Datasheet

REV. B
–6–
AD768
I
REFIN
– mA
POWER – mW
550
500
300
1.0 7.02.0 3.0 4.0 5.0 6.0
450
400
350
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.
ERROR – LSB
I
REFIN
– mA
10
0
1.0 7.02.0 3.0 4.0 5.0 6.0
9
6
3
2
1
8
7
5
4
INL
DNL
Figure 4. INL/DNL vs. I
REFIN
Current
I
REFIN
– mA
SFDR – dB
–85
–55
1.0 7.02.0 3.0 4.0 5.0 6.0
–80
–75
–70
–65
–60
CLOCK = 10 MSPS
CLOCK = 20 MSPS
CLOCK = 30 MSPS
CLOCK = 40 MSPS
Figure 5. SFDR (to Nyquist) vs. I
REFIN
@ F
OUT
= 1 MHz
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,
I
OUT
= (DAC CODE/65536) × (I
REFIN
× 4)
In voltage output mode,
V
OUT
= I
OUT
× R
LOAD
iR
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
iR
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
iR
LAD
)/R
REF
, allowing for cancel-
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 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
1kΩ.
5
6
REFCOM
IREFIN
V
EE
V
EE
IFB
5mA
Figure 2. Equivalent Reference Input Circuit
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.