24/7 Data Acquisition Wildcard Users Guide

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Setting the Gain

The gain option is specified as a constant to Start_Conversion or Start_Conv_With_Values. Valid constants are GAIN_1, GAIN_2, GAIN_4, GAIN_8, GAIN_16, GAIN_32, GAIN_64, or GAIN_128. Saturation will occur if the absolute value of the input signal multiplied by the gain is greater than 2.5 volts.

Choosing Bipolar or Unipolar Conversion

Passing the UNIPOLAR or BIPOLAR constant to Start_Conversion or Start_Conv_With_Values sets the polarity option for the module. The 24/7 Data Acquisition Wildcard accepts either unipolar or bipolar input voltage ranges. Bipolar input ranges do not imply that the part can handle negative voltages below -20 mV on its analog inputs! The input channels are arranged in pairs with an Analog In + and Analog In −. As a result, the voltage to which the unipolar and bipolar signals on the Analog In + input are referenced is the voltage on the respective Analog In − input. For example, if Analog In − is +2.5 V and the module is configured for unipolar operation with a gain of 2 and a reference voltage of +2.5 V, the input voltage range on the Analog In + input is +2.5 V to +3.75 V. If Analog In − is +2.5 V and the module is configured for bipolar mode with a gain of 2 and a reference voltage of +2.5 V, the analog input range on the Analog In + input is +1.25 V to +3.75 V (i.e., 2.5 V +/- 1.25 V).

Specifying 16- or 24-bit Resolution

The 24/7 Data Acquisition Wildcard has two resolution settings: 16 and 24 bit. The 16 bit resolution option is used when the constant WORD_16BIT is passed to Start_Conversion or Start_Conv_With_Values. The 24 bit option is used when the constant WORD_24BIT is passed to Start_Conversion or Start_Conv_With_Values. Changing the resolution only changes the data size; it does not affect the conversion process.

Turning the Burnout Current Option On/Off

The analog to digital converter contains two 1 ?A currents, one source current from the analog power supply to the current Analog In + and one sink from Analog In − to analog ground. The currents are either both on or both off depending on the constant, BO_ON or BO_OFF, that is passed to Start_Conversion or Start_Conv_With_Values. You can use these currents to see if a transducer has burned out or gone open-circuit before attempting to take measurements on that channel. If the transducer becomes an open-circuit with the burnout currents on, the input voltage will be measured as full scale. If the transducer becomes a short circuit with the burnout currents on, the input voltage will be measured as zero.

Synchronizing Conversions

The synchronization option allows you to reset the modulator and the digital filter of the analog to digital converter without affecting any other settings such as gain, channel, polarity, or calibration values. This allows conversions to be started at a precisely known time. To start a synchronous conversion, pass FSYNC_ON to Start_Conv_With_Values or call Sync, which synchronizes the conversion through a hardware pin. A valid sample will be available 3 sample periods (3/sample frequency) later. For example, if the sample frequency is 10 Hz, the channel has already been calibrated, and Sync is called, the next sample will be available after 300 ms (3 / 10 Hz). The synchronization option should not be used when a Background Calibration is operating.

Specifying the Source Channels

Table 1 lists the options available for channel selection.