User Manual

26 DS658F4
CS5345
4.4 Analog Input Multiplexer, PGA, and Mic Gain
The CS5345 contains a stereo 6-to-1 analog input multiplexer followed by a programmable gain amplifier
(PGA). The input multiplexer can select one of six possible stereo analog input sources and route it to the
PGA. Analog inputs 4A and 4B are able to insert a +32 dB gain stage before the input multiplexer, allowing
them to be used for microphone-level signals without the need for any external gain. The PGA stage pro-
vides 12 dB of gain or attenuation in 0.5 dB steps. Figure 9 shows the architecture of the input multiplexer,
PGA, and microphone gain stages. .
The “Analog Input Selection (Bits 2:0)” on page 36 outlines the bit settings necessary to control the input
multiplexer and mic gain. “Channel B PGA Control - Address 07h” on page 34 and “Channel A PGA Control
- Address 08h” on page 35 outline the register settings necessary to control the PGA. By default, line-
level input 1 is selected, and th e PGA is set to 0 dB.
4.5 Input Connections
The analog modulator samples the input at 6.144 MHz (MCLK=12.288 MHz). The digital filter will reject sig-
nals within the stopband of the filter. However, there is no rejection for input signals which are
(n
 6.144 MHz) the digital passband frequency, where n=0,1,2,... Refer to the Typical Connection Diagram
for the recommended analog input circuit that will attenuate noise energy at 6.144 MHz. The use of capac-
itors which have a large voltage coefficient (such as general-purpose ceramics) must be avoided since
these can degrade signal linearity. Any unused analog input pairs should be left unconnected.
4.6 PGA Auxiliary Analog Ou tput
The CS5345 includes an auxiliary analog output through the PGAOUT pins. These pins can be configured
to output the analog in put to the ADC as selected b y the input MUX and gained or attenuated with the PGA,
or alternativ ely, th ey ma y be set t o high -imp eda nce. See the “ “PGAOut Source Select (Bit 6)” on page 34”
for information on configuring the PGA auxiliary analog output.
The PGA auxiliary analog output can source very little current. As current from the PGAOUT pins increases,
distortion will increase. For this reason, a high-input impedance buffer must be used on the PGAOUT pins
to achieve full performance. Refer to the table in “PGAOUT Analog Ch ar acteristics” o n page 12 for accept-
able loading conditions.
PGA
MUX
+32 dB
AIN1A
AIN2A
AIN3A
AIN4A/MICIN1
AIN5A
AIN6A
PGA
MUX
+32 dB
AIN1B
AIN2B
AIN3B
AIN4B/MICIN2
AIN5B
AIN6B
Analog Input
Selection Bit s
Channel A
PGA Gain Bits
Channel B
PGA Gain Bits
Out to ADC
Channel A
Out to ADC
Channel B
Figure 9. Analog Input Architecture