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1070_0_Product_Manual - October 28, 2010 8:52 AM
Setting Ratiometric causes the reference to the internal Analog to Digital Converter to be set to the power supply
voltage level. When Ratiometric is enabled, the maximum voltage returned on the Analog Input should be the +5V
nominal power provided by the PhidgetInterfaceKit.
Non-Ratiometric Configuration
If Ratiometric is false, the ADC reference is set to a 5.0V 0.5% stable voltage reference. The maximum voltage
returned on the Analog Input should be maximum 5.0V. Note that the Analog Input power supply voltage is not
affected by the setting of the Ratiometric property.
Factors that can affect Accuracy
High Output Impedance
- Sensors that have a high output impedance will be distorted by the 900K input
impedance of the Analog Input. If your output impedance is high, it is possible to correct for this distortion to some
extent in your software application.
Power Consumption
- Sensor cables have some resistance, and the power consumption of the sensor will cause
the sensor to have a slightly different ground from the Analog Input on the PhidgetInterfaceKit. The more power
consumed by the sensor, and the longer the sensor cable, the more pronounced this effect will be.
Intrinsic Error In Sensors
- For many sensors, the error is quite predictable over the life of the sensor, and it can
be measured and calibrated out in software.
Non-Ratiometric Configuration
- Voltage Reference error. The 5.0VDC voltage reference is accurate to 0.5%.
This can be a significant source of error in some applications, but can be easily measured and compensated for.
Connecting non-Phidget devices to the Analog Inputs
Here are some circuit diagrams that illustrate how to connect various non Phidgets devices to the analog inputs on
your Phidget.
Sensing the value of a variable resistance sensor
In this diagram, an FSR (Force Sensitive Resistor) is shown.
Sensing the position of a potentiometer
1
1
2
2
3
3
4
4
D
D
C
C
B
B
A
A
20pF
1K
1M
+V
ANALOG
GROUND
Phidget
Analog
Input x1
Detail of Analog Input
INPUT
5V PW R
1K
SAMPL ING SWITCH
ANALOG
GROUND
INPUT
5V PW R
Phidget
Analog
Input
4K
Sensing the value of a variable resistance sensor
FSR
In this case, an FSR (force sensitive resistor) is shown.
1K
ANALOG
GROUND
INPUT
5V PW R
Phidget
Analog
Input
Sensing the position of a potentiometer
ANALOG
GROUND
INPUT
5V PW R
Phidget
Analog
Input
Interfacing to an arbitrary sensor
GND
3
VOUT
2
VCC
1
100nF
1K
100nF
Note the use of power supply decoupling and the RC Filter on the output.
The RC filter also prevents VOUT from oscillating on many sensors.
1
1
2
2
3
3
4
4
D
D
C
C
B
B
A
A
20pF
1K
1M
+V
ANALOG
GROUND
Phidget
Analog
Input x1
Detail of Analog Input
INPUT
5V PW R
1K
SAMPL ING SWITCH
ANALOG
GROUND
INPUT
5V PW R
Phidget
Analog
Input
4K
Sensing the value of a variable resistance sensor
FSR
In this case, an FSR (force sensitive resistor) is shown.
1K
ANALOG
GROUND
INPUT
5V PW R
Phidget
Analog
Input
Sensing the position of a potentiometer
ANALOG
GROUND
INPUT
5V PW R
Phidget
Analog
Input
Interfacing to an arbitrary sensor
GND
3
VOUT
2
VCC
1
100nF
1K
100nF
Note the use of power supply decoupling and the RC Filter on the output.
The RC filter also prevents VOUT from oscillating on many sensors.
