Quantum QT310 DATA SHEET

LQ
P
ROGRAMMABLE
Single channel digital advanced capacitance sensor IC Spread spectrum burst modulation for high EMI rejection Full autocal capability User programmable via cloning process Internal eeprom storage of user setups, cal data Variable drift compensation & recalibration times BG and OBJ cal modes for learn-by-example Sync pins for daisy-chaining or noise suppression Variable gain via Cs capacitor change Selectable output polarity, high or low Toggle mode (optional via setups) Push-pull output Completely programmable output behavior
via cloning process from a PC
HeartBeat™ health indicator (can be disabled)
APPLICATIONS
ROX
QT310
QP
C
APACITANCE SENSOR
IC
Fluid level sensorsIndustrial panels
This device requires only a few external passive parts to operate. It uses spread-spectrum burst modulation to dramatically reduce interference problems.
The QT310 charge-transfer (“QT’”) touch sensor IC is a self-contained digital IC capable of detecting proximity, touch, or fluid level when connected to a corresponding type of electrode. It projects sense fields through almost any dielectric, like glass, plastic, stone, ceramic, and wood. It can also turn metal-bearing objects into intrinsic sensors, making them respond to proximity or touch. This capability coupled with its ability to self calibrate continuously or to have fixed calibration by example can lead to entirely new product concepts.
It is designed specifically for advanced human interfaces like control panels and appliances or anywhere a mechanical switch or button may be found; it can also be used for material sensing and control applications, and for point-level fluid sensing.
The ability to daisy-chain permits electrodes from two or more QT310’s to be adjacent to each other without interference. The burst rate can be programmed to a wide variety of settings, allowing the designer to trade off power consumption for response time.
The IC’s RISC core employs signal processing techniques pioneered by Quantum; these are specifically designed to make the device survive real-world challenges, such as ‘stuck sensor’ conditions and signal drift. All operating parameters can be user-altered via Quantum’s cloning process to alter sensitivity, drift compensation rate, max on-duration, output polarity, calibration mode, Heartbeat™ feature, and toggle mode. The settings are permanently stored in onboard eeprom.
The Quantum-pioneered HeartBeat™ signal is also included, allowing a host controller to monitor the health of the QT310 continuously if desired.
By using Quantum’s advanced, patented charge transfer principle, the QT310 delivers a level of performance clearly superior to older technologies yet is highly cost-effective.
Appliance controlsSecurity systems
Access controlsMicro-switch replacement
Material detectionToys & games
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AVAILABLE OPTIONS
A
Copyright © 2002 QRG Ltd QT310/R1.03 21.09.03
8-PIN DIPSOICT
QT310-D-00C to +700C
-QT310-IS-400C to +850C
Table 1-1 Pin Descriptions
5Vd
c
FunctionNamePin
Ext Cal, latch clear input/CAL_CLR1
Sync Output/SYNC_O2 Sense 1 lineSNS13
Negative supply (ground)VSS4
Sense 2 lineSNS25
Sync Input/SYNC_I6
Detection outputOUT7
Positive supplyVDD8
Alternate Pin Functions for Cloning
Serial clone data clockSCK3
Serial clone data outSDO6
Serial clone data inSDI7
1 - OVERVIEW
The QT310 is a digital burst mode charge-transfer (QT) sensor designed for touch controls, level sensing and proximity sensing; it includes all hardware and signal processing functions necessary to provide stable sensing under a wide variety of changing conditions. Only one low cost sampling capacitor is required for operation.
A unique aspect of the QT310 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 the part. For production, the parts can be cloned in-circuit or can be procured from Quantum pre-cloned.
Figure 1-1 shows the basic QT310 circuit using the device, with a conventional output drive and power supply connections.
1.1 BASIC OPERATION
The QT310 employs bursts of charge-transfer cycles to acquire its signal. Burst mode permits power consumption in the microamp range, dramatically reduces RF emissions,
Result
SNS1
r e
l
Sta rt
l o
r t
n o
C t s
r
Done
u B
ingle-Slope S
Switched Capacitor ADC
Charge
Amp
Cs Cx
SNS2
lowers susceptibility to EMI, and yet permits excellent response time. Internally the signals are digitally processed to reject impulse noise, using a 'consensus' filter which requires several consecutive confirmations of a detection before the output is activated.
A unique cloning process allows the internal eeprom of the device to be programmed to permit unique combinations of sensing and processing functions.
+2to
100nF
8
VDD
12
/CAL
SYNC_O
6
SYNC_I
7
OUT SNS2
VSS
4
SNS1
3
5
C
4.7nF
ELECTRO DE
s
C
Calibration
10K
10K
Figure 1-1 Basic QT310 circuit
1.2 ELECTRODE DRIVE
1.2.1 S
The IC implements 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 10nF to 200nF.
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 5-2).
As Cx increases, the length of the burst decreases resulting in lower signal numbers.
The electrode should always be connected to SNS1; connections to SNS2 are also possible but this can cause the signal to be susceptible to noise.
It is important to limit the amount of stray Cx capacitance on both SNS terminals, especially if the Cx load is already large.
WITCHING OPERATION
x
Figure 1-2 Internal Switching
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This can be accomplished by minimising trace lengths and widths.
1.2.2 C
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.
Multiple electrodes can be connected, for example to create a control button on both sides of an object, however it is impossible for the sensor to distinguish between the two electrodes.
ONNECTION TO ELECTRODE
Figure 1-3 Mesh Electrode Geometry
1.3.2 K
Like all capacitance sensors, the QT310 relies on Kirchoff’s Current Law (Figure 1-4) to detect the change in capacitance of the electrode. This law as applied to capacitive sensing requires that the sensor’s field current must complete a loop, returning back to its source in order for capacitance to be sensed. Although most designers relate to Kirchoff’s law with regard to hardwired circuits, it applies equally to capacitive field flows. By implication it requires that the signal ground and the target object must both be coupled together in some manner for a capacitive sensor to operate properly. Note that there is no need to provide actual hardwired ground connections; capacitive coupling to ground (Cx1) is always sufficient, even if the coupling might seem very tenuous. For example, powering the sensor via an isolated transformer will provide ample ground coupling, since there is capacitance between the windings and/or the transformer core, and from the power wiring itself directly to 'local earth'. Even when battery powered, just the physical size of the PCB and the object into which the electronics is embedded will generally be enough to couple a few picofarads back to local earth.
The implications of Kirchoff’s law can be most visibly demonstrated by observing the E3B eval board’s sensitivity change between laying the board on a table versus holding the board in your hand by it’s batteries. The effect can also be observed by holding the board by the electrode ‘Sensor1’, letting it recalibrate, then touching the battery end; the board will work quite well in this mode.
IRCHOFF’S CURRENT LAW
1.2.3 B
The acquisition process occurs in bursts (Figure 1-7) of variable length, in accordance with the single-slope CDC method. The burst length depends on the values of Cs and Cx. Longer burst lengths result in higher gains and more sensitivity for a given threshold setting, but consume more average power and are slower.
Burst mode operation acts to lower average power while providing a great deal of signal averaging inherent in the CDC process, making the signal acquisition process more robust.
The QT method is a very low impedance method of sensing as it loads Cx directly into a very large capacitor (Cs). This results in very low levels of RF susceptibility.
URST MODE OPERATION
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 QT310 will operate equally well with a long, thin electrode as with a round or square one; 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. Smaller electrodes have less sensitivity than large ones.
If a relatively large electrode surfaces are desired, and if tests show that an electrode has a high Cx capacitance that reduces the sensitivity or prevents proper operation, the electrode can be made into a mesh (Figure 1-3) which will have a lower Cx than a solid electrode area.
LECTRODE GEOMETRY AND SIZE
1.3.3 V
When detecting human contact (e.g. a fingertip), grounding of the person is never required, nor is it necessary to touch an exposed metal electrode. The human body naturally has several hundred picofarads of ‘free space’ capacitance to the local environment (Cx3 in Figure 1-4), which is more than two orders of magnitude greater than that required to create a return path to the QT310 via earth. The QT310's PCB however can be physically quite small, so there may be little ‘free space’ coupling (Cx1 in Figure 1-4) between it and the environment to complete the return path. If the QT310 circuit ground cannot be grounded via the supply connections, then a ‘virtual capacitive ground’ may be required to increase return coupling.
IRTUAL CAPACITIVE GROUNDS
Figure 1-4 Kirchoff’s Current Law
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Figure 1-5 Shielding Against Fringe Fields
A ‘virtual capacitive ground’ can be created by connecting the QT310’s own circuit ground to:
(1) A nearby piece of metal or metallized housing; (2) A floating conductive ground plane; (3) A fastener to a supporting structure; (4) A larger electronic device (to which its output might be
connected anyway).
Because the QT310 operates at a relatively low frequency, about 500kHz, even long inductive wiring back to ground will usually work fine.
Free-floating ground planes such as metal foils should maximise exposed surface area in a flat plane if possible. A square of metal foil will have little effect if it is rolled up or crumpled into a ball. Virtual ground planes are more effective and can be made smaller if they are physically bonded to other surfaces, for example a wall or floor.
of the QT310, sensitivity can be high enough (depending on Cx and Cs) that 'walk-by' signals are a concern; if this is a problem, then some form of rear shielding may be required.
1.4 SENSITIVITY ADJUSTMENTS
There are three variables which influence sensitivity:
1. Cs (sampling capacitor)
2. Cx (unknown capacitance)
3. Signal threshold value
There is also a sensitivity dependence of the whole device on Vdd. Cs and Cx effects are covered in Section 1.2.1.
The threshold setting can be adjusted independently from 1 to 255 counts of signal swing (Section 2.3).
Note that sensitivity is also a function of other things like electrode size, shape, and orientation, the composition and aspect of the object to be sensed, the thickness and composition of any overlaying panel material, and the degree of mutual coupling of the sensor circuit and the object (usually via the local environment, or an actual galvanic connection).
Threshold levels of less than 5 counts in BG mode are not advised; if this is the case, raise Cs so that the threshold can also be increased.
1.4.1 I
NCREASING SENSITIVITY
In some cases it may be desirable to greatly increase sensitivity, for example when using the sensor with very thick panels having a low dielectric constant, or when sensing low capacitance objects.
Sensitivity can be increased by using a bigger electrode, reducing panel thickness, or altering panel composition. Increasing electrode size can have diminishing returns, as high values of Cx load will also reduce sensor gain (Figures 5-1 and 5-2). The value of Cs also has a dramatic effect on sensitivity, and this can be increased in value up to a limit.
Increasing electrode surface area will not substantially increase sensitivity if its area is already larger than the object to be detected. The panel or other intervening material can be made thinner, but again there are diminishing rewards for
1.3.4 F
The electrode can be prevented from sensing in undesired directions with the assistance of metal shielding connected to circuit ground (Figure 1-5). For example, on flat surfaces, the field can spread laterally and create a larger touch area than desired. To stop field spreading, it is only necessary to surround the touch electrode on all sides with a ring of metal connected to circuit ground; the ring can be on the same or opposite side from the electrode. The ring will kill field spreading from that point outwards.
If one side of the panel to which the electrode is fixed has moving traffic near it, these objects can cause inadvertent detections. This is called ‘walk-by’ and is caused by the fact that the fields radiate from either surface of the electrode equally well. Again, shielding in the form of a metal sheet or foil connected to circuit ground will prevent walk-by; putting a small air gap between the grounded shield and the electrode will keep the value of Cx lower and is encouraged. In the case
IELD SHAPING
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Figure 1-6 Burst Detail
4 QT310/R1.03 21.09.03
Figure 1-7 Burst when SC is set to 1
(Observed using a 750K resistor in series with probe)
Figure 1-8 Burst when SC is set to 0 (no sleep cycles)
(Observed using a 750K resistor in series with probe)
doing so. Panel material can also be changed to one having a higher dielectric constant, which will help propagate the field. Locally adding some conductive material to the panel (conductive materials essentially have an infinite dielectric constant) will also help; for example, adding carbon or metal fibers to a plastic panel will greatly increase frontal field strength, even if the fiber density is too low to make the plastic electrically conductive.
1.4.2 D
In some cases the circuit may be too sensitive, even with high signal threshold values. In this case gain can be lowered by making the electrode smaller, using sparse mesh with a high space-to-conductor ratio (Figure 1-3), and most importantly by decreasing Cs. Adding Cx capacitance will also decrease sensitivity.
It is also possible to reduce sensitivity by making a capacitive divider with Cx by adding a low-value capacitor in series with the electrode wire.
ECREASING SENSITIVITY
1.5 TIMING
Figure 1-7 and 1-8 shows the basic timing parameters of the QT310. The basic QT310 timing parameters are:
Tbd Burst duration (1.5.1) Tbs Burst spacing (1.5.2) Tsc Sleep Cycle duration (1.5.2) Tmod Max On-Duration (1.5.3) Tdet Detection response time (1.5.4)
1.5.1 B
The burst duration depends on the values of Cs and Cx, and to a lesser extend, Vdd. The burst is normally composed of hundreds of charge-transfer cycles (Figure 1-6) operating at about 240kHz. This frequency varies by about ±7% during the burst in a spread-spectrum modulation pattern. See Section
3.5.2 page 13 for more information on spread-spectrum.
URST FREQUENCY AND DURATION
The number of pulses in a burst and hence its duration increases with Cs and decreases with Cx.
1.5.2 B
Between acquisition bursts, the device can go into a low power sleep mode. The duration of this is a multiple of Tsc, the basic sleep cycle time. Tsc depends heavily on Vdd as shown in Figure 5-4, page 16. The parameter SC calls out how many of these cycles are used. More SC means lower power but also slower response time.
Tbs is the spacing from the start of one burst to the start of the next. This timing depends on the burst length Tbd and the dead time between bursts, i.e. Tsc.
The resulting timing of Tbs is:
If SC = 0, the device never sleeps between bursts (example: Figure 1-8). In this case the value of Tsc is fixed at about
2.25ms, but this time is not spent in Sleep mode and maximal power is consumed.
if SC >> 0 (example: SC=15), the device will spend most of its time in sleep mode and will consume very little power, but it will be much slower to respond.
By selecting a supply voltage and a value for SC, it is possible to fine-tune the circuit for the desired speed / power trade-off.
1.5.3 MAX ON-D
The Max On-Duration is the amount of time required for sensor to recalibrate itself when continuously detecting. This parameter is user-settable by changing MOD and SC (see Section 2.6).
Tmod restarts if the sensor becomes inactive before the end of the Max On Duration period.
URST SPACING
Tbs = Tbd + (SC x Tsc) where SC > 0
-or­Tbs = Tbd + 2.25ms where SC = 0
: TBS, T
URATION
, T
SC
MOD
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1.5.4 R
Response time Tdet from the onset of detection to the OUT pin becoming active depends on:
If the control bit DIS is normal (0), then Tdet depends on the rate at which the bursts are acquiring, and the value of DIT. A DIT number of bursts must confirm the detection before the OUT line becomes active:
If DIS is set to ‘fast’, then Tdet is computed as:
Quantum’s QT3View software calculates an estimate of response time based on this formula.
ESPONSE TIME
Tbs Burst spacing (Section 1.5.2) DIT Detection Integrator Target (user setting) DIS Detect Integration Speed (user setting) Tbd Burst duration (if DIS is set to ‘fast’)
Tdet = Tbs x DIT (normal DIS)
Tdet = (SC x Tsc) + (DIT x (Tbd + 2.25ms)) (fast DIS)
, T
DET
1.6 EXTERNAL RECALIBRATION
The /CAL_CLR pin can be used to recalibrate the sensor on demand. A low pulse of at least Tbs (burst spacing) duration is require to initiate a recalibration. The calibration occurs just after /CAL_CLR returns high.
In BG1 mode (Section 2.8.4), the calibration data is not stored in EEPROM, and the part will recalibrate after each power up.
In BG1 mode, if the device has been set for Toggle Latch output mode, the /CAL_CLR pin becomes an output reset control and the part cannot be recalibrated via /CAL_CLR. However the part can be recalibrated by powering it down and back up again (Section 2.7.3).
In BG2 mode, the calibration data is stored in EEPROM, and the part will not recalibrate after power up, using instead the stored calibration data. The internal eeprom has a life expectancy of 100,000 erase/write cycles.
In OBJ mode, the part stores the calibration data into EEPROM and the part will not recalibrate after power up, using instead the stored calibration data.
In both BG2 and OBJ mode, the device must be calibrated using the /CAL_CLR input, or the calibration data can be set via cloning process, otherwise the calibration data will be invalid.
are crowded together with a rep rate that depends entirely on the burst lengths (Section 1.5.1).
Response time, drift compensation rate, max on-duration, and power consumption are all affected by this parameter. A high value of SC will allow the device to consume very low power but it will also be very slow.
2.2 DRIFT COMPENSATION (PDC, NDC)
Signal drift can occur because of changes in Cx, Cs, Vdd, electrode contamination and ageing effects. It is important to compensate for drift, otherwise false detections and sensitivity shifts can occur.
Drift compensation is performed by making the signal’s reference level slowly track the raw signal while no detection is in effect. The rate of adjustment must be performed slowly, otherwise legitimate detections could be affected. The device compensates using a slew-rate limited change to the signal reference level; the threshold and hysteresis points are slaved to this reference.
Once an object is detected, drift compensation stops since a legitimate signal should not cause the reference to change.
Positive and negative drift compensation rates (PDC, NDC) can be set to different values (Figure 2-1). This is invaluable for permitting a more rapid reference recovery after the device has recalibrated while an object was present and then removed.
Positive drift occurs when the Cx slowly increases. Negative drift occurs when Cx slowly decreases (see Section 2.8.1).
PDC+1 sets the number of burst spacings, Tbs, that determines the interval of drift compensation, where:
Tbs = Tbd + (SC x Tsc)
where SC > 0 (Section 1.5.2)
-or-
Tbs = Tbd + 2.25ms
where SC = 0 (Section 1.5.2)
Example: PDC = 9, (user setting)
Tbs = 100ms
then
Tpdc = (9+1) x 100ms = 1 sec
NDC operates in exactly the same way as PDC.
2 - Control & Processing
All acquisition functions are digitally controlled and can be altered via the cloning process.
Signals are processed using 16 bit integers, using Quantum-pioneered algorithms specifically designed to provide for high survivability.
2.1 SLEEP CYCLES (SC)
Range: 0..255; Default: 1 Affects speed & power of entire device.
Refer to Section 1.5.2 for more information on the effect of Sleep Cycles.
SC changes the number of intervals Tsc separating two consecutive burst (Figure 1-7 and 1-8). SC = 0 disables sleep intervals and bursts
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Figure 2-1 Drift Compensation
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