DSP56156ROM datasheet(70/76 Pages) MOTOROLA

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Design Considerations

MOTOROLA

DSP56156 Data Sheet

Analog I/O Considerations

Figure 44 describes the recommended analog I/O and power supply configurations. The two

analog inputs are electrically identical. When one is not used, it can be left floating. When

used, an AC coupling capacitor is required. The value of the capacitor along with the input

impedance of the pin determine the cut off frequency of a high pass filter. The input imped-

ance of the MIC and AUX varies as a function of the sigma-delta (²ý) modulator master clock.

78 k

Ω is a typical value at 2 MHz. An AC capacitor of 1µF defines a high pass filter pole of 2

Hz. A smaller capacitor value will move this pole higher in frequency.

Figure 44 Recommended Analog I/O Configuration

Σ∆

modulator

2.0 V

±10%

(≤ ±1mA)

MIC

VREF

AUX

Bias

Š10

µF

-6 dB

6 dB

MGS1-0 bits

3 POLE

2 ZERO

Low Pass

Filter (LPF)

VC3-VC0

(to microphone)

SPKP

(2/5 VCC)

digital VCC

INS bit

17 dB

VDIV

ð50 nF

µF

600

Ω

0.001

µF

600

Ω

RBias

µF

0.1

µF

+5 dB

54 K

Ω

36 K

Ω

Š1 K

Ω

10 K

Ω

VREF

0.001

µF

VCCA

GNDA

µF

0.1

µF

GNDA

+5 V

220

µF

Analog Decoupling

near DSP

Single trace

External

External Supply

GND

digital GND

Single trace

GNDA

VCCA

0.01

µF

SPKM

VREF

GND

Analog I/O Considerations

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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