ETC CAV424

Converter IC for Capacitive Signals
CAV424
FEATURES
GENERAL DESCRIPTION
• Ratiometric Supply Voltage: 5V ± 5%
• Wide Operating Temperature Range:
–40°C...+85°C
• High Detection Sensitivity of Relative
Capacitive Changes: 5% – 100%
• Detection Frequency up to 2kHz
• Differential Output Signal with Great
Voltage Swing
• Integrated Temperature Sensor
• Adjustable with only two Resistors
The CAV424 is an integrated C/V converter and
contains the complete signal processing unit for
capacitive signals on chip. The CAV424 detects
the relative capacitive change of a measuring
capacity to a fixed reference capacity. The IC is
optimised for capacities in the wide range of
10pF to 2nF with possible changes of capacity of
5% to 100% of the reference capacity. The differential voltage output signal can be directly
connected to a following A/D converter or another signal conditioning IC from Analog Microelectronics. Using the integrated temperature
sensor, digital adjustable systems can be built
easily.
APPLICATIONS
•
•
•
•
•
Industrial Process Control
Distance Measurement
Pressure Measurement
Humidity Measurement
Level Control
DELIVERY
• DIL16 packages
• SO16(n) packages
• Dice put on 5“ blue foil
BLOCK DIAGRAM
RCX1 RCX2 RCOSC
VTEMP
7
3
2
1
VCC
CAV424
11
T Sensor
Current Reference
COSC
12
Reference
Oscillator
6
VM
CX1
16
Signal Conditioning
Integrator 1
CX2
14
Integrator 2
5
LPOUT
10
GND
15
CL1
13
CL2
4
RL
Figure 1: block diagram CAV424
analog microelectronics
Analog Microelectronics GmbH
An der Fahrt 13, D – 55124 Mainz
Internet: http://www.analogmicro.de
Phone: +49 (0)6131/91 073 – 0
Fax:
+49 (0)6131/91 073 – 30
E–mail: [email protected]
January 2002
1/7
Rev. 1.3
Converter IC for Capacitive Signals
CAV424
ELECTRICAL SPECIFICATIONS
Tamb = 25°C, VCC = 5V (unless otherwise noted)
Parameter
Symbol
Conditions
Min.
Typ.
Max.
Unit
4.75
5.00
5.25
V
17
V
1.4
mA
Supply
Supply Voltage
VCC
Maximum Supply Voltage
VCCmax
Quiescent Current
ICC
ratiometric range
Tamb = –40 ... 85°C, GLP = 1
0.6
1.0
Temperature Specifications
Operating
Tamb
–40
85
°C
Storage
Tst
–55
125
°C
Junction
Tj
Thermal Resistance
Θja
DIL16 plastic package
70
150
°C/W
°C
Θja
SO16 (n) plastic package
140
°C/W
Oscillator Capacitor Range
COSC
COSC = 1.6 ⋅ CX1
Oscillator Frequency Range
fOSC
Oscillator Current
IOSC
Reference Oscillator
14
1800
pF
1
130
kHz
10.75
µA
1000
pF
5.38
µA
ROSC = 250kΩ
9.5
10
Capacitive Integrator 1 and 2
Capacitor Range 1
CX1
10
Capacitive Integrator Current 1
IX1
RCX1 = 500kΩ
4.75
Capacitor Detection Sensitivity
∆ CX
∆ CX = (CX2 − CX1 )/CX1
5
100
%
Capacitor Range 2
CX2
CX2 = CX1 ⋅ (1 + ∆ CX )
10.5
2000
pF
Capacitive Integrator Current 2
IX2
RCX2 = 500kΩ
4.75
Detection Frequency
fDET
CL1 = CL2 =1nF
5
5
5.38
µA
2
kHz
Lowpass
Adjustable Gain
GLP
Output Voltage
VLPOUT
Corner Frequency 1
fC1
R01 = 20kΩ, CL1 =1nF
Corner Frequency 2
fC2
R02 = 20kΩ, CL2 =1nF
Resistive Load at PIN LPOUT
RLOAD
Capacitive Load at PIN LPOUT
CLOAD
Temperature Coefficient VDIFF (together
with Input Stages)
dVDIFF /dT
Internal Resistor 1 and 2
R01, R02
Temperature Coefficient R01,02
dR01,02 /dT
Ratiometric Error of VLPOUT
[email protected]*
1
10
1.1
VCC – 1.1
V
8
kHz
8
kHz
200
kΩ
50
VDIFF = VLPOUT - VM ,
Tamb = –40 ... 85°C
Tamb = –40 ... 85°C
pF
±100
ppm/°C
20
kΩ
1.9
10-3/°C
0.11
%FS
* RAT @ VDIFF = 2 [1.05 VDIFF(VCC = 5V) – VDIFF(VCC = 5.25V)]/[VDIFF(VCC = 5V) + VDIFF(VCC = 5.25V)]
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Converter IC for Capacitive Signals
Parameter
Symbol
Conditions
CAV424
Min.
Typ.
Max.
Unit
Voltage Reference VM
Voltage
VM
VM vs. Temperature
dVM /dT
Tamb = –40...+85°C
Current
IVM
Source
IVM
Sink
Load Capacitance
CVM
Ratiometric Error of VM
[email protected]**
2.5
V
±20
80
100
±50
ppm/°C
16
µA
–16
µA
120
nF
0.007
%FS
Temperature Sensor VTEMP
Voltage
VTEMP
RTEMP ≥ 50MΩ
Sensitivity
dVTEMP/dT
RTEMP ≥ 50MΩ
Thermal Nonlinearity
2.20
2.32
RTEMP ≥ 50MΩ, end point method
2.45
V
8
mV/°C
0.5
%FS
** RAT @ VM = 2 [1.05 VM(VCC = 5V) – VM(VCC = 5.25V)]/[VM(VCC = 5V) + VM(VCC = 5.25V)]
Note:
1) The oscillator capacity has to be chosen in the following way: COSC = 1.6 ⋅ CX1
2) The capacitor range of CX1 and CX2 can be extended whereby the system performance is reduced and the electrical limits
are exceeded.
3) Currents flowing into the IC, are negative.
4) RTEMP is the minimum load resistance at pin VTEMP
BOUNDARY CONDITIONS
Parameter
Symbol
Min.
Typ.
Max.
Unit
Current Definition of Ref. Oscillator
RCOSC
235
250
265
kΩ
Current Adjustment of Cap. Integrator 1
RCX1
475
500
525
kΩ
500
Current Adjustment of Cap. Integrator 2
RCX2
475
RL1 + RL2
90
Reference Voltage 2.5V (only for internal use)
CVM
80
100
Lowpass Capacitance 1
CL1
100⋅CX1
200⋅CX1
Lowpass Capacitance 2
CL2
100⋅CX1
200⋅CX1
Oscillator Capacitance
COSC
COSC =1.55⋅CX1
COSC =1.60⋅CX1
Output Stage Resistor Sum
525
kΩ
200
kΩ
120
nF
COSC =1.65⋅CX1
Note: The system performance over temperature forces that the resistors RCX1, RCX2 and ROSC have the same temperature coefficient and a very close placement of them in the circuit. The capacities CX1, CX2 and COSC are also forced to have
the same temperature coefficient and a very close placement of them in the circuit.
FUNCTIONAL DESCRIPTION
The CAV424 functions according to the following principle. A variable reference oscillator, whose frequency is set via capacitance COSC, drives two symmetrical integrators which are phase-locked and
clock-synchronised. The amplitudes of the two driven integrators are determined by capacitances CX1
and CX2, where CX1 is designated as the (measurement signal) reference capacitance and CX2 as the
measurement signal capacitance. With high common-mode rejection ratio and a high resolution, com-
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Converter IC for Capacitive Signals
CAV424
parison of the two amplitudes proV
duces a signal which corresponds to
V
the change in capacitance of CX1 and
CX2 relative to one another. This difference signal is rectified in an ensuing low pass. The filtered DC signal
V
is transferred to the differential, adjustable output stage. Individual circuit variables, such as filter constants
T
Time t
3T
T
2T
and amplification, can be set with just
2
4
a few external components. By using
Figure 2: oscillator voltage curve
the integrators and their capacitances
CX1 and CX2, swings in capacitance of 5% to 100% in relation to the measurement reference capacitance
can be measured. As CX1 can be varied in a range of 10 pF to 1 nF, the range of measurement for the
measurement signal capacitance is 0-10.5 pF to 0-2 nF.
OSC
OSC,HIGH
OSC,LOW
The way a capacitive sensor functions whose signal can be conditioned with a CAV424 is described in
detail in the following section. Simple dimensional requirements are given, permitting a sensor system
to be assembled.
The CAV424 reference oscillator
The reference oscillator charges up
and then discharges the external
oscillator capacitance COSC, the
internal parasitic capacitance of the
IC, COSC,PAR,INT, and the external
parasitic capacitance COSC,PAR,EXT
(from a printed board assembly, for
example). Oscillator capacitance COSC
is dimensioned as follows:
VOSC
VCX1
VCX2
VCLAMP
T
2
COSC = 1.6 ⋅ C X 1 ,
3T
4
T
2T
Time t
Figure 3: integrator voltage curve
where CX1 is the fixed capacitance
(reference capacitance) of a capacitive sensing element.
The reference oscillator current IOSC is determined via external resistance ROSC and reference voltage
VM:
I OSC =
VM
ROSC
The frequency of the reference oscillator fOSC is given by
f OSC =
I OSC
2 ⋅ ∆VOSC ⋅ (COSC + COSC , PAR , INT + COSC , PAR , EXT )
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Converter IC for Capacitive Signals
CAV424
where ∆VOSC is the difference between the thresholds (VOSC,HIGH and VOSC,LOW) of the internal reference
oscillator. ∆VOSC is defined via internal resistances and has a value of 2.1V @ VCC = 5V. The oscillator
voltage curve is shown in Figure 2.
Capacitive integrators
The built-in capacitive integrators function in much the same way as the reference oscillator. One
difference lies in the discharge time, which here is twice as long as the charge-up period. Furthermore,
the discharge voltage is clamped to an internal fixed voltage, VCLAMP. The signal voltage of capacitances
CX1 and CX2 is outlined in Figure 3.
The capacitive integrator current ICX is set by external resistance RCX and reference voltage VM:
V
I CX = M
RCX
Capacitance CX is charged up to maximum voltage VCX and can be calculated as follows:
I CX
VCX =
+ VCLAMP
2 ⋅ f OSC ⋅ (C X + C X , PAR , INT + C X , PAR , EXT )
The two voltages across capacitances CX1 and CX2 are subtracted from one another. Applied to the
reference voltage VM the resulting differential voltage is:
VCX ,DIFF = (VCX 1 − VCX 2 ) + V M
Differential voltage VCX,DIFF is applied to a second-order low-pass filter. The 3dB cut-off frequencies
of the two stages, fC1 and fC2, are defined by external capacitances CL1 and CL2 and internal resistances
R01 and R02 (typically 20kΩ). The 3dB cut-off frequencies must be selected with regard to the reference
oscillator frequency fOSC and the required detection frequency of the overall sensor system (fDET). Here,
the following inequality of the various frequencies must be adhered to:
f DET < f C << f OSC
The external capacitance for the required cut-off frequency fC amounts to
1
CL =
2π ⋅ R0 ⋅ f C
The output signal of the low-pass filter tracing the ideal curve shown in Figure 3 is calculated as
3
VLPOUT = VDIFF ,0 + VM
with
VDIFF , 0 = ⋅ (VCX 1 − VCX 2 )
8
Should the differential output voltage VDIFF,0 be too small it can be amplified using the non-inverting
output amplifier, with the degree of amplification being determined by resistances RL1 and RL2.
The amplification of the stage is
R
GLP = 1 + L1
RL 2
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Converter IC for Capacitive Signals
CAV424
It thus follows that the output signal of the low-pass stage is
VLPOUT = VDIFF + VM
with
3
VDIFF = GLP ⋅ VDIFF ,0 = GLP ⋅ ⋅ (VCX 1 − VCX 2 )
8
In order to reduce the number of external components needed for the sensor system a temperature
acquisition sensor was integrated. With the aid of a processor, this sensor can be used to compensate for
the temperature error of the entire sensor system, for example.
FUNCTIONAL DIAGRAM
RCX1
7
ROSC
RCX2
3
2
1
VCC
CAV424
11
T Sensor
Current Reference
COSC
12
Reference
Oscillator
16
Integrator 1
14
Integrator 2
6
CVM
Signal Conditioning
VDIFF
CX1
5
CX2
10
15
CL1
13
CL2
4
RL1
RL2
GND
Figure 4: functional diagram CAV424
Adjustment:
The zero-adjustment is made by the resistors RCX1 or RCX2 for the case that the varying capacitance CX2
has nearly the same (and its smallest) value as the fixed capacitance CX1 (reference capacitance). Therefore one of this resistors is varied until the differential voltage
VDIFF = VLPOUT − VM
is zero:
VDIFF = 0
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Converter IC for Capacitive Signals
CAV424
Application Example:
The following values are given:
• fixed capacitance CX1:
50pF
• varying capacitance CX2:
50 ... 100pF
Calculation:
With the equations given in the boundary conditions, the following values for the devices can be calculated:
• COSC:
80pF
• CL1:
10nF
• CL2:
10nF
PINOUT
RCOSC
1
16
CX1
RCX1
2
15
CL1
RCX2
3
14
CX2
RL
4
13
CL2
LPOUT
5
12
COSC
VM
6
11
VTEMP
7
10
VCC
GND
N.C.
8
9
N.C.
Figure 5: pinout CAV424
DELIVERY
PIN
NAME
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
RCOSC
RCX1
RCX2
RL
LPOUT
VM
VTEMP
N.C.
N.C.
GND
VCC
COSC
CL2
CX2
CL1
CX1
DESRIPTION
Current Definition of Ref. Oscillator
Current Adjustment of Cap. Integrator 1
Current Adjustment of Cap. Integrator 2
Gain Adjustment
Output
Reference Voltage 2.5V
Temperature Sensor
Not Connected
Not Connected
IC Ground
Supply Voltage
Capacitor of Reference Oscillator
Corner Frequency of Lowpass 2
Integrator Capacitor 2
Corner Frequency of Lowpass 1
Integrator Capacitor 1
The CAV424 is available in version:
• 16 pin DIL
• SO 16 (n) (maximum power dissipation PD = 300mW)
• Dice on 5“ blue foil
The information provided herein is believed to be reliable; however, Analog Microelectronics assumes no responsibility for inaccuracies or omissions. Analog Microelectronics assumes no responsibility for the
use of this information, and all use of such information shall be entirely at the user's own risk. Prices and specifications are subject to change without notice. No patent rights or licences to any of the circuits
described herein are implied or granted to any third party. Analog Microelectronics does not authorise or warrant any Analog Microelectronics product use in life support devices and/or systems.
analog microelectronics
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