AD7568 Просмотр технического описания (PDF) - Analog Devices

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AD7568

LC2MOS Octal 12-Bit DAC

Analog Devices 

AD7568

UNIPOLAR BINARY OPERATION

(2-Quadrant Multiplication)

Figure 15 shows the standard unipolar binary connection dia-

gram for one of the DACs in the AD7568. When VIN is an ac

signal, the circuit performs 2-quadrant multiplication. Resistors

R1 and R2 allow the user to adjust the DAC gain error. Offset

can be removed by adjusting the output amplifier offset voltage.

A1 should be chosen to suit the application. For example, the

AD OP07 or OP177 are ideal for very low bandwidth applica-

tions while the AD843 and AD845 offer very fast settling time

in wide bandwidth applications. Appropriate multiple versions

of these amplifiers can be used with the AD7568 to reduce

board space requirements.

The code table for Figure 15 is shown in Table III.

R2 10Ω

R1 20Ω

VIN

RFBA

DAC A

I OUT1 A

C1

A1

VOUT

VREF A

AD7568

I OUT2 A

A1: OP-177

SIGNAL

ADOP-07

GND

AD711

NOTES

1. ONLY ONE DAC IS SHOWN FOR CLARITY.

AD843

AD845

2. DIGITAL INPUT CONNECTIONS ARE OMITTED.

3. C1 PHASE COMPENSATION (5–15pF) MAY BE

REQUIRED WHEN USING HIGH SPEED AMPLIFIER, A1.

Figure 15. Unipolar Binary Operation

Table III. Unipolar Binary Code Table

Digital Input

MSB………LSB

Analog Output

(VOUT As Shown in Figure 15)

1111 1111 1111

1000 0000 0001

1000 0000 0000

0111 1111 1111

0000 0000 0001

0000 0000 0000

–VREF (4095/4096)

–VREF (2049/4096)

–VREF (2048/4096)

–VREF (2047/4096)

–VREF (1/4096)

–VREF (0/4096) = 0

NOTE

Nominal LSB size for the circuit of Figure 15 is given by:

VREF (1/4096).

BIPOLAR OPERATION

(4-Quadrant Multiplication)

Figure 16 shows the standard connection diagram for bipolar

operation of any one of the DACs in the AD7568. The coding is

offset binary as shown in Table IV. When VIN is an ac signal,

the circuit performs 4-quadrant multiplication. To maintain the

gain error specifications, resistors R3, R4 and R5 should be ra-

tio matched to 0.01%.

R4

R2 10Ω

R1 20Ω

VIN

RFB A

DAC A

I OUT1 A

VREF A

I OUT2 A

AD7568

20kΩ

C1

A1

R3

10kΩ

R5

20kΩ

VOUT

A2

NOTES

SIGNAL

GND

1. ONLY ONE DAC IS SHOWN FOR CLARITY.

2. DIGITAL INPUT CONNECTIONS ARE OMITTED.

3. C1 PHASE COMPENSATION (5–15pF) MAY BE

REQUIRED WHEN USING HIGH SPEED AMPLIFIER, A1.

Figure 16. Bipolar Operation (4-Quadrant Multiplication)

Table IV. Bipolar (Offset Binary) Code Table

Digital Input

Analog Output

MSB . . . . . LSB (VOUT As Shown in Figure 16)

1111 1111 1111

1000 0000 0001

1000 0000 0000

0111 1111 1111

0000 0000 0001

0000 0000 0000

+VREF (2047/2048)

+VREF (1/2048)

+VREF (0/2048) = 0

–VREF (1/2048)

–VREF (2047/2048)

–VREF (2048/2048) = –VREF

NOTE

Nominal LSB size for the circuit of Figure 16 is given by:

VREF (1/2048).

SINGLE SUPPLY CIRCUITS

The AD7568 operates from a single +5 V supply, and this

makes it ideal for single supply systems. When operating in such

a system, it is not possible to use the standard circuits of Figures

15 and 16 since these invert the analog input, VIN. There are

two alternatives. One of these continues to operate the DAC as

a current-mode device, while the other uses the voltage switch-

ing mode.

VIN

VREF A

RFB A

IOUT1 A

DAC A

A1

AD7568

IOUT2 A

VOUT

NOTES

VBIAS

1. ONLY ONE DAC IS SHOWN FOR CLARITY.

2. DIGITAL INPUT CONNECTIONS ARE OMITTED.

3. C1 PHASE COMPENSATION (5–15pF) MAY BE

REQUIRED WHEN USING HIGH SPEED AMPLIFIER, A1.

Figure 17. Single Supply Current-Mode Operation

–8–

REV. C

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