AD9772A
DAC OPERATION
The 14-bit DAC along with the 1.2 V reference and reference
control amplifier is shown in Figure 14. The DAC consists of a
large PMOS current source array capable of providing up to
20 mA of full-scale current, I
OUTFS
. The array is divided into
thirty-one equal currents that make up the five most significant
bits (MSBs). The next four bits, or middle bits, consist of 15
equal current sources whose values are 1/16th of an MSB
current source. The remaining LSBs are binary weighted frac-
tions of the middle-bits’ current sources. All of these current
sources are switched to one or the other of two output nodes
(i.e., I
OUTA
or I
OUTB
) via PMOS differential current switches.
Implementing the middle and lower bits with current sources,
instead of an R-2R ladder, enhances its dynamic performance
for multitone or low amplitude signals and helps maintain the
DAC’s high output impedance.
2.7V TO 3.6V
As previously mentioned, I
OUTFS
is a function of the reference
current I
REF
, which is nominally set by a reference voltage
V
REFIO
, and external resistor, R
SET
. It can be expressed as:
I
OUTFS
= 32
×
I
REF
where
I
REF
=
V
REFIO
/R
SET
(4)
The two current outputs will typically drive a resistive load
directly or via a transformer. If dc coupling is required, I
OUTA
and I
OUTB
should be directly connected to matching resistive
loads, R
LOAD
, that are tied to analog common, ACOM. Note
that R
LOAD
may represent the equivalent load resistance seen by
I
OUTA
or I
OUTB
as would be the case in a doubly terminated 50
Ω
or 75
Ω
cable. The single-ended voltage output appearing at the
I
OUTA
and I
OUTB
nodes is simply:
V
OUTA
=
I
OUTA
×
R
LOAD
V
OUTB
=
I
OUTB
×
R
LOAD
(5)
(6)
(3)
REFLO
+1.2V REF
REFIO
0.1 F
R
SET
2k
FSADJ
I
REF
SEGMENTED
SWITCHES
250pF
AVDD
ACOM
CURRENT
SOURCE
ARRAY
I
OUTA
LSB
SWITCHES
I
OUTB
I
OUTA
V
DIFF
= V
OUTA
– V
OUTB
R
LOAD
R
LOAD
Note that the full-scale value of
V
OUTA
and
V
OUTB
should not
exceed the specified output compliance range of 1.25 V to pre-
vent signal compression. To maintain optimum distortion and
linearity performance, the maximum voltages at V
OUTA
and
V
OUTB
should not exceed
±
500 mV p-p.
The differential voltage, V
DIFF
, appearing across I
OUTA
and
I
OUTB
, is:
V
DIFF
= (I
OUTA
– I
OUTB
)
×
R
LOAD
Substituting the values of
I
OUTA
,
I
OUTB
and I
REF
; V
DIFF
can be
expressed as:
V
DIFF
= [(2
DAC CODE
– 16383)/16384]
×
(32
R
LOAD
/R
SET
)
×
V
REFIO
(8)
(7)
I
OUTB
AD9772A
INTERPOLATED
DIGITAL DATA
Figure 14. Block Diagram of Internal DAC, 1.2 V
Reference, and Reference Control Circuits
The full-scale output current is regulated by the reference control
amplifier and can be set from 2 mA to 20 mA via an external
resistor, R
SET
, as shown in Figure 14. R
SET
, in combination
with both the reference control amplifier and voltage reference,
REFIO, sets the reference current, I
REF
, which is mirrored to
the segmented current sources with the proper scaling factor.
The full-scale current, I
OUTFS
, is exactly thirty-two times the
value of I
REF
.
DAC TRANSFER FUNCTION
The last two equations highlight some of the advantages of
operating the AD9772A differentially. First, the differential
operation will help cancel common-mode error sources such as
noise, distortion and dc offsets associated with I
OUTA
and I
OUTB
.
Second, the differential code-dependent current and subsequent
voltage, V
DIFF
, is twice the value of the single-ended voltage
output (i.e., V
OUTA
or V
OUTB
), thus providing twice the signal
power to the load.
Note that the gain drift temperature performance for a single-
ended (V
OUTA
and V
OUTB
) or differential output (V
DIFF
) of the
AD9772A can be enhanced by selecting temperature tracking
resistors for R
LOAD
and R
SET
due to their ratiometric relation-
ship as shown in Equation 8.
REFERENCE OPERATION
The AD9772A provides complementary current outputs, I
OUTA
and I
OUTB
. I
OUTA
will provide a near full-scale current output,
I
OUTFS
, when all bits are high (i.e., DAC CODE = 16383) while
I
OUTB
, the complementary output, provides no current. The
current output appearing at I
OUTA
and I
OUTB
is a function of
both the input code and I
OUTFS
and can be expressed as:
I
OUTA
= (DAC
CODE/16384)
×
I
OUTFS
I
OUTB
= (16383 –
DAC CODE)/16384
×
I
OUTFS
(1)
(2)
where
DAC CODE
= 0 to 16383 (i.e., Decimal Representation).
The AD9772A contains an internal 1.20 V bandgap reference
that can easily be disabled and overridden by an external
reference. REFIO serves as either an
output
or
input,
depending
on whether the internal or external reference is selected. If
REFLO is tied to ACOM, as shown in Figure 15, the internal
reference is activated, and REFIO provides a 1.20 V output. In
this case, the internal reference
must
be compensated externally
with a ceramic chip capacitor of 0.1
µF
or greater from REFIO
to REFLO. If any additional loading is required, REFIO should
be buffered with an external amplifier having an input bias cur-
rent less than 100 nA.
–16–
REV. A
AD [ ANALOG DEVICES ]
