8.4.3 AC-Bridge Excitation Mode

Resistive bridge sensors are excited by dc or ac voltages; or by dc or ac currents. DC voltage excitation is the most common type of excitation. AC excitation reverses the polarity of the excitation voltage by the use of external switching components. Similar in concept to chop mode, the result of the voltage reversal removes offset voltage in the connections leading from the bridge to the ADC inputs. This also includes the offset voltage of the ADC itself. The ADC provides the signals necessary to drive the external switching components in order to reverse the bridge voltage.

The timing of the drive signals is synchronized to the ADC conversion phase. During one conversion phase, the voltage polarity is normal. For the alternate conversion phase, the voltage polarity is reversed. The ADC compensates the reversed polarity conversion by internal reversing the reference voltage. The ADC subtracts the data corresponding to the normal and reverse phases in order to remove offset voltage from the input.

The ADC output drive signals are non-overlapping in order to avoid bridge cross-conduction that can otherwise occur during excitation voltage reversal. The switch rate of the ac-excitation drive signals are performed at the data rate to avoid unnecessary fast switching. See Figure 7 for output drive timing.

Table 7 shows the ac-bridge excitation drive signals and the associated GPIO pins. Program the ac-bridge excitation mode using the CHOP[1:0] bits in register MODE1. AC-bridge excitation can be programmed for two-wire or four-wire drive mode. For two-wire operation, two drive signals are provided on the GPIOs. If needed, use two external inverters to derive four signals to drive discrete transistors. The GPIO drive levels are referred to the 5-V analog supply. Be aware that the ac-bridge excitation mode changes the nominal data rate, depending on the order of the sinc filter. See the Chop Mode section for details of the effective data rate.