Figure 6-5. The Effects of Range on ADC Precision

Limit Settings 0 to 5 V

Figure 6-6. The Effects of Limit Settings on ADC Precision

Considerations for Selecting Analog Input Settings

The resolution and device range of a DAQ device determine the smallest detectable change in the input signal. You can calculate the smallest detectable change, called the code width, using the following formula.

device range code width = ----------------------------------

2resolution

For example, a 12-bit DAQ device with a 0 to 10 V input range detects a 2.4 mV change, while the same device with a –10 to 10 V input range detects only a change of 4.8 mV.

A high-resolution A/D converter provides a smaller code width given the device voltage ranges shown above.

The smaller your code width, the more accurate your measurements will be.

There are times you must know whether your signals are unipolar or bipolar. Unipolar signals are signals that range from 0 value to a positive value (for example, 0 to 5 V). Bipolar signals are signals that range from a negative to a positive value (for example, –5 to 5 V). To achieve a smaller code width if your signal is unipolar, specify that the device range is unipolar, as shown previously. If your signal range is smaller than the device range, set your limit settings to values that more accurately reflect your signal range. Table 6-1 shows how the code width of the 12-bit DAQ devices varies with device ranges and limit settings, because your limit settings automatically adjust the gain on your device.