DAC TRANSFER FUNCTION
The total output current, I
OUTFS
, of the DAC904 is the sum-
mation of the two complementary output currents:
I
OUTFS
= I
OUT
+ I
OUT
(1)
+V
A
DAC904
The individual output currents depend on the DAC code and
can be expressed as:
I
OUT
= I
OUTFS
• (Code/16384)
I
OUT
= I
OUTFS
• (16383 – Code/16384)
(2)
I
OUT
I
OUT
R
L
(3)
R
L
where ‘Code’ is the decimal representation of the DAC data
input word. Additionally, I
OUTFS
is a function of the reference
current I
REF
, which is determined by the reference voltage
and the external setting resistor, R
SET
.
I
OUTFS
= 32 • I
REF
= 32 • V
REF
/R
SET
(4)
FIGURE 2. Equivalent Analog Output.
+V
D
= 5V. Note that the compliance range decreases to
about 1V for a selected output current of I
OUTFS
= 2mA.
Care should be taken that the configuration of the DAC904
does not exceed the compliance range to avoid degradation
of the distortion performance and integral linearity.
Best distortion performance is typically achieved with the
maximum full-scale output signal limited to approximately
0.5V. This is the case for a 50Ω doubly-terminated load and
a 20mA full-scale output current. A variety of loads can be
adapted to the output of the DAC904 by selecting a suitable
transformer while maintaining optimum voltage levels at
I
OUT
and I
OUT
. Furthermore, using the differential output
configuration in combination with a transformer will be instru-
mental for achieving excellent distortion performance. Com-
mon-mode errors, such as even-order harmonics or noise,
can be substantially reduced. This is particularly the case
with high output frequencies and/or output amplitudes below
full-scale.
For those applications requiring the optimum distortion and
noise performance, it is recommended to select a full-scale
output of 20mA. A lower full-scale range down to 2mA may
be considered for applications that require a low power
consumption, but can tolerate a reduced performance level.
In most cases the complementary outputs will drive resistive
loads or a terminated transformer. A signal voltage will
develop at each output according to:
V
OUT
= I
OUT
• R
LOAD
V
OUT
= I
OUT
• R
LOAD
(5)
(6)
The value of the load resistance is limited by the output
compliance specification of the DAC904. To maintain speci-
fied linearity performance, the voltage for I
OUT
and I
OUT
should not exceed the maximum allowable compliance range.
The two single-ended output voltages can be combined to
find the total differential output swing:
(7)
V
OUTDIFF
=
V
OUT
– V
OUT
=
(2
•
Code – 16383)
•
I
OUTFS
•
R
LOAD
16384
ANALOG OUTPUTS
The DAC904 provides two complementary current outputs,
I
OUT
and I
OUT
. The simplified circuit of the analog output
stage representing the differential topology is shown in
Figure 2. The output impedance of 200kΩ 12pF for I
OUT
and I
OUT
results from the parallel combination of the differen-
tial switches, along with the current sources and associated
parasitic capacitances.
The signal voltage swing that may develop at the two
outputs, I
OUT
and I
OUT
, is limited by a negative and positive
compliance. The negative limit of –1V is given by the break-
down voltage of the CMOS process, and exceeding it will
compromise the reliability of the DAC904, or even cause
permanent damage. With the full-scale output set to 20mA,
the positive compliance equals 1.25V, operating with
INPUT CODE (D13 - D0)
11 1111 1111 1111
10 0000 0000 0000
00 0000 0000 0000
I
OUT
20mA
10mA
0mA
I
OUT
0mA
10mA
20mA
TABLE I. Input Coding versus Analog Output Current.
OUTPUT CONFIGURATIONS
The current output of the DAC904 allows for a variety of
configurations, some of which are illustrated below. As men-
tioned previously, utilizing the converter’s differential outputs
will yield the best dynamic performance. Such a differential
output circuit may consist of an RF transformer (see Figure 3)
or a differential amplifier configuration (see Figure 4). The
DAC904
SBAS095C
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