Diamond Power Products MM-48-AT user manual Analog Input Ranges and Resolution

7. ANALOG INPUT RANGES AND RESOLUTION

7.1 Resolution

Diamond-MM-48-AT uses a 16-bit A/D converter. This means that the analog input voltage can be measured to the precision of a 16-bit binary number. The maximum value of a 16-bit binary number is 216 - 1, or 65535, so the full range of numerical values that you can get from a Diamond-MM-48-AT analog input channel is 0 - 65535.

The smallest change in input voltage that can be detected is 1/(216), or 1/65536, of the full- scale input range. This smallest change results in an increase or decrease of 1 in the A/D code, and so this change is referred to as 1 LSB, or 1 least significant bit.

7.2 Input Ranges

Diamond-MM-48-AT is available in two models. The default model has bipolar inputs of +/- 10V and +/-5V, selectable with jumper block J10. An optional model with a slightly different component configuration has unipolar inputs of 0-5V.

7.3 Single Ended and Differential Inputs

A single-ended input is a two-wire input (one input signal and ground) that is referenced to analog ground on the board. This means that the input voltage will be measured with respect to the board’s analog ground. A differential input is a three-wire input (input +, input -, and ground), and the board will measure the difference between the voltages of the two inputs. Diamond-MM-48-AT works only with single-ended inputs.

In some cases, a differential input can be simulated by connecting the high and low input wires to two adjacent analog input channels, measuring both of them with and A/D scan operation, and subtracting the low reading from the high reading. This will work only for very slow changing (or DC) signals with very low noise. The problem with this technique is that the two signals are sampled at different times, either 5µs or 9.3µs apart depending on the setting of the SCANINT register bit. This time delay can lead to measurement errors due to the change in the input signal during the delay. For example, an input signal consisting of a 1KHz sine wave with 10V amplitude has a maximum slope of 10V x 2 x π x 1000Hz = 63,000 V/sec, or 63 mV/µs. This represents a change of 315mV during the 5µs delay between the sampling of the high input and the low input. For a 16-bit A/D converter on a +/-10V input range, this is an error of over 1,000 counts.

Diamond-MM-48-AT User Manual V1.01 Page 27