Cirrus Logic CRD49530-USB - Manual (Page 37)

Detailed Schematic Descriptions
CRD49530-USB User’s Manual
DS705RD3 Copyright 2008 Cirrus Logic 5-4
Each output of the CS42448 has an output filter that consists of an AC-coupling cap (3.3
μ
F), a pull-down
resistor to prevent the output from floating when not connected to a load, a series resistor (470
Ω
) to
provide a voltage drop when the muting transistor is enabled, and a mute transistor that will pull the output
low when the mute control signal is enabled. The series resistor is small enough that it does not affect the
signal in normal operation, assuming a load of at least 10 k
Ω
is connected to the analog output of the
board.
5.2.7 Microphone Preamplifier
Figure 5-7 shows the microphone preamplifier on the CRD49530-USB, which has a 1/8” microphone input
jack to allow direct connection to an encapsulated condenser microphone (ECM). Because the output of
the ECM is so small, a pre-amplifier is needed to boost the signal to a line-level voltage.
These specifications for the amplifier are noted on the schematic page. These parameters should be
considered when choosing the microphone to be connected to the CRD49503-USB. Too large of a signal
on the CS42448 analog input will result in distortion of the sampled signal.
It is important to note that although the amplifier circuit shown is non-inverting (the input to U12-B is the
same polarity as the output from U12-C), the output of an ECM is inherently inverted since it acts as an
open-collector device. Therefore the microphone signal driven to the CS42448 should be considered an
inverted signal for processing purposes.
5.2.8 Power and Connectors
The audio input connectors consist of:
• 6 RCA jacks for analog input
• 2 RCA jacks for coaxial S/PDIF input
• 2 optical jacks for optical S/PDIF input
The audio output connectors consist of:
• 8 RCA jacks for analog output
• 1 optical jack for S/PDIF output
There is one control connector on the board, J11. This 50-pin connector provides pins for the following
functions:
• Serial control interface for configuring the DSP, codec, and S/PDIF RX
• Reset lines for the DSP and other board devices
• Pins to provide power to the CRD USB Master USB control board
• An interface for delivering audio data from the USB board (feature not yet available)
The DC input connector (J25) for the CRD49530 can accept 9 to 12 V DC , and the power supply should be
capable of supplying at least 1 amp of current.
The 3 voltage regulators on the CRD49530 generate the 1.8V, 3.3V, and 5V necessary for powering all of
the ICs on the board. Note that the 5 V and 3.3 V regulators run directly off the DC input supply connected
to the CRD49530, while the 1.8 V regulator is dependent upon 3.3 V system power (not necessarily the
3.3 V regulator).
The 3 power jumpers (J17, J18, J19) are used to choose between the on-board regulators, and an
external source for 5 V, 3.3 V, and 1.8 V. This is a feature intended only for special applications, so these
jumpers should be left in the ‘REG’ position for normal operation.