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2.4
Analog Inputs
Hardware Configuration
The left and right channel analog input sources for the PCM4222 are provided through connectors J1 and
J2, respectively. The J1 and J2 connectors accommodate both 3-pin balanced XLR and 1/4-inch balanced
TRS input connections. Pin 1 for both connector J1 and J2 include a ground lift jumper.
Each analog input is buffered by an input circuit employing the OPA1632 fully differential audio amplifier,
selected for its low noise and distortion, and fully-differential input-to-output architecture. The buffer circuit
provides attenuation (nominal gain = 0.482, or –6.34dB) and low-pass antialiasing filter functions. The
printed circuit board (PCB) layout supports limited board stuff options for experimentation. The full-scale
input voltage for the input buffer circuit is approximately 4.2V
RMS
(or +14.6845dBu) differential, given a
nominal VCC supply of +4.0V for the PCM4222 and a source impedance of 40
Ω
. Due to the full-scale
input voltage varying slightly from one PCM4222 device to another, the full-scale input swing should be
calibrated for each EVM individually, adjusting the input voltage level until a 0dBFS output level is
indicated.
The common-mode bias for each OPA1632 is provided by the corresponding VCOML or VCOMR outputs
from the PCM4222. The VCOML and VCOMR outputs are buffered by
low-noise precision op
amps, configured as voltage followers. A buffer is required because of the low input impedance of the
OPA1632 V
OCM
pin.
8
PCM4222EVM User's Guide
SBAU124 – December 2006