ratio of about 1:1000.
It is important to minimize the amount of unnecessary stray
capacitance Cx, for example by minimizing trace lengths and
widths and backing off adjacent ground traces and planes so
as keep gain high for a given value of Cs, and to allow for a
larger sensing electrode size if so desired.
The PCB traces, wiring, and any components associated with
or in contact with SNS1 and SNS2 will become touch sensitive
and should be treated with caution to limit the touch area to the
desired location.
1.3 ELECTRODE DESIGN
1.3.1 E
There is no restriction on the shape of the electrode; in most
cases common sense and a little experimentation can result in
a good electrode design. The QT110A will operate equally well
R
E
OUTPUT = DC
TIMEOUT = 10 Secs
TOGGLE = OFF
GAIN = HIGH
LECTRODE
Figure 1-1 Standard mode options
ELECTRODE
SENSING
2
3
4
OUT
OPT1
OPT2
C
+2.5 ~ +5
x
+2 ~ +5
G
1
8
Vdd
Vss
EOMETRY AND
with long, thin electrodes as with round
or square ones; even random shapes
are acceptable. The electrode can also
be a 3-dimensional surface or object.
Sensitivity is related to electrode surface
area, orientation with respect to the
object being sensed, object
composition, and the ground coupling
quality of both the sensor circuit and the
sensed object.
1.3.2 K
Like all capacitance sensors, the
QT110A relies on Kirchoff’s Current Law
(Figure 1-5) to detect the change in
capacitance of the electrode. This law
as applied to capacitive sensing
SNS2
GAIN
SNS1
7
5
6
2nF - 500nF
IRCHOFF
QT110A_1R0.01_0408
S
Cs
IZE
’
S
Rs
C
R
URRENT
E
L
ELECTRODE
SENSING
AW
C
x