7.3.9 Input Impedance and VCM Control

The TLV320AIC3109-Q1 includes several programmable settings to control analog input pins, particularly when these inputs are not selected for connection to a PGA. The default option allows unselected inputs to be put into a high-impedance state such that the input impedance of the device is extremely high. However, 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 begin conducting current, resulting in an effective impedance that no longer appears as a high-impedance state.

In most cases, ac-couple the analog input pins on the TLV320AIC3109-Q1 to the analog input sources, except if an ADC is used for dc voltage measurement. The ac-coupling capacitor causes a high-pass 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 a PGA varies with the setting of the input level control, starting at approximately 20 kΩ with an input level control setting of 0 dB, and increasing to approximately 80 kΩ 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 high-pass filter pole of 80 Hz when the 0-dB input level control setting is selected.