High-Pass Filter
3-7
THS6132EVM Applications
3.4
High-Pass Filter
Because ADSL CPE is designed to transmit from 25.875 kHz to 138 kHz, C2
and R23 can be used to implement an HPF function. These are selected to be
20X lower than 25 kHz (1.25 kHz). Some designs use a capacitor—some do
not. This path allows for a common gain setting between the two channels.
This helps (but does not assure) the signals are truly differential.
Figure 3-4 compares the frequency spectrum of ADSL to a simulation of the
high-pass filter on the THS6132EVM.
Figure 3-4. ADSL Spectrum and High-Pass Filter Response
(Above not on a logarithmic scale)
Note that the high-pass filter function is not a true high-pass filter. C2 in series
with R23 creates a zero at about 10 Hz. As the frequency decreases from
about 3 kHz to 10 Hz, the circuit changes from a gain stage into two unity gain
buffers.
