AD768 Analog Devices, AD768 Datasheet - Page 6

• 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

AD768
DAC TRANSFER FUNCTION
The AD768 may be used in either current-output mode with the
output connected to a virtual ground, or voltage-output mode
with the output connected to a resistive load.
In current output mode,
In voltage output mode,
where:
DAC CODE is the decimal representation of the DAC inputs;
an integer between 0 and 65535.
I
V
Substituting for I
V
These equations clarify an important aspect of the AD768
transfer function; the full-scale current output of the DAC is
proportional to a current input. The voltage output is then a
function of the ratio of (R
lation of resistor drift by selection of resistors with matched
characteristics.
REFERENCE INPUT
The IREFIN pin is a current input node with low impedance to
REFCOM. This input current sets the magnitude of the DAC
current sources such that the full-scale output current is exactly
four times the current applied at IREFIN. For the nominal in-
put current of 5 mA, the nominal full-scale output current is
20 mA.
The 5 mA reference input current can be generated from the
on-chip 2.5 V reference with an external resistor of 500
REFOUT to IREFIN. If desired, a variety of external reference
voltages may be used based on the selection of an appropriate
resistor. However, to maintain stability of the reference ampli-
fier, the external impedance at IREFIN must be kept below
1 k .
The I
subsequently will result in a proportional change in the DAC
full-scale. Since the operating currents within the DAC vary
with I
that relationship.
REFIN
REF
OUT
/R
= –V
REFIN
REFIN
is the current applied at the IREFIN pin, determined by
REF
Figure 2. Equivalent Reference Input Circuit
.
REF
, so does the power dissipation. Figure 3 illustrates
I
OUT
current can be varied from 1 mA to 7 mA which
(DAC CODE/65536)
= (DAC CODE/65536) (I
REFCOM
OUT
IREFIN
V
OUT
and I
= I
5
6
REFIN
LOAD
OUT
V
EE
,
R
LAD
R
LOAD
)/R
4
REF
R
V
LAD
EE
, allowing for cancel-
[(R
REFIN
IFB
5mA
LOAD
4)
R
LAD
)/R
from
REF
]
–6–
Note the AD768 is optimized for operation at an input current
of 5 mA. Both linearity and dynamic performance at other input
currents may be somewhat degraded. Figure 4 shows typical dc
linearity over a range of input currents. Figure 5 shows typical
SFDR (to Nyquist) performance over a range of input currents
and CLOCK input rates for a 1 MHz output frequency.
Figure 5. SFDR (to Nyquist) vs. I
Figure 3. Power Dissipation vs. I
550
500
450
400
350
300
10
–85
–80
–75
–70
–65
–60
–55
9
8
7
6
5
4
3
2
1
0
1.0
1.0
1.0
Figure 4. INL/DNL vs. I
2.0
2.0
2.0
3.0
CLOCK = 30 MSPS
3.0
3.0
I
REFIN
I
I
REFIN
REFIN
CLOCK = 40 MSPS
4.0
4.0
CLOCK = 10 MSPS
4.0
– mA
– mA
– mA
REFIN
REFIN
CLOCK = 20 MSPS
5.0
5.0
5.0
REFIN
Current
INL
@ F
OUT
6.0
6.0
6.0
Current
DNL
= 1 MHz
7.0
7.0
7.0
REV. B