Datasheet

AD9772A
Rev. C | Page 22 of 40
DAC OPERATION
The 14-bit DAC, along with the 1.2 V reference and reference
control amplifier, is shown in
Figure 37. 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 31
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 1/16
th
of an MSB current
source. The remaining LSBs are binary-weighted fractions of
the middle bits’ current sources. All of these current sources are
switched to one of the two output nodes (that is, I
OUTA
or I
OUTB
) via
the 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 high output impedance of the DAC.
REFIO
FSADJ
250pF
REFLO AVDD
AD9772A
R
SET
2kΩ
0.1µF
ACOM
CURRENT
SOURCE
ARRAY
I
OUTA
I
OUTB
INTERPOLATED
DIGITAL DATA
R
LOAD
R
LOAD
V
DIFF
= V
OUTA
– V
OUTB
I
OUTA
I
OUTB
SEGMENTED
SWITCHES
LSB
SWITCHES
1.2V REF
2.7V TO 3.6V
I
REF
02253-037
Figure 37. 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 37. 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 32 times
the value of I
REF
.
DAC TRANSFER FUNCTION
The AD9772A provides complementary current outputs, I
OUTA
and I
OUTB
. IB
OUTA
provides a near full-scale current output, I
OUTFS
,
when all bits are high (that is, DAC CODE = 16,383), whereas
I
OUTB
B, 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
B
OUTFS
and can be expressed as
I
OUTA
= (DAC CODE/16,384) × I
OUTFS
(1)
I
OUTB
= (16,383 − DAC CODE)/16,384 × I
OUTFS
(2)
where DAC CODE = 0 to 16,383 (that is, decimal representation).
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 an external resistor (R
SET
). It can be expressed as
I
OUTFS
= 32 × I
REF
(3)
where:
I
REF
= V
REFIO
/R
SET
(4)
The two current outputs 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
can
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
(5)
V
OUTB
= I
OUTB
× R
LOAD
(6)
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
prevent 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. B
The differential voltage, V
DIFF
, appearing across I
OUTA
and I
OUTB
is B
V
DIFF
= (I
OUTA
− I
OUTB
) × R
LOAD
(7)
Substituting the values of I
OUTA
, I
OUTB
, and IB
REF
, V
DIFF
can be
expressed as
V
DIFF
= [(2 DAC CODE − 16,383)/16,384] ×
(32 × R
LOAD
/R
SET
) × V
REFIO
(8)
The last two equations highlight some of the advantages of
operating the AD9772A differentially. First, the differential
operation helps 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 (that is, V
OUTA
or V
OUTB
), thus providing twice the
signal power to the load.
B
Note that the gain drift temperature performance for a single-
ended (V
OUTA
and V
OUTB
) or differential output (VB
DIFF
) of the
AD9772A can be enhanced by selecting temperature tracking
resistors for R
LOAD
and R
SET
due to their ratiometric relationship,
as shown in Equation 8.
REFERENCE OPERATION
The AD9772A contains an internal 1.20 V band gap reference
that can easily be disabled and overridden by an external
reference. REFIO serves as either an output or input, depend-
ing on whether the internal or external reference is selected. If
REFLO is tied to ACOM, as shown in
Figure 38, 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
current less than 100 nA.