ADS1261: AC Excitation programming setup

Hi Dennis,

Good to hear from you! Let me try my best to clarify...

QUESTION: Because 4-wire AC excitation is enabled, will enabling only AIN4 and AIN5 as GPIO outputs also enable AIN2 and AIN3 as GPIO outputs?

No, enabling the GPIO outputs and enabling the ac-excitation mode are two separate settings. Both must be enabled to see the ac-excitation drive signals on the GPIO outputs.

QUESTION: What do you mean by "by not enabling AIN2 and AIN3 this is an alternate way of performing 2-wire AC excitation"? Do you mean that if you were to enable 2-wire AC excitation, and configure AIN4 and AIN5 as GPIO outputs, AIN4 and AIN5 would become the drivers for the AC switching?

According to the ADS1261's register settings "2-wire ac-excitation mode" will only enable the drive signals on AIN2 and AIN3 (assuming these pins are also configured as GPIO outputs).

Likewise, according to the ADS1261's register settings "4-wire ac-excitation mode" will enable the drive signals on AIN2, AIN3, AIN4, and AIN5 (assuming these pins are also configured as GPIO outputs). However, since AIN4 and AIN5 are inverted versions of AIN2 and AIN3, you may choose to only enable the GPIO outputs on AIN4 and AIN5 and just use these signals as the drive signals (which is a kind of 2-wire implementation).

The ADS1261EVM was configured to demo this "alternate" two wire ac-excitation mode; however, whichever implementation you want to use is up to you. However, the benefit of this "alternate" mode is that is leaves AIN2 and AIN3 free to be used as a seconed external reference input if needed. If you don't need a second external reference.

QUESTION: In the AC-Excitation Timing Characteristics (FIG 7 page 14 in the ADS1261 data sheet), it shows ACX1 (GPIO2, AIN4) starting out low. By setting GPIO2 (AIN4, ACX1) high, does this mean that all ACX signals will be initialized opposite of what is shown in FIG 7? The reason I ask this is because I'm using 2-wire AC excitation and the TI TMUX1511 to due the AC switching. I have inadvertently connected !ACX1 and !ACX2 to the wrong switches. I therefore would like to flip the initial !ACX polarity. Can this be done?

Unfortunately, you'll have to make a change to the pin connections. Once you enable one of the ac-excitation modes, the internal logic takes over control of the ACXn pins, no matter their initial state.

QUESTION: It appears that an A written to the REF register (REG 6) will choose AIN2 as the positive reference input and the internal reference negative as the negative reference input. Is this correct or am I missing something?

Writing 0x0A to the REF register configures AIN0 as the positive reference input and AIN1 as the negative reference input.

Writing 0x1C would configure the reference inputs as you described.

QUESTION: How can AIN 2 and AIN3 be used as inputs when they are being used for 4-wire excitation switching?

As long as you do not enable the GPIO connections to AIN2 and AIN3, you can use their other functions. The input MUX, reference MUX, and GPIO MUX switches are all independent, as hinted by the Functional block diagram: