10.1.5 AC-Excitation

Figure 97 shows a example of an AC-excited bridge measurement system. The example shown omits optional filter components for clarity. The transistors switch the bridge excitation voltage by drive signals provided by the GPIO drivers through the analog input pins. The timing of the drive signals are synchronized to the ADC conversions. The drive signals do not overlap in order to avoid bridge commutation during the switching phase of the drive signal. The transistors gate resistors bias the transistors off at power-on. At host start-up, the host configures the ADC to the AC-excitation mode. See Figure 7 for timing of the drive signals.

Figure 97. 4-Wire Drive, AC-Excitation Example

The recommended sequence AC-excitation configuration is as follows:

1. Stop conversions by taking the START pin low, or by control of conversions in software mode; send the STOP command
2. Program the input and reference MUX, gain, data rata, filter mode and other configurations as needed
3. Program the 2-wire or 4-wire AC-excitation mode
4. Program the 2 GPIOs or 4 GPIOs internal connection to the analog input pins
5. Program the 2 GPIOs or 4 GPIOs as outputs to enable drive signals at the analog input pins.

Start the conversions. Adjust the time delay parameter as necessary based on the time constant of the input and reference filters.