SLAS510G March 2007 – February 2021 TLV320AIC3104
The TLV320AIC3104 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 high-impedance state, 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 begin conducting current, resulting in an effective impedance that no longer appears as a high-impedance state.
In most cases, the analog input pins on the TLV320AIC3104 should be ac-coupled to analog input sources, the exception to this being if an ADC is being 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 an ADC 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.