9.1.5 AC-Bridge Excitation Example

Figure 82 shows an ac-excited bridge measurement system in the 4-wire, GPIO-control mode. Signal and reference-input filter components are omitted for clarity. The transistors switch the polarity of the excitation voltage provided to the bridge by the drive signals from the ADC GPIO drivers via the spare analog input pins. The timing of the drive signals are synchronized to the ADC conversions. The drive signals are non-overlapping in order to avoid commutation errors that can occur during the switching phase. The resistors located at the gates of each transistor maintain the transistors off at power-on, while the ADC drive signals are initialized by the host after system power up. See Figure 7 for timing of the drive signals.

Figure 82. AC-Bridge Excitation Application

The recommended configuration sequence for ac-bridge excitation mode 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 signal and reference input multiplexers, gain, data rata, filter mode and other configurations as needed
3. Program the 2-wire or 4-wire ac-bridge excitation mode. 2-wire mode requires complementary output switching devices
4. Program the GPIO internal connection to the analog input pins
5. Program the GPIO as outputs to enable drive signals at the analog input pins. The bridge output drive signals appear on the GPIO pins.

Start the conversions. Adjust the time delay parameter as necessary to provide sufficient bridge switch delay. The delay is based on the time constant of the input and reference filters.