Datasheet

17
Signal Chain Design Guide
Oscillator Circuits for Sensors
RC oscillators can accurately and quickly measure resistive
and capacitive sensors. The oscillator period (or frequency)
is measured against a reference clock signal, so no
analog-to-digital convertor is needed.
State-Variable Oscillators
State-variable oscillators have reliable start-up, low
sensitivity to stray capacitances and multiple output
configurations (sine wave or square wave). They can use
one or two resistive sensors, and they can use one or two
capacitive sensors.
Some of their advantages and features are:
■ Precision
■ Reliable oscillation startup
■ Sine or square wave output
■ Frequency ∝ 1/(R
1
R
2
C
1
C
2
)
1/2
Relaxation Oscillators
Relaxation oscillators have reliable start-up, low cost and
square wave output. They can use a resistive sensor or a
capacitive sensor.
Oscillator Circuits For Sensors
Oscillator Circuits for Resistive Sensors
Oscillator Circuits for Capacitive Sensors
VOUT
C2
C1
VREF
–
+
–
+
–
+
–
+
C
4
R4
R1
R2
VREF
V
REF
R3 R7 R8
V
REF
Notes: In AN895, R1 = RTDA and R2 = RTDA. A resistive divider to VDD
sets V
REF
(V
DD
/2 is recommended).
MCP6XX4 MCP6XX4 MCP6XX4
C1
VDD
–
+
R2
R3
MCP65X1
MCP65X1
U1a
U1b
U1c
U2
State Variable Oscillator:
–
+
. . .
V
REF
MCP6XX4
Relaxation Oscillator:
U1
R1
R4
VOUT
Note: In AN895, R1 = RTD.
U1d
V
OUT
C2
C1
VREF
–
+
–
+
–
+
–
+
C4
R4
R1
R2
VREF
V
REF
R3 R7 R8
V
REF
Notes: In AN895, R1 = RTDA and R2 = RTDA. A resistive divider to VDD
sets VREF (VDD/2 is recommended).
MCP6XX4 MCP6XX4 MCP6XX4
C1
VDD
–
+
R2
R3
MCP65X1
MCP65X1
U1a
U1b
U1c
U2
State Variable Oscillator:
–
+
. . .
V
REF
MCP6XX4
Relaxation Oscillator:
U1
R1
R4
VOUT
Note: In AN895, R1 = RTD.
U1d
VOUT
C2
C1
VREF
–
+
–
+
–
+
–
+
C
4
R4
R1
R2
V
REF
VREF
R3 R7 R8
VREF
Notes: A resistive divider to VDD sets VREF (VDD/2 is recommended).
MCP6XX4 MCP6XX4 MCP6XX4
C1
VDD
–
+
R2
R3
MCP65X1
MCP65X1
U1a
U1b
U1c
U2
State Variable Oscillator:
–
+
. . .
VREF
MCP6XX4
Relaxation Oscillator:
U1
R1
R4
V
OUT
U1d
VOUT
C2
C1
VREF
–
+
–
+
–
+
–
+
C4
R4
R1
R2
V
REF
VREF
R3 R7 R8
VREF
Notes: A resistive divider to VDD sets VREF (VDD/2 is recommended).
MCP6XX4 MCP6XX4 MCP6XX4
C1
VDD
–
+
R2
R3
MCP65X1
MCP65X1
U1a
U1b
U1c
U2
State Variable Oscillator:
–
+
. . .
VREF
MCP6XX4
Relaxation Oscillator:
U1
R1
R4
VOUT
U1d
Some of their advantages and features are:
■ Low cost
■ Reliable oscillation startup
■ Square wave output
■ Frequency ∝ 1/(R
1
C
1
)
Sensors and Applications
These oscillator circuits are applicable to various type of
sensors.
Resistive Sensors
■ RTDs
■ Thermistors
■ Humidity
Capacitive Sensors
■ Humidity
■ Pressure (e.g., absolute quartz)
■ Fluid Level
Related Application Notes:
AN895: Oscillator Circuits for RTD Temperature Sensors
AN866: Designing Operational Amplifier Oscillator Circuits
for Sensor Applications
Available on the Microchip web site at: www.microchip.com.