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PDF QT301 Data sheet ( Hoja de datos )
| Número de pieza | QT301 | |
| Descripción | CAPACITANCE TO DIGITAL CONVERTER | |
| Fabricantes | QUANTUM | |
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QT301
CAPACITANCE TO ANALOG CONVERTER
Capacitance to Analog Converter (CAC) IC
Patented charge-transfer conversion method
Sub-ranging Direct-to-Analog conversion
Rescaleable PWM: wide dynamic range
End-to-end calibration (gain, span) via CAL pins
100 kHz PWM
Spread spectrum acquisition bursts for low noise
Sample on demand via Sync pin
Only one external sample capacitor
SYNC
CAL_DN
SNS1
VSS
1
2
3
4
8 VDD
7 CAL_UP
6 PWM
5 SNS2
APPLICATIONS
Fluid level sensors
Proximity sensors
Moisture detection
Position sensing
Transducer driver
Material sensors
The QT301 charge-transfer (QT) IC is a self-contained Capacitance-to-Analog-Converter (CAC) capable of detecting
femotofarad level changes in capacitance. This part is designed primarily for stand-alone instrumentation applications.
Primary applications include fluid level sensors, distance sensors, material detectors, transducer amplifiers for pressure and
humidity sensing functions, and other uses requiring quantified capacitance data.
Unlike other Quantum products, the QT301 does not process its acquired data. Its only output is raw, unprocessed data in
filterable PWM form that can be translated into an analog voltage by a simple RC network. This allows the designer to treat
the device as a CAC for measurement applications.
The PWM range is set via two inputs that control the starting and ending point of the conversion range. For example, if the
capacitance range of Cx is from 27pF to 38pF, the QT301 can be calibrated so that the PWM zero point occurs at 27pF, and
the endpoint (255) occurs at 38pF. In this way, the PWM range is optimized for the zone of interest. These calibration points
are stored in internal EEPROM and do not have to be reacquired after a power reset. This means that the resolution of the
part can be compared easily to other methods that might otherwise require 12 or more bits of overall resolution.
The device operates on demand via a sync input pin. The sync input can also be used to avoid external noise sources and
cross-interference from adjacent QRG capacitive sensors. Unique among capacitance sensors, this device features
spread-spectrum burst modulation, permitting extremely high noise rejection characteristics for very robust signals even in
high EMI environments.
The device requires only a single sampling capacitor (Cs) to acquire signals. The value of this capacitor controls the gain of
the sensor, and it can be adjusted over 2½ decades of range from 1nF to 500nF. No external switches, opamps, or other
components are required.
AVAILABLE OPTIONS
TA
00C to +700C
-400C to +850C
SOIC
-
QT301-IS
8-PIN DIP
QT301-D
-
LQ
Copyright © 2003 QRG Ltd
QT301 R1.06 12/03
1 page  
The CAL_UP pin should be used to calibrate the signal when
the electrode is at its maximum useable level of Cx, for
example with a level probe when the fluid is at the top.
It does not matter whether CAL_DN or CAL_UP are applied
first. After calibration is complete, either CAL_DN or CAL_UP
can be asserted again to obtain a fresh calibration for the
corresponding end point, without affecting the other end
point.
4.2 Calibration Process
The CAL pins are inputs used to trigger a CAL process on
the upper (max Cx) or lower (min Cx) capacitance endpoints.
These pins must be pulled low via a pulldown resistor on
each, to prevent damage.
Figure 4-1 Calibration Process
User sets CAL_DN
high
User sets CAL_UP
high
2.5ms Delay
2.5ms Delay
CAL_DN forced high
by QT301
CAL_UP forced high
by QT301
Calibration starts
Calibration starts
To calibrate either endpoint, assert either CAL pin high using
an open-source output from a mosfet or microcontroller, or, a
collector from a PNP transistor whose emitter is connected to
Vdd. Hold this level high for 2.5ms minimum (preferably, 3ms
to be safe). Then release the pin to try to float down.
User floats CAL_DN
NO
QT301 Cal done?
User floats CAL_UP
NO
QT301 Cal done?
The QT301 will continue to hold the pin high starting at the
2.5ms point. There should be no contention problem with an
external voltage plus the QT301 both holding this pin high.
When the QT301 is done calibrating, it will release the CAL
pin in question to float low. A host controller can use this
feature to check when the calibration process has completed.
YES
QT301 floats
CAL_DN pin again
YES
QT301 floats
CAL_UP pin again
Calibration takes 15 acquisition burst samples to complete.
The new calibration data is stored in internal EEPROM when
the host releases the CAL pin to float low again; the chip also
begins to operate normally again at this time.
Figure 4-1 shows the control flows for calibration.
The capacitive signal on the electrode should be as stable
and noise-free as possible during the CAL intervals to ensure
accurate calibration points.
During a CAL cycle, the PWM output functions normally
using the last known good calibration data and signal value.
The PWM output will change again only when the CAL
process is complete.
Note: The CAL pins should never be driven low. Driving
either of the CAL pins low will short circuit the chip.
5 - CIRCUIT GUIDELINES
5.1 Sample Capacitor
The charge sampler capacitor (Cs) can be virtually any
plastic film or low to medium-K ceramic capacitor. The
acceptable Cs range is from 1nF to 500nF depending on the
sensitivity required; larger values of Cs demand higher
stability to ensure reliable sensing. Acceptable capacitor
types include plastic film (especially PPS film) and NP0/C0G
ceramic. X7R ceramic can also be used but this type is less
stable over temperature.
5.2 Power supply, PCB Layout
The QT301 makes use of the power supply as a reference
voltage. The acquired signal will shift slightly with changes in
VDD; fluctuations in VDD often happen when additional loads
are switched on or off such as LEDs etc.
Care should be taken when designing the power supply, as
any change in VDD will affect the PWM level.
If the power supply is shared with another electronic system,
make sure the supply is free of spikes, sags, and surges.
The supply is best locally regulated using a conventional
78L05 type regulator, or almost any 3-terminal LDO device
from 3V to 5V.
For proper operation, a 0.1µF or greater bypass capacitor
must be used between VDD and VSS; the bypass cap should
be placed very close to the device pins. The PCB should if
possible include a copper pour under and around the IC, but
not extensively under the SNS pins or lines.
5.3 ESD Protection
In cases where the electrode is 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
electrodes 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 SNS pins that absorb
most induced discharges (up to 20mA), and protect the
device. The usefulness of the internal clamping will depend
on the dielectric properties, panel thickness, and rise time of
the ESD transients. In extreme cases, ESD dissipation can
be aided further by adding a resistor in series with the
electrode.
The charge pulse can be a minimum of 1µs and therefore the
circuit can tolerate values of series-R up to 18k in cases
where electrode Cx load is below 10pF. Extra diode
protection may be used at the electrodes but this often leads
to additional RFI problems as the diodes will rectify RF
signals into DC; this will disturb the sensing signals.
Series-R’s should be low enough to permit at least six RC
time-constants (i.e. a net RC timeconstant of 1/6 µs) to occur
during the charge pulse, where R is the added series-R and
LQ
5
QT301 R1.06 12/03
5 Page 
lQ
Copyright © 2003 QRG Ltd. All rights reserved.
Patented and patents pending
Corporate Headquarters
1 Mitchell Point
Ensign Way, Hamble SO31 4RF
Great Britain
Tel: +44 (0)23 8056 5600 Fax: +44 (0)23 8045 3939
www.qprox.com
North America
651 Holiday Drive Bldg. 5 / 300
Pittsburgh, PA 15220 USA
Tel: 412-391-7367 Fax: 412-291-1015
The specifications set out in this document are subject to change without notice. All products sold and services supplied by QRG are subject
to our Terms and Conditions of sale and supply of services which are available online at www.qprox.com and are supplied with every order
acknowledgement. QProx, QTouch, QMatrix, QLevel, and QSlide are trademarks of QRG. QRG products are not suitable for medical
(including life-saving equipment), safety or mission critical applications or other similar purposes. Except as expressly set out in QRG's
Terms and Conditions, no licenses to patents or other intellectual property of QRG (express or implied) are granted by QRG in connection
with the sale of QRG products or provision of QRG services. QRG will not be liable for customer product design and customers are entirely
responsible for their products and applications which incorporate QRG's products.
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| Páginas | Total 11 Páginas | |
| PDF Descargar | [ Datasheet QT301.PDF ] | |
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