QT300_03 Datasheet, PDF (2/12 Pages) Quantum Research Group – CAPACITANCE TO DIGITAL CONVERTER

Table 1-1 SPI Mode Pin Description
Pin
Name
Function
1
/DRDY
Data Ready
2
SCK
Serial data clock
3
SNS1
Sense 1 line
4
VSS
Negative supply (ground)
5
SNS2
Sense 2 line
6
/REQ
Request input
7
SDO
Serial data out
8
VDD
Positive supply
DRDY
Host MicrSoCK
REQ
SDI
Vdd
100nF
QT 300 8
Vcc
1
DRDY
2
3
SCK SNS 1
6
5
REQ SNS 2
CS
7
SDO
GND
4
SENSOR
Table 1-2 1W UART Mode Pin Description
Pin
Name
Function
1
-
Connect to Vdd or Vss
2
-
Connect to Vdd or Vss
3
SNS1
Sense 1 line
4
VSS
Negative supply (ground)
5
SNS2
Sense 2 line
6
1W
1W UART Line
7
-
8
VDD
Connect to Vdd or Vss
Positive supply
Table 1-3 Alternate Cloning Pin Functions
Pin
Name
Function
2
SCK
6
SDI
Serial clone data clock
Serial clone data in
7
SDO
Serial clone data out
1 - OVERVIEW
The QT300 is a digital burst mode charge-transfer (QT)
capacitance-to-digital converter (CDC) designed for
applications requiring raw signal information such as fluid
level sensing and distance gauging; it outputs raw digital
signal data over a serial interface. The output data is in a
16-bit format; signal levels depend on load (Cx) and the
sampling capacitor value (Cs).
1.1 Basic Operation
The QT300 does no internal signal processing; data is simply
returned via one of two serial port types.
There are two basic types of serial interface: 4-wire SPI and
a simple single wire (‘1W’) UART. The SPI interface allows
multiple devices to be connected on one SPI bus, while
the1W UART requires that the controller have one dedicated
pin for each QT300. There are two types of SPI mode,
master and slave.
The type of serial port and its mode can be selected via the
cloning process using a QTM300CA programming adapter.
The QT300 operates only on request from a host device.
After initiation via a trigger signal, the QT300 generates an
acquisition burst and sends the resulting raw signal data
back via one of the serial modes.
Figure 1-1 Basic QT300 Circuit in SPI mode.
Host Micro
(1W UART)
Rx
QT300
8
1
Vdd
SNS1 ³
2
7
SNS2 ⁵
6 _1W
Vss
4
Electrode
Figure 1-2 Basic QT300 Circuit in UART mode.
1.2 CS / CX Dependency
The value returned is a direct function of Cs, the fixed sample
capacitor and Cx, the unknown or variable capacitance.
These two values influence device sensitivity and response
time, making them very important parameters.
Sensitivity is also a function of electrode size, shape,
orientation, the composition and aspect of the object being
sensed, the thickness and composition of any dielectric
overlaying the electrode, and the degree of mutual coupling
between the electrode and the object being sensed.
The response follows a logarithmic curve (Figures 7-4, 7-5,
Page 10); each doubling of Cs increases the signal level and
differential sensitivity by a factor of 2. Likewise, doubling Cx
reduces the signal level and differential sensitivity by a factor
of 2.
2 - Timing
Figure 2-1 shows the basic QT300 acquisition timing
parameters. The basic timing parameters are:
Tbd
Tacq
Tbs
Burst duration
Acquire response time
Burst Spacing
(2.1)
(2.2)
(2.3)
2.1 Tbd - Burst duration
The burst duration depends on the values of Cs and Cx and
to a lesser extent, Vdd. The burst is composed of
charge-transfer cycles operating at about 240kHz.
LQ
2
QT300 R1.01 21/09/03