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參數(shù)資料
| 型號(hào)： | QT301 |
| 廠商： | Electronic Theatre Controls, Inc. |
| 英文描述： | CAPACITANCE TO ANALOG CONVERTER |
| 中文描述： | 電容模擬轉(zhuǎn)換器 |
| 文件頁(yè)數(shù)： | 4/10頁(yè) |
| 文件大?。?/td> | 247K |
| 代理商： | QT301 |
3 PWM Output
The PWM output is a 100KHz ±7% square wave.
The PWM can be filtered using a simple RC circuit, or fed
directly into a timer circuit that can measure its duty cycle
be at least 10K ohms to reduce pin loading errors.
The PWM duty cycle is defined as follows:
D
PWM
=
T
PWM
_
high
T
PWM
_
Period
If an RC circuit is used, it is often best to put a voltage
follower circuit on the output of the filter to buffer the output
voltage (Figure 2-6).
Note that the PWM output is not linear with changes in Cx
capacitance from end to end. The transfer function for the
QT301 is a logarithmic response (Section 2.3).
During CAL, the PWM output value is locked in place with
the value just prior to when the CAL process was triggered.
Only after CAL is complete is the PWM updated with the new
results.
4 Calibration
The QT301 should be calibrated end to end to have an
effective, properly scaled PWM output. The calibration is
done on a ‘learn by example’ basis. Each end is calibrated
separately while the appropriate end-point signal level is
applied. After the Cal process, the PWM signal will scale
itself to reflect these endpoints with the best resolution
possible.
4.1 Calibration Pins
The CAL_DN pin should be used to calibrate the signal when
the electrode is at its lowest level of Cx, for example with a
level probe when the fluid is at a minimum.
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.
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.
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.
L
Q
4
QT301 R1.04 21/09/03
Figure 2-6 Line Sync and PWM Output Filter
7
1
2
8
6
CAL_UP
PWM
SNS2
SNS1
VDD
5
3
SYNC
VSS
+3 to 5.5V
Electrode
Cs
Cx
CAL_DN
Rs
0.1uf
4
10K
10K
Upper Cx Cal
Lower Cx Cal
Line
Frequency
20pF
100K
0.1uF
IN4148
1M
Analog
Output
Figure 4-1 Calibration Process
CAL_DN forced high
by QT301
User sets CAL_DN
high
2.5ms Delay
Calibration starts
User floats CAL_DN
QT301 Cal done
NO
YES
QT301 floats
CAL_DN pin again
CAL_UP forced high
by QT301
User sets CAL_UP
high
2.5ms Delay
Calibration starts
User floats CAL_UP
QT301 Cal done
NO
YES
QT301 floats
CAL_UP pin again
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