DS658F2

25

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 35” outlines the bit settings necessary to control the input

multiplexer and mic gain. “Channel B PGA Control - Address 07h” on page 33 and “Channel A PGA Control

- Address 08h” on page 34 outline the register settings necessary to control the PGA. By default, line-

level input 1 is selected, and the 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

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 Output

The CS5345 includes an auxiliary analog output through the PGAOUT pins. These pins can be configured

to output the analog input to the ADC as selected by the input MUX and gained or attenuated with the PGA,

or alternatively, they may be set to high-impedance. See the ““PGAOut Source Select (Bit 6)” on page 33”

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 Characteristics” on 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 Bits

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