AD9772EB Analog Devices, AD9772EB Datasheet - Page 16

• Manufacturer Part Number: AD9772EB
• Description: 14-Bit/ 160 MSPS TxDAC+ with 2x Interpolation Filter
• Manufacturer: Analog Devices

AD9772
The current output appearing at IOUTA and IOUTB is a func-
tion of both the input code and I
where DAC CODE = 0 to 16383 (i.e., Decimal Representation).
As previously mentioned, I
current I
V
where
The two current outputs will typically drive a resistive load
directly or via a transformer. If dc coupling is required, IOUTA
and IOUTB should be directly connected to matching resistive
loads, R
that R
IOUTA or IOUTB as would be the case in a doubly terminated
50
at the IOUTA and IOUTB nodes is simply:
Note that the full-scale value of V
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
V
The differential voltage, V
IOUTB, is:
Substituting the values of IOUTA, IOUTB and I
be expressed as:
The last two equations highlight some of the advantages of
operating the AD9772 differentially. First, the differential
operation will help cancel common-mode error sources such as
noise, distortion and dc offsets associated with IOUTA and
IOUTB. Second, the differential code-dependent current and
subsequent voltage, V
voltage output (i.e., V
signal power to the load.
Note that the gain drift temperature performance for a single-
ended (VOUTA and VOUTB) or differential output (V
the AD9772 can be enhanced by selecting temperature tracking
resistors for R
ship as shown in Equation 8.
REFERENCE OPERATION
The AD9772 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 32, 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 F or greater from REFIO
REFIO
OUTB
V
V
OUTA
OUTB
IOUTA = (DAC CODE/16384)
IOUTB = (16383 – DAC CODE)/16384
I
I
V
V
(32 R
OUTFS
REF
or 75
LOAD
DIFF
DIFF
, and external resistor, R
should not exceed 500 mV p-p.
LOAD
= V
= IOUTA
= IOUTB
REF
LOAD
= (IOUTA – IOUTB)
= [(2 DAC CODE – 16383)/16384]
= 32
may represent the equivalent load resistance seen by
, which is nominally set by a reference voltage
, that are tied to analog common, ACOM. Note
REFIO
cable. The single-ended voltage output appearing
LOAD
/R
SET
/R
I
and R
REF
)
SET
R
R
DIFF
OUTA
LOAD
LOAD
V
REFIO
SET
, is twice the value of the single-ended
DIFF
OUTFS
or V
due to their ratiometric relation-
, appearing across IOUTA and
SET
OUTB
OUTFS
is a function of the reference
OUTA
. It can be expressed as:
R
LOAD
), thus providing twice the
I
and can be expressed as:
and V
OUTFS
OUTB
I
OUTFS
REF
OUTA
should not
; V
DIFF
DIFF
and
) of
can
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
–16–
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.
The internal reference can be disabled by connecting REFLO to
AVDD. In this case, an external 1.2 V reference such as the
AD1580 may then be applied to REFIO as shown in Figure 33.
The external reference may provide either a fixed reference
voltage to enhance accuracy and drift performance or a varying
reference voltage for gain control. Note that the 0.1 F compen-
sation capacitor is not required since the internal reference is
disabled, and the high input impedance of REFIO minimizes
any loading of the external reference.
REFERENCE CONTROL AMPLIFIER
The AD9772 also contains an internal control amplifier that is
used to regulate the DAC’s full-scale output current, I
The control amplifier is configured as a V-I converter, as shown
in Figure 33, such that its current output, I
the ratio of the V
in Equation 4. I
sources with the proper scaling factor to set I
Equation 3.
The control amplifier allows a wide (10:1) adjustment span of
I
62.5 A and 625 A. The wide adjustment span of I
provides several application benefits. The first benefit relates
directly to the power dissipation of the AD9772’s DAC, which
is proportional to I
tion). The second benefit relates to the 20 dB adjustment, which
is useful for system gain control purposes.
I
Figure 34 for a fixed R
ence is disabled, and the voltage of REFIO is varied over its
compliance range of 1.25 V to 0.10 V. REFIO can be driven
ADDITIONAL
OUTFS
REF
AD1580
can be controlled using the single-supply circuit shown in
LOAD
over a 2 mA to 20 mA range by setting I
Figure 33. External Reference Configuration
Figure 32. Internal Reference Configuration
10k
REF BUFFER
EXTERNAL
OPTIONAL
R
SET
REF
REFIO
I
V
REF
REFIO
OUTFS
V
is copied over to the segmented current
REFIO
=
2k
0.1 F
/R
and an external resistor, R
SET
SET
(refer to the Power Dissipation sec-
. In this example, the internal refer-
REFIO
FSADJ
REFIO
FSADJ
REFLO
REFLO
AD9772
AD9772
+1.2V REF
+1.2V REF
REF
REFERENCE
AMPLIFIER
CONTROL
OUTFS
250pF
, is determined by
250pF
REF
SET
+2.7V TO +3.6V
+2.7 TO +3.6V
as stated in
between
AVDD
AVDD
, as stated
CURRENT
CURRENT
SOURCE
SOURCE
ARRAY
ARRAY
OUTFS
OUTFS
REV. 0
.
A
A