What makes this data collection possible is the analog-to-digital, or A/D, converter. As shown in Figure 1, the analog-to-digital converter takes the analog voltage at its input and outputs a digital word whose value is proportional to the input voltage. The resolution of these A/D converters can be as low as eight bits (256 discrete values) or as high as 24 bits (16,777,216 values).

There are two types of A/D converters - integrating and non- integrating.  The non-integrating flash converter compares the input voltage (Vin) to a set of known (reference) voltages using comparators and outputs a digital value (0 or 1) based on the comparison. Although flash converters are very fast—hence the name—they tend to be expensive. To make accurate conversions, they must use very accurate voltage references, which can be costly. Another disadvantage is that they have poor noise rejection. If any noise is present on the signal at the time it is digitized, the digital word will include that noise.

Another type of non-integrating A/D, the successive approximation A/D, is less expensive than a flash converter. It uses a single comparator and generates its own reference voltages, comparing each one to the input signal. While these are less expensive than flash converters, successive approximation converters maybe produce erroneous results if the input signal varies while a conversion is taking place.

Integrating A/Ds

In an integrating A/D converter, the input signal is connected to an integrator, whose output is connected to a comparator. In a single-slope integrating A/D converter the time that it takes for the output of the integrator to reach the reference voltage will be proportional to the input voltage. The integrating A/D converter measures this time and then outputs a digital word.

A dual-slope integrator first integrates the input voltage for a fixes amount of time and then “de-integrates a known, fixed value until the output reaches zero. The “de-integration” time will be proportional to input voltage. The A/D converter measures this time and outputs a digital word that is proportional to the de-integration time.

Integrating A/D converters are slower than non-integrating A/D converters, but they generally provide higher resolution than non-integrating A/D converters. In addition, they provide better noise rejection.

When making measurements with a data acquisition system, it's important to choose the A/D converter that's right for the measurements that you're trying to make. For more information about A/D converters and AMETEK VTI data acquisition systems, phone 949.955.1894 or send an email to vti.sales@ametek.com.