AD768
DAC TRANSFER FUNCTION
550
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,
POWER – mW
500
I
OUT
= (DAC CODE/65536)
×
(I
REFIN
×
4)
In voltage output mode,
V
OUT
= I
OUT
×
R
LOAD
R
LAD
where:
DAC CODE
is the decimal representation of the DAC inputs;
an integer between 0 and 65535.
I
REFIN
is the current applied at the IREFIN pin, determined by
V
REF
/R
REF
.
Substituting for I
OUT
and I
REFIN
,
V
OUT
= –V
REF
×
(DAC
CODE/65536)
×
4
×
[(R
LOAD
R
LAD
)/R
REF
]
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
LOAD
R
LAD
)/R
REF
, allowing for cancel-
lation of resistor drift by selection of resistors with matched
characteristics.
REFERENCE INPUT
450
400
350
300
1.0
2.0
3.0
4.0
I
REFIN
– mA
5.0
6.0
7.0
Figure 3. Power Dissipation vs. I
REFIN
Current
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.
10
9
8
7
ERROR – LSB
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
Ω
from
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 kΩ.
REFCOM
IREFIN
5
6
5
4
3
2
1
0
1.0
2.0
3.0
4.0
I
REFIN
– mA
5.0
6.0
7.0
DNL
INL
Figure 4. INL/DNL vs. I
REFIN
Current
6
–85
IFB
5mA
V
EE
V
EE
CLOCK = 10 MSPS
–80
–75
SFDR – dB
Figure 2. Equivalent Reference Input Circuit
CLOCK = 20 MSPS
The I
REFIN
current can be varied from 1 mA to 7 mA which
subsequently will result in a proportional change in the DAC
full-scale. Since the operating currents within the DAC vary
with I
REFIN
, so does the power dissipation. Figure 3 illustrates
that relationship.
–70
CLOCK = 30 MSPS
–65
CLOCK = 40 MSPS
–60
–55
1.0
2.0
3.0
4.0
I
REFIN
– mA
5.0
6.0
7.0
Figure 5. SFDR (to Nyquist) vs. I
REFIN
@ F
OUT
= 1 MHz
–6–
REV. B
AD [ ANALOG DEVICES ]
