AD768 datasheet(15/20 Pages) AD

15 / 20 page

AD768

REV. B

–15–

AD768 EVALUATION BOARD

GENERAL DESCRIPTION

The AD768-EB is an evaluation board for the AD768 16-bit

30 Msps D/A converter. Careful attention to layout and circuit

design combined with analog and digital prototyping areas al-

lows the user to easily and effectively evaluate the AD768 in any

application where high resolution, high speed conversion is

required.

The digital inputs to the AD768-EB may be driven directly us-

ing the standard 40-pin IDC connector. An external clock is

also required. These signals may be applied from a user’s

bench, or they can be generated from a circuit built on the

prototyping area. The analog outputs from the AD768-EB are

available on BNC connectors. These outputs may be configured

to use either resistors, op amps, or a transformer.

OPERATING PROCEDURE AND FUNCTIONAL

DESCRIPTION

Power

Power may be supplied to the AD768-EB by applying either

wires or banana plugs to the metal binding posts included on the

printed circuit board.

DGND. Digital Ground. The digital ground and the analog

ground are connected together underneath the AD768. Optimal

performance can be obtained with separate analog and digital

supplies. For evaluation purposes, a single-supply which makes

a second analog and digital ground connection at the supply is

acceptable.

+5D. The +5 V (

±5%) digital supply should be capable of sup-

plying 50 mA.

–5A. The –5 V (

±5%) analog supply should be capable of sup-

plying –75 mA.

AGND. Analog ground. The analog ground and the digital

ground are connected together underneath the AD768. Optimal

performance can be obtained with separate analog and digital

supplies. For evaluation purposes, a single-supply which makes

a second analog and digital ground connection at the supply is

acceptable.

–VEE. Negative analog supply; typically –5 V to –15 V. This

supply is used as the negative supply rail for the external op

amps. For the AD811 supplied with the AD768-EB, a supply

capable of supplying –20 mA (excluding external load require-

ments) is required.

+VCC. Positive analog supply; typically +5 V to +15 V. This

supply is used as the positive supply rail for the external op

amps. For the AD811 supplied with the AD768-EB, a supply

capable of supplying +20 mA (excluding external load require-

ments) is required.

Analog Outputs

The analog output(s) from the AD768-EB are available on BNC

jacks “A” and “B.” The complementary current outputs from

the AD768 can be configured using either resistors, op amps, or

a transformer. Only the “A” portion of the AD768-EB is popu-

lated and shipped from the factory. The “B” side, or comple-

mentary output, may be populated by the user if so desired.

JP1.

Buffered op amp output “A”. Jumper JP1 should be

installed if the buffered op amp output is desired. When JP1

is installed, JP2 and JP3 must be removed for proper

operation. The output, available on the “A” connector, has a

nominal voltage swing of 0 V to 2 V and is in-phase with the

digital input. This is the factory default setting.

JP2. Bipolar 50

Ω transformer output. If jumper JP2 is in-

stalled, a transformer coupled output is available on the “A”

connector. When JP2 is installed, JP1 and JP3 must be re-

moved for proper operation. The transformer acts both as a

differential-to-single-ended converter and as an impedance

transformer. For proper operation, the transformer must be

terminated with a 50

Ω resistor. R2 must be replaced with

the 100

Ω resistor, R7. An additional 100 Ω resistor and the

transformer are included with the AD768-EB. The additional

100

Ω resistor must be soldered into the appropriate position la-

beled “R3” and the transformer must be inserted into the

socket labeled “T1.” The nominal output voltage into a 50

Ω

load is 1 V p-p centered on a common-mode voltage of 0 V.

JP3. Resistor output “A.” JP3 is used to connect the resistor

R2 to the “A” output. U2 should be removed from its socket.

Using a 24.9

Ω resistor for R2, the output is an unbuffered 0 V

to –0.5 V output that is out of phase with the digital input. Re-

sistor R2 may be replaced with other values, but careful atten-

tion to the recommended output compliance range should be

observed. When JP3 is installed, JP1 and JP2 must be re-

moved for proper operation.

JP4. Resistor output “B.” JP4 is used to connect the resistor

R3 to the “A” output. U3 should be removed from its socket.

The AD768-EB is shipped from the factory with resistor R3

shorted to ground. A different value selected by the user can be

installed for R3 to generate an unbuffered output that is in-

phase with the digital input. Careful attention to the recom-

mended output compliance range should be observed when

selecting the value of R3. When JP4 is installed, JP5 must

be removed for proper operation.

JP5. Buffered op amp output “B.” Jumper JP5 should be in-

stalled if the buffered op amp output is desired. When JP5 is

installed, JP4 must be removed for proper operation.

The output is available on the “B” connector and has a nominal

voltage swing determined by the combination of resistors R3,

R9, and R10. This op amp is not provided with the AD768-EB.

Reference

Either the internal reference of the AD768 or an external refer-

ence may be selected on the AD768-EB. R12 is used to adjust

the full-scale output current of the AD768.

SW2. Internal/External reference select switch. When SW2 is

in position 1, the internal reference of the AD768 is selected.

When SW2 is in position 2, an external reference must be pro-

vided by the user.

Level-Shifting the Analog Output

Resistor sockets R8 and R6 can be populated with an appropri-

ately valued resistor to add dc offset current to an output which

uses the op amp configuration. As an example, to generate a

bipolar output signal, a 1.25 k

Ω resistor installed into the “R8”

socket level-shifts the normally unipolar output by –1 V. The

factory defaults for R8 and R6 are open circuits.

AD768 Datasheet(PDF) 15 Page - Analog Devices

AD768 Datasheet(HTML) 15 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL