INPUT IMPEDANCE AND VCM CONTROL
SLAS553A – NOVEMBER 2008 – REVISED SEPTEMBER 2009...................................................................................................................................... www.ti.com
Inputs can be selected as single-ended instead of fully differential, and mixing or multiplexing into the ADC PGAs
is also possible in this mode. It is not possible, however, for an input pair to be selected as fully-differential for
connection to one ADC PGA and simultaneously selected as single-ended for connection to the other ADC PGA
channel. However, it is possible for an input to be selected or mixed into both left and right channel PGAs, as
long as it has the same configuration for both channels (either both single-ended or both fully differential).
The TLV320ADC3101 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 will begin conducting current, resulting in an effective impedance that no longer
appears as a high-impedance state.
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 band-gap 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 1 / register 52 through page 1 / register 57. The user should make
sure this option is disabled when an input is selected for connection to an ADC PGA or selected for the analog
input bypass path, because it can corrupt the recorded input signal if left operational when an input is selected.
In most cases, the analog input pins on the TLV320ADC3101 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 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 35 k
with an input-level control setting of
0 dB, and 62.5-k
when the input-level control is set at –6 dB. For example, using a 0.1-
F ac-coupling capacitor
at an analog input results in a high-pass filter pole of 45.5 Hz when the 0-dB input-level control setting is
selected. To set a high-pass corner for the application, the following input-impedance table has been provided
with various mixer gains and microphone PGA ranges.
Table 5. Single-Ended Input Impedance vs PGA Ranges (1)
Mixer Gain (dB)
Microphone PGA Range (dB)
Input Impedance (Ohms)
0
0–5.5
35,000
0
6–11.5
38,889
0
12–17.5
42,000
0
18–23.5
44,074
0
24–29.5
45,294
0
30–35.5
45,960
0
36–40
46,308
–6
0–5.5
62,222
–6
6–11.5
70,000
–6
12–17.5
77,778
–6
18–23.5
84,000
–6
24–29.5
88,148
–6
30–35.5
90,588
–6
36–40
91,919
(1)
Valid when only one input is enabled
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