CDB4227 datasheet(13/36 Pages) CIRRUS

13 / 36 page

CS4227

DS281PP2

2.2.2

Adjustable Input Gain

The signals from the line inputs are routed to a pro-

grammable gain circuit which provides up to 9 dB

of gain in 3 dB steps, adjustable through the Input

Control Byte. Right and left channel gain settings

are controlled independently with the GNR1/0 and

GNL1/0 bits. To minimize audible artifacts, level

changes should be done with the channel muted, as

the changes occur immediately on register updates.

The ADC Status Report Byte provides feedback of

input level for each ADC channel. This register

continously monitors the ADC output and records

the peak output level since the last register read.

Reading this register causes it to reset to 0, where-

upon peak monitoring begins again.

2.2.3

High Pass Filter

The operational amplifiers in the input circuitry

driving the CS4227 may generate a small DC offset

into the A/D converter. The CS4227 includes a

high pass filter after the decimator to remove any

DC offset which could result in recording a DC lev-

el, possibly yielding "clicks" when switching be-

tween devices in a multichannel system.

The characteristics of this first-order high pass fil-

ter are outlined below for an output sample rate of

44.1 kHz. This filter response scales linearly with

sample rate.

Table 2. High Pass Filter Characteristics

2.3

Analog Outputs

2.3.1

Line Level Outputs

The CS4227 contains an on-chip buffer amplifier

producing single-ended outputs capable of driving

10 k

Ω loads. Each output (AOUT 1-6) will produce

a nominal 2.83 Vpp (1 Vrms) output with a 2.3 volt

quiescent voltage for a full scale digital input. The

recommended off-chip analog filter is a 2nd order

Butterworth with a -3 dB corner at Fs (see

Figure 7). This filter provides out-of-band noise at-

tenuation along with a gain of 2, providing a 2 Vrms

output signal. A 3rd order Butterworth filter with a

-3 dB corner at 0.75 Fs can be used if greater out of

band noise filtering is desired. The CS4227 DAC

interpolation filter is a linear phase design which

has been pre-compensated for an external 2nd or-

der Butterworth filter to provide a flat frequency re-

sponse and linear phase response over the

passband. If this filter is not used, small frequency

response magnitude and phase errors will occur.

Frequency Response

-3dB @ 3.4 Hz

-0.13 dB @ 20 Hz

Phase Deviation

10 degrees @ 20 Hz

Passband Ripple

None

22 k

Ω

150pF

3.9 k

Ω

11 k

Ω

1000pF

CMOUT

5 k

Ω

0.47

µF

Example

Op-Amps

are

MC33078

5.85 k

Ω

560 pF

1.21 k

Ω

1.1 k

Ω

5600 pF

CMOUT

5 k

Ω

0.47

µF

AOUT

4.75 k

Ω

5600 pF

2-Pole Butterworth Filter

3-Pole Butterworth Filter

Figure 7. Butterworth Filters

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