User manual

Table Of Contents

Product information30

When selecting a sensor for an application you should consider the following:

The input is 0-2.5 V.

1. Is the sensor Active or Passive? If Active then can it be powered by the USB

DrDAQ?

2. What is the output of the sensor? Can the sensor be plugged directly into the USB

DrDAQ?

3. Is the sensor already available on the USB DrDAQ, or are there more suitable

sensors out there?

3.7.3

Scaling

Sensor Scaling - Software

It is necessary to create a scaling file so that the software can extract details about

the sensor. Details on creating scaling files can be found in the DrDAQ Scaling Files (.

DDS) topic. Scaling in software also provides an opportunity to perform a

calibration on the sensor, which can compensate for any manufacturing

irregularities.

Sensor Scaling - Hardware

With both these types of sensors it is essential that the USB DrDAQ displays an

accurate representation of the property to be measured. There are many factors to

take into account when designing scaling circuitry:

Sensitivity

The USB DrDAQ has 12-bit resolution over the 0 to 2.5 V input range. This means the

sensitivity is:

2500 mV / 2^12 = 0.611 mV

To make the most of the resolution, the signal output from the sensor should use as

much of the input range of the USB DrDAQ as possible.

Linearity

As illustrated below, not all sensors have a linear response.

Linear Response

An Example of Non-Linear

Response

A linear response is one whose output-versus-input graph is a straight line. An

example of this type of response is a photodiode.

Note: a non-linear response curve cannot be calculated using a simple y = ax + b

formula. An example of this type of response is an LDR.