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Número de pieza QT310-IS
Descripción PROGRAMMABLE CAPACITANCE SENSOR IC
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QT310-IS datasheet

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QT310-IS pdf
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 DECREASING SENSITIVITY
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.
1.5 TIMING
Figure 1-7 and 1-8 shows the basic timing parameters of the
QT310. The basic QT310 timing parameters are:
Tbd
Tbs
Tsc
Tmod
Tdet
Burst duration
Burst spacing
Sleep Cycle duration
Max On-Duration
Detection response time
(1.5.1)
(1.5.2)
(1.5.2)
(1.5.3)
(1.5.4)
The number of pulses in a burst and hence its duration
increases with Cs and decreases with Cx.
1.5.2 BURST SPACING: TBS, TSC
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:
Tbs = Tbd + (SC x Tsc)
-or-
Tbs = Tbd + 2.25ms
where SC > 0
where SC = 0
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.
1.5.1 BURST FREQUENCY AND DURATION
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.
1.5.3 MAX ON-DURATION, TMOD
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.
LQ
5 QT310/R1.03 21.09.03

5 Page

QT310-IS arduino
In addition, the OUT pin can be made either active low or
active high (Section 2.7.1).
2.8.4.1 BG1 Mode (volatile reference)
In BG1 mode, the reference is set via recalibration initiated
using the /CAL_CLR pin or on power-up. The resulting
reference level is not stored into EEPROM. Max On-Duration
and drift compensation are able to function normally.
BG1 mode is useful when the signal can change slightly over
time and temperature, and it is useful to track these changes
without a loss of sensitivity.
2.8.4.2 BG2 Mode (stored reference)
In BG2 mode, the reference level is fixed and stored in
internal EEPROM. Drift compensation (Section 2.2) can be
used, but changes to the reference due to drift compensation
are not updated to EEPROM. Max On-Duration can also be
enabled (Section 2.6); if a MOD timeout occurs, the new
reference will be stored in EEPROM.
The reference is normally set during recalibration when the
/CAL_CLR pin pulses low (Section 1.6); the resulting
reference value is then stored in EEPROM. At power-up the
part automatically restores this reference level and runs
without another recalibration.
The reference value can also be entered numerically via the
cloning process (Table 4-1, page 14) to precisely replicate the
calibration point across many devices.
BG2 mode is useful when it is desired to lock in the reference
to prevent changes on startup, for example to replace
mechanical switches in process controls.
Vdd Open Loop
Closed Loop
Vdd
1 /CAL
U1
OUT 7
6 /SYNC_I SNS1 3
2 /SYNC_O SNS2 5
OUT1
CS1
Vdd
1 /CAL
U2
OUT 7
6 /SYNC_I SNS1 3
2 /SYNC_O SNS2 5
OUT2
CS2
Vdd
1 /CAL
Un
OUT 7
6 /SYNC_I SNS1 3
2 /SYNC_O SNS2 5
OUT_N
CS3
SENSOR 1
SENSOR 2
SENSOR N
Figure 2-3 Daisy chain wiring
2.8.5 OBJ (OBJECT) DETECTION MODE
This mode is useful to do a ‘learn by example’ calibration.
Typically, a test object is placed at the electrode in such a
way as to create a 50% signal level change relative to a
normal, full presentation of the object. The QT310 is then
calibrated in OBJ mode. Calibration in OBJ mode should
never be done with a full presentation of signal, as this will
create a marginal, unreliable detection.
This mode is suited to material detection, fluid level sensing,
and similar applications.
In OBJ mode, on calibration the current signal value is
recorded as a fixed threshold point and stored to EEPROM.
The hysteresis level is made relative to the fixed threshold,
and can be altered as with the BG modes. If hysteresis is too
large, the sensor can ‘stick’ on; hysteresis should normally be
set to a small value, just enough to prevent output chatter.
Hysteresis can also be made intentionally large, for example
for ‘bang-bang’ fluid level sensing, where an ‘upper’ level is
calibrated using OBJ, and a ‘lower’ cut-out level is defined by
the hysteresis value. The sensor must have SD = positive for
this mode (Section 2.8.2).
OBJ mode does not make use of a reference level and does
not allow drift compensation or Max On-Duration to operate.
The threshold point is fixed for all time until another
/CAL_CLR signal is received.
The OBJ threshold value can also be entered numerically via
the cloning process (Table 4-1, page 14) to precisely replicate
the threshold point across many devices.
Positive, negative detection mode behavior: In OBJ mode
OUT can be made active on either positive or negative signal
changes (Section 2.8.2). The signal direction selection affects
which side of the threshold the hysteresis level is placed after
calibration.
The OUT pin can be made either active low or active high
(Section 2.7.1).
2.9 SYNCHRONISATION
The synchronization feature allows a QT310 to generate its
burst on demand from an external trigger rather than of its
own accord. This feature is made possible by the fact that the
QT310 operates in burst mode, rather than continuously.
Sync is a powerful feature that permits two important
operating modes: Daisy-chaining, and noise synchronization.
Daisy-chaining allows several QT310 or similar devices to
coexist in close proximity to each other without cross
interference. Noise synchronization allows a QT310 to lock
onto the fundamental frequency of an external interference
source, such as 50/60Hz, to correlate the noise with the
signal and thus eliminate alias frequencies from the acquired
signal. These are extremely powerful noise reduction
methods.
The SYNC_I pin is used to trigger the QT310 to generate a
burst. The sleep timer will always wake the part if a sync
pulse has not been received before the sleep time expires.
The sleep timer is always restarted when a sync pulse is
received.
The pulse applied to SYNC_I must be normally high,
negative-going, and of >15µs pulse duration. SYNC_O emits
an 80µs pulse at the end of each burst.
LQ
10 QT310/R1.03 21.09.03

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