Cirrus Logic AN241 - Manual (Page 5)

AN241
5
Figure 3. Single-Ended to Differential Input Buffer
3.4 Overview of the Filter Topology
3.4.1 High Pass Filter and DC Biasing
The first stage of the buffer forms a high pass filter and provides the proper biasing to the positive termi-
nals of the op-amps. The value of the resistors denoted as “R” will vary depending on the optimal DC bias
for the given converter. Typically this is around half of the VA voltage supply. The high pass filter is formed
from the combination of the AC-coupling capacitor along with the resistor that connects the positive ter-
minal of the op-amp to the DC bias voltage. The 3dB corner of the high pass filter can be calculated as
follows:
VA
+
+
-
-
1 µF
100 kΩ
100 kΩ
R kΩ
3.3 kΩ
0.01 µF 100 µF
470 pF
470 pF
C0G
C0G
634 Ω
634 Ω
634 Ω
91 Ω
91 Ω
2700 pF
C0G
0.01 µF 1 µF
AIN+
AIN-
VQ or
VCOM
CSxxxx
AIN+
AIN-
VA
+
+
-
-
1 µF
100 kΩ
100 kΩ
R kΩ
3.3 kΩ
0.01 µF 100 µF
470 pF
470 pF
C0G
C0G
634 Ω
634 Ω
634 Ω
91 Ω
91 Ω
2700 pF
C0G