CDB42428 Cirrus Logic Inc, CDB42428 Datasheet - Page 5

• Manufacturer Part Number: CDB42428
• Description: BOARD EVAL FOR CS42428 CODEC
• Manufacturer: Cirrus Logic Inc

Specifications of CDB42428

• Main Purpose: Audio, CODEC
• Embedded: Yes, FPGA / CPLD
• Utilized Ic / Part: CS42428/26/18/16
• Primary Attributes: 6 Single-Ended Analog Inputs and 8 Outputs, S/PDIF Digital Audio Transmitter and Receiver
• Secondary Attributes: Graphic User Interface
• Product: Audio Modules
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Contains lead / RoHS non-compliant
• Other names: 598-1499

The 2-pole low pass filter provides an example of an inexpensive circuit with good distortion and dynamic range per-
formance. It is designed to have the in-band impedance matched between the positive and negative legs. It also
provides a balanced to single-ended conversion for standard un-balanced outputs. Evaluate this circuit by placing
the FILT jumpers (three per output channel) to position 1 (selectable by J9, J10 & J11 for OUTA1, etc.).
The instrumentation amplifier is optionally inserted before the LPF by changing the FILT jumpers to position 2. The
instrumentation amplifier incorporates a 5x gain (+14dB) which effectively lowers the noise contribution of the fol-
lowing 2-pole LPF. This improves the overall dynamic range of the system. The gain of this stage is determined from
the following equation:
The resistor designated by R
feedback resistors on the two sides of the instrumentation amp ‘R’ must be equal.
A resistor divider pad (parallel combination of R59 // R56 // R80 and R93 for AOUTA1) has been placed after the
low pass filter to bring the circuit back to unity gain (selectable with jumper J12 for AOUTA1).
In the resistor divider pad, three 3.01 kΩ, 1/4 W, 1210 size resistors are used in parallel to provide a combined re-
sistance of 1 kΩ and a combined power handling of 3/ 4W. This is done to provide sufficient power handling capa-
bility to accommodate the high signal levels output from the instrumentation amplifier stage. When not using the
instrumentation amplifier, these resistors may be reduced to a single 1kΩ, 1/10 W, 0805 size resistor (for muting
attenuation purposes).
In certain places throughout the output circuit, 1 /8W, 1206 size and 1/4W, 1210 size resistors are used. Similar to
the parallel resistors in the resistor divider pad, these are used to provide sufficient power handling capability in order
to accommodate the high signal levels output from the instrumentation amplifier stage. When not using the instru-
mentation amplifier, these resistors may all be replaced with 1/ 10W, 0805 size resistors.
The attenuation provided by the output mute transistor (Q2 for OUTA1) is determined by the resistor-divider formed
between the collector-emitter on-resistance and the output resistor of the LPF. The greater the output resistor, the
greater the attenuation will be for a given transistor. The trade off is that a high output impedance is not usually de-
sirable, and may affect the voltage transfer to the next stage based upon its input impedance.
The same resistor that affects the transistor mute level also affects the HPF formed with the output DC-block capac-
itor (C71 for OUTA1). For LPF configuration 2, the values for the DC-block capacitor and output resistor pad were
chosen to give uniform distortion performance across the audio bandwidth, particularly at low frequency. The HPF
formed by this R-C pair must be such that the voltage across the aluminum electrolytic DC-block capacitor must be
a minimum at 20 Hz. This keeps the distortion due to the electrolytic's dielectric absorption properties to a minimum.
For a design utilizing only LPF configuration 1, there is no post-LPF resistor-divider pad, and a much smaller value
capacitor can be used.
2
Figure 1. Instrumentation Amplifier Configuration
(see Figure 1) can be adjusted to change the gain of the instrumentation amp. The
IN+
IN-
R2
Gain
=
R
R
1
+
2 R
------------
R 2
( )
OUT+
OUT-
CDB42428
5