Which Range and Resolution to Choose?
Thermocouples, strain gauge bridges, gas concentation probes and most other analogue sensors produce a voltage signal. This is converted to a form the computer can understand by an analogue-to-digital (A-D) coverter.
The input range (or gain range) of an A-D coverter refers to the maximum and minimum voltage that it can accept.
An input range may be bipolar, covering a range from -10 mV to +10 mV for example; or unipolar, perhaps covering a range of 0 to 10 mV. Many A-D converters have an actual range of 0 to 10 V, so an input signal with a range of 0 to 10 mV needs to be amplified by a gain of 1000 before it can be converted. Many systems offer a choice of ranges, and you can select the most appropriate using software like Windmill.
You should always choose the smallest range that encompasses your signal, as this optimises the resolution.
The resolution is the number of steps into which the input range is divided. The resolution is usually expressed as bits (n) and the number of steps is 2n-1. This equates to (2n) values. A converter with 12-bit resolution, for instance, divides the range into 212, or 4096, values. In this case a 0-10 V range will be resolved to 2.5 mV, and a 0-100 mV range will be resolved to 0.025 mV.
Although the resolution increases when you narrow the range, there is no point in trying to resolve signals below the noise level of the system: all you will get are unstable readings.
Some A-D converters have a choice of resolutions (offering 12-, 13-, 14-, 15- and 16-bit for example). You can choose the most suitable for your application, balancing speed against accuracy.