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USB DrDAQ User's Guide 27
Copyright © 2013 Pico Technology Ltd. All rights reserved. usbdrdaq.en r4
Offsets
The above linear and non-linear responses could have a voltage, current, resistive or
other type of offset that may need taking into account. It is quite usual to see offsets
in output signals from sensors.
Sensor output
The USB DrDAQ requires an input signal in DC volts or resistance. The sensor in
question could give an output in other units such as resistance, current or AC volts.
Drift
It is possible that the sensor output drifts over time. This may involve recalibration of
the sensor at set time intervals, or some self-calibration.
Hysteresis
The sensor may give a different output value when rising to a voltage than when
falling to a voltage. This is difficult to eliminate and should be taken into account.
Once the sensor has been selected and all of the above has been considered, the
design of some scaling circuitry can begin. There are many ICs on the market that can
be purchased cheaply that deal with the above potential problems. Look into these
before re-inventing the wheel and spending expensive development time designing a
circuit that can be purchased for a few pounds.
Some simple scaling examples:
Example 1: Interfacing a Resistive sensor
Notes:
Light-dependent resistor (LDR) can be
used in conjunction with a fixed
resistor to measure light level. A
suitable LDR sensor can be obtained
from Maplin Electronics (part number
N53AY). A resistor (R) of around 500k
is suitable.
Example 2: Measuring 4-20 mA Current