Data Acquisition Boards: Input Ranges

When choosing a DAQ system, one of the first things to consider is the voltage range you will be measuring. First, go through the specifications of all the sensors and other input devices you are using. This may include RTDs, pressure transducers, strain gauges and load cells. Then, determine the output voltages these devices will provide (hint: consult the spec sheet, often available on the manufacturer's web site).

Input ranges for any given DAQ board is listed in its data (specifications) sheet. These ranges can be unipolar (e.g. 0-10 Volts for positive voltages only) or bipolar (±10 Volts, for both positive and negative voltages). Choose a board that covers a range you want to measure with as little overlap as possible. Doing so will give you the greatest quantity of data points in the range you need to measure, thus assuring excellent accuracy. e.g., suppose that a sensor output varies from 1 to 3 Volts; thus, you choose a board with a 0-5 Volts range. This allows twice as many valid data points as would be possible had you chosen a board with an input range of ± 5 Volts or 0-10 Volts. Why? Because the counts for ranges -5 Volts to 0 Volts and +5 Volts to +10 Volts would not be used. Alternatively, a lower-voltage range (e.g. 0-2.5 Volts) would not allow you to measure the full output of the sensor.

Fortunately, most DAQ boards provide multiple input ranges by using software-programmable gain amplification. e.g., a vendor may provide multiple levels of gain (e.g. 1X, 2X, 4X, and 8X) allowing one to measure input signals as low as 1.25 Volts with full resolution. Low-level gain DAQ boards (10 to 500X) will allow one to measure signals as small as 20mV with full resolution. Some signal conditioning may be necessary.

Programmable gain, which allows you to change input range via software, does have a disadvantage: the extra software processing can drag down high-speed data acquisition. The solution: A channel-gain list -- a feature on many high-performance DAQ boards. The list is basically a small onboard memory buffer which may be pre-loaded with up to 32,768 channel numbers and their respective gains. While a DAQ board is working, the channel-gain list automatically selects channel and gain values in hardware without curtailing throughput.

below: Channel-Gain List

Input Types and Modules