參數(shù)資料
型號： AD9772AST
廠商： ANALOG DEVICES INC
元件分類： DAC
英文描述： 14-Bit, 150 MSPS TxDAC⑩ with 2x Interpolation Filter
中文描述： PARALLEL, WORD INPUT LOADING, 0.011 us SETTLING TIME, 14-BIT DAC, PQFP48
封裝： PLASTIC, LQFP-48
文件頁數(shù)： 16/30頁
文件大?。?/td> 340K
代理商： AD9772AST
REV. 0
AD9772
–16–
The current output appearing at IOUTA and IOUTB is a func-
tion of both the input code and I
OUTFS
and can be expressed as:
IOUTA
= (
DAC CODE/
16384)
×
I
OUTFS
IOUTB
= (16383 –
DAC CODE
)/16384
×
I
OUTFS
where
DAC CODE
= 0 to 16383 (i.e., 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 external resistor, R
SET
. It can be expressed as:
I
OUTFS
= 32
×
I
REF
where
I
REF
=
V
REFIO
/
R
SET
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
LOAD
, that are tied to analog common, ACOM. Note
that R
LOAD
may represent the equivalent load resistance seen by
IOUTA or IOUTB as would be the case in a doubly terminated
50
or 75
cable. The single-ended voltage output appearing
at the IOUTA and IOUTB nodes is simply:
V
OUTA
=
IOUTA
×
R
LOAD
V
OUTB
=
IOUTB
×
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 IOUTA and
IOUTB, is:
V
DIFF
= (
IOUTA – IOUTB
)
×
R
LOAD
Substituting the values of IOUTA, IOUTB and I
REF
; V
DIFF
can
be expressed as:
V
DIFF
= [(2
DAC CODE
– 16383)/16384]
×
(32
R
LOAD
/
R
SET
)
×
V
REFIO
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
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 (VOUTA and VOUTB) or differential output (V
DIFF
) of
the AD9772 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.
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(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 1.20V output. In
this case, the internal reference
must
be compensated externally
with a ceramic chip capacitor of 0.1
μ
F 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.
+1.2V REF
REFIO
FSADJ
CURRENT
SOURCE
ARRAY
250pF
REFLO
AVDD
AD9772
2k
V
0.1
m
F
ADDITIONAL
LOAD
OPTIONAL
EXTERNAL
REF BUFFER
+2.7V TO +3.6V
A
Figure 32. Internal Reference Configuration
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
μ
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.
+1.2V REF
REFIO
FSADJ
CURRENT
SOURCE
ARRAY
250pF
REFLO
AVDD
AD9772
AD1580
+2.7 TO +3.6V
A
REFERENCE
CONTROL
AMPLIFIER
R
SET
I
REF
=
V
REFIO
/R
SET
10k
V
V
REFIO
Figure 33. External Reference Configuration
REFERENCE CONTROL AMPLIFIER
The AD9772 also contains an internal control amplifier that is
used to regulate the DAC’s full-scale output current, I
OUTFS
.
The control amplifier is configured as a V-I converter, as shown
in Figure 33, such that its current output, I
REF
, is determined by
the ratio of the V
REFIO
and an external resistor, R
SET
, as stated
in Equation 4. I
REF
is copied over to the segmented current
sources with the proper scaling factor to set I
OUTFS
as stated in
Equation 3.
The control amplifier allows a wide (10:1) adjustment span of
I
OUTFS
over a 2mA to 20 mA range by setting I
REF
between
62.5
μ
A and 625
μ
A. The wide adjustment span of I
OUTFS
provides several application benefits. The first benefit relates
directly to the power dissipation of the AD9772’s DAC, which
is proportional to I
OUTFS
(refer to the Power Dissipation sec-
tion). The second benefit relates to the 20 dB adjustment, which
is useful for system gain control purposes.
I
REF
can be controlled using the single-supply circuit shown in
Figure 34 for a fixed R
SET
. In this example, the internal refer-
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
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