ADSP-BF504 datasheet(63/80 Pages) AD

63 / 80 page

Preliminary Technical Data

Rev. PrC

Page 63 of 80

January 2010

ADSP-BF504/F,ADSP-BF506F

Analog Inputs

The ADC has a total of 12 analog inputs. Each on-board ADC

has six analog inputs that can be configured as six single-ended

channels, three pseudo differential channels, or three fully dif-

ferential channels. These may be selected as described in the

Analog Input Selection section.

Single-Ended Mode

The ADC can have a total of 12 single-ended analog input chan-

nels. In applications where the signal source has high

impedance, it is recommended to buffer the analog input

before applying it to the ADC. The analog input range can be

programmed to be either 0 to VREF or 0 to 2 × VREF.

If the analog input signal to be sampled is bipolar, the internal

reference of the ADC can be used to externally bias up this sig-

nal to make it correctly formatted for the ADC. Figure 74 shows

a typical connection diagram when operating the ADC in sin-

gle-ended mode.

Differential Mode

The ADC can have a total of six differential analog input pairs.

Differential signals have some benefits over single-ended sig-

nals, including noise immunity based on the device’s common-

mode rejection and improvements in distortion performance.

Figure 75 (Differential Input Definition) defines the fully differ-

ential analog input of the ADC.

The amplitude of the differential signal is the difference between

the signals applied to the VIN+ and VIN– pins in each differential

pair (VIN+ VIN–). VIN+ and VIN– should be simultaneously driven

by two signals each of amplitude VREF (or 2 × VREF, depending

on the range chosen) that are 180° out of phase. The amplitude

of the differential signal is, therefore (assuming the 0 to VREF

range is selected) –VREF to +VREF peak-to-peak (2 × VREF),

regardless of the common mode (CM).

The common mode is the average of the two signals

(VIN+ + VIN–)/2

and is, therefore, the voltage on which the two inputs are

centered.

This results in the span of each input being CM ± VREF/2. This

voltage has to be set up externally and its range varies with the

reference value, VREF. As the value of VREF increases, the com-

mon-mode range decreases. When driving the inputs with an

amplifier, the actual common-mode range is determined by the

amplifier’s output voltage swing.

Figure 76 (Input Common-Mode Range vs. VREF (0 to VREF

Range, VDD = 5 V)) and Figure 77 (Input Common-Mode

Range vs. VREF (2 × VREF Range, VDD = 5 V)) show how the

common-mode range typically varies with VREF for a 5 V power

Figure 72. THD vs. Analog Input Frequency for

Various Source Impedances, Differential Mode

Figure 73. THD vs. Analog Input Frequency for Various Supply Voltages

INPUT FREQUENCY (kHz)

600

700

800

900 1000

200

100

400

300

500

–60

–65

–70

–75

–80

–85

–90

FSAMPLE = 1.5MSPS

VDD = 3V

RANGE = 0V TO VREF

RSOURCE = 300

RSOURCE = 0

RSOURCE = 10

RSOURCE = 47

RSOURCE = 100

INPUT FREQUENCY (kHz)

600

700

800

900 1000

200

100

400

300

500

–50

–60

–55

–65

–70

–75

–80

–85

–90

VDD = 3V

SINGLE-ENDED MODE

VDD = 5V

SINGLE-ENDED MODE

VDD = 3V

DIFFERENTIAL MODE

VDD = 5V

DIFFERENTIAL MODE

FSAMPLE = 1.5MSPS/2MSPS

VDD = 3V/5V

RANGE = 0 TO VREF

Figure 74. Single-Ended Mode Connection Diagram

Figure 75. Differential Input Definition

VIN

+1.25V

–1.25V

REF

(DCAPA/DCAPB)

VA1

ADC1

VB6

+2.5V

0.47μF

1ADDITIONAL PINS OMITTED FOR CLARITY.

VIN+

ADC1

VIN–

VREF p-p

COMMON

MODE

VOLTAGE

1ADDITIONAL PINS OMITTED FOR CLARITY.

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