The TLV320AIC3106 includes several programmable settings to control analog input pins, particularly when they are not selected for connection to an ADC PGA. The default option allows unselected inputs to be put into a 3-state condition, such that the input impedance seen looking into the device is extremely high. Note, however, that the pins on the device do include protection diode circuits connected to AVDD and AVSS. Thus, if any voltage is driven onto a pin approximately one diode drop (~0.6 V) above AVDD or one diode drop below AVSS, these protection diodes will begin conducting current, resulting in an effective impedance that no longer appears as a 3-state condition.

Another programmable option for unselected analog inputs is to weakly hold them at the common-mode input voltage of the ADC PGA (which is determined by an internal bandgap voltage reference). This is useful to keep the ac-coupling capacitors connected to analog inputs biased up at a normal DC level, thus avoiding the need for them to charge up suddenly when the input is changed from being unselected to selected for connection to an ADC PGA. This option is controlled in Page-0/Reg-20 and 23. The user should ensure this option is disabled when an input is selected for connection to an ADC PGA or selected for the analog input bypass path, since it can corrupt the recorded input signal if left operational when an input is selected.

In most cases, the analog input pins on the TLV320AIC3106 should be ac-coupled to analog input sources, the only exception to this generally being if an ADC is being used for DC voltage measurement. The ac-coupling capacitor will cause a highpass filter pole to be inserted into the analog signal path, so the size of the capacitor must be chosen to move that filter pole sufficiently low in frequency to cause minimal effect on the processed analog signal. The input impedance of the analog inputs when selected for connection to an ADC PGA varies with the setting of the input level control, starting at approximately 20 k? with an input level control setting of 0-dB, and increasing to approximately 80-k? when the input level control is set at –12 dB. For example, using a 0.1 μF ac-coupling capacitor at an analog input results in a highpass filter pole of 80 Hz when the 0 dB input level control setting is selected.