AD768 Analog Devices, AD768 Datasheet - Page 15

• Manufacturer Part Number: AD768
• Description: 16-Bit, 30 MSPS D/A Converter
• Manufacturer: Analog Devices

Specifications of AD768

• Resolution (bits): 16bit
• Dac Update Rate: 30MSPS
• Dac Settling Time: 25ns
• Max Pos Supply (v): +5.25V
• Single-supply: No
• Dac Type: Current Out
• Dac Input Format: Par

REV. B
AD768 EVALUATION BOARD
GENERAL DESCRIPTION
The AD768-EB is an evaluation board for the AD768 16-bit
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.
–V
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.
+V
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.
30 Msps D/A converter. Careful attention to layout and circuit
EE
CC
. Negative analog supply; typically –5 V to –15 V. This
. Positive analog supply; typically +5 V to +15 V. This
–15–
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
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
the 100
transformer are included with the AD768-EB. The additional
100
beled “R3” and the transformer must be inserted into the
socket labeled “T1.” The nominal output voltage into a 50
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
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 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.
resistor must be soldered into the appropriate position la-
resistor, R7. An additional 100
resistor for R2, the output is an unbuffered 0 V
transformer output. If jumper JP2 is in-
resistor. R2 must be replaced with
resistor and the
AD768