ADSP-BF504 datasheet(64/80 Pages) AD

64 / 80 page

Rev. PrC

Page 64 of 80

January 2010

ADSP-BF504/F,ADSP-BF506F

Preliminary Technical Data

supply using the 0 to VREF range or 2 × VREF range, respectively.

The common mode must be in this range to guarantee the func-

tionality of the ADC.

When a conversion takes place, the common mode is rejected,

resulting in a virtually noise free signal of amplitude –VREF to

+VREF corresponding to the digital codes of 0 to 4096. If the 2 ×

VREF range is used, then the input signal amplitude extends from

– 2 VREF to +2 VREF after conversion.

Driving Differential Inputs

Differential operation requires that VIN+ and VIN– be simulta-

neously driven with two equal signals that are 180° out of phase.

The common mode must be set up externally. The common-

mode range is determined by VREF, the power supply, and the

particular amplifier used to drive the analog inputs. Differential

modes of operation with either an ac or dc input provide the

best THD performance over a wide frequency range. Because

not all applications have a signal preconditioned for differential

operation, there is often a need to perform single-ended-to-dif-

ferential conversion.

Using an Op Amp Pair

An op amp pair can be used to directly couple a differential sig-

nal to one of the analog input pairs of the ADC. The circuit

configurations illustrated in Figure 78 (Dual Op Amp Circuit to

Convert a Single-Ended Unipolar Signal into a Differential Sig-

nal) and Figure 79 (Dual Op Amp Circuit to Convert a Single-

Ended Bipolar Signal into a Differential Unipolar Signal) show

how a dual op amp can be used to convert a single-ended signal

into a differential signal for both a bipolar and unipolar input

signal, respectively.

The voltage applied to Point A sets up the common-mode volt-

age. In both diagrams, it is connected in some way to the

reference, but any value in the common-mode range can be

input here to set up the common mode. The AD8022 is a suit-

able dual op amp that can be used in this configuration to

provide differential drive to the ADC.

Take care when choosing the op amp; the selection depends on

the required power supply and system performance objectives.

The driver circuits in Figure 78 (Dual Op Amp Circuit to Con-

vert a Single-Ended Unipolar Signal into a Differential Signal)

and Figure 79 (Dual Op Amp Circuit to Convert a Single-Ended

Bipolar Signal into a Differential Unipolar Signal) are optimized

for dc coupling applications requiring best distortion

performance.

The circuit configuration shown in Figure 78 (Dual Op Amp

Circuit to Convert a Single-Ended Unipolar Signal into a Differ-

ential Signal) converts a unipolar, single-ended signal into a

differential signal.

The differential op amp driver circuit shown in Figure 79 (Dual

Op Amp Circuit to Convert a Single-Ended Bipolar Signal into a

Differential Unipolar Signal) is configured to convert and level

shift a single-ended, ground-referenced (bipolar) signal to a dif-

ferential signal centered at the VREF level of the ADC.

Pseudo Differential Mode

The ADC can have a total of six pseudo differential pairs. In this

mode, VIN+ is connected to the signal source that must have an

amplitude of VREF (or 2 × VREF, depending on the range chosen)

Figure 76. Input Common-Mode Range vs. VREF (0 to VREF Range, VDD = 5 V)

Figure 77. Input Common-Mode Range vs. VREF (2 × VREF Range, VDD = 5 V)

VREF (V)

5.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

3.5

3.0

2.5

2.0

1.5

1.0

0.5

TA = 25°C

VREF (V)

2.5

0.5

1.0

1.5

2.0

5.0

4.0

4.5

3.0

3.5

2.0

2.5

0.5

1.0

1.5

TA = 25°C

Figure 78. Dual Op Amp Circuit to Convert a Single-Ended Unipolar Signal

into a Differential Signal

GND

2× VREF p–p

V+

V–

V+

V–

VREF

2.5V

3.75V

1.25V

2.5V

3.75V

1.25V

VREF

(DCAPA/DCAPB)

VIN+

ADC1

VIN–

440

220

0.47μF

1ADDITIONAL PINS OMITTED FOR CLARITY.

220

10k

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