Choosing the Right Range and Resolution

June 29th, 2006

From Monitor - #95, an e-mail newsletter of Windmill Software.

Most analogue sensors produce a voltage, including thermocouples, strain gauge bridges, and gas concentration probes. This is converted to a form the computer can understand by an analogue-to-digital (A-D) converter.

The input range (or gain range) of an A-D converter refers to the maximum and minimum voltage that it can accept.

An input range may be bipolar, covering a range from -50 mV to +50 mV, for example, or unipolar, perhaps covering a range of 0 to 50 mV. Many A-D converters have an actual range of 0 to 10 V, so an input signal with a range of 0 to 50 mV needs to be amplified by a gain of 200 before it can be converted. Many systems offer a choice of ranges, and you can select the most appropriate using your data acquisition software.

You should always choose the smallest range that encompasses your signal, as this optimizes 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 ((2 to the power n) - 1). This equates to (2 to the power n) 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 is unstable readings.

Some A-D converters have a choice of resolutions (offering 12-, 13-, 14-, 15- and 16-bit for example). In general, the higher the resolution, the slower the conversion. You can choose the most suitable for your application, balancing speed against accuracy.

Entry Filed under: Data Acquisition