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參數(shù)資料
| 型號(hào)： | AD9744-EB |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | 14-Bit, 165 MSPS TxDAC D/A Converter |
| 中文描述： | 14位，165 MSPS的TxDAC系列D / A轉(zhuǎn)換 |
| 文件頁數(shù)： | 11/28頁 |
| 文件大?。?/td> | 1142K |
| 代理商： | AD9744-EB |
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REV. A
AD9744
–11–
REFERENCE CONTROL AMPLIFIER
The AD9744 contains a control amplifier that is used to regu-
late the full-scale output current, I
OUTFS
. The control amplifier
is configured as a V-I converter, as shown in Figure 4, so 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 to the segmented current sources with the proper scale
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 2 mA to 20 mA range by setting I
REF
between
62.5
m
A and 625
m
A. The wide adjustment span of I
OUTFS
pro-
vides several benefits. The first relates directly to the power
dissipation of the AD9744, which is proportional to I
OUTFS
(refer to the Power Dissipation section). The second relates to
the 20 dB adjustment, which is useful for system gain control
purposes.
The small signal bandwidth of the reference control amplifier is
approximately 500 kHz and can be used for low frequency small
signal multiplying applications.
DAC TRANSFER FUNCTION
Both DACs in the AD9744 provide complementary current
outputs, IOUTA and IOUTB. IOUTA provides a near full-
scale current output, I
OUTFS
, when all bits are high (i.e., DAC
CODE = 16383), while IOUTB, the complementary output,
provides no current. The current output appearing at
IOUTA
and
IOUTB
is a function of both the input code and
I
OUTFS
and
can be expressed as
=
(
IOUTB
where DAC CODE = 0 to 16383 (i.e., decimal representation).
As mentioned previously,
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
REF
=
32
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
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
W
or 75
W
cable. The single-ended voltage output appearing
at the
IOUTA
and
IOUTB
nodes is simply
=
=
Note that the full-scale value of
V
OUTA
and
V
OUTB
should not
exceed the specified output compliance range to maintain speci-
fied distortion and linearity performance.
=
(
IOUTA
DAC CODE
I
OUTFS
)
￥
)
/16384
(1)
DAC CODE
I
OUTFS
=
(
￥
16383
16384
–
/
(2)
I
￥
(3)
I
V
R
REF
REFIO
SET
(4)
V
V
IOUTA
R
OUTA
LOAD
￥
￥
(5)
IOUTB
R
OUTB
LOAD
(6)
V
IOUTA
IOUTB
R
DIFF
LOAD
)
￥
–
(7)
Substituting the values of
IOUTA
,
IOUTB
,
I
REF
, and
V
DIFF
can
be expressed as
(
￥
32
/
V
(
DAC CODE
)
￥
R
R
V
DIFF
LOAD
SET
REFIO
=
￥
{
}
2
16383 16384
–
/
(8)
Equations 7 and 8 highlight some of the advantages of operating
the AD9744 differentially. First, the differential operation helps
cancel common-mode error sources associated with IOUTA and
IOUTB, such as noise, distortion, and dc offsets. 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
AD9744 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.
ANALOG OUTPUTS
The complementary current outputs in each DAC, IOUTA,
and IOUTB may be configured for single-ended or differential
operation. IOUTA and IOUTB can be converted into comple-
mentary single-ended voltage outputs, V
OUTA
and V
OUTB
, via a
load resistor, R
LOAD
, as described in the DAC Transfer Func-
tion section by Equations 5 through 8. The differential voltage,
V
DIFF
, existing between V
OUTA
and V
OUTB
, can also be con-
verted to a single-ended voltage via a transformer or differential
amplifier configuration. The ac performance of the AD9744 is
optimum and specified using a differential transformer-coupled
output in which the voltage swing at IOUTA and IOUTB is
limited to
±
0.5 V.
The distortion and noise performance of the AD9744 can be
enhanced when it is configured for differential operation. The
common-mode error sources of both IOUTA and IOUTB can
be significantly reduced by the common-mode rejection of a
transformer or differential amplifier. These common-mode error
sources include even-order distortion products and noise. The
enhancement in distortion performance becomes more signifi-
cant as the frequency content of the reconstructed waveform
increases and/or its amplitude decreases. This is due to the first
order cancellation of various dynamic common-mode distortion
mechanisms, digital feedthrough, and noise.
Performing a differential-to-single-ended conversion via a trans-
former also provides the ability to deliver twice the reconstructed
signal power to the load (assuming no source termination).
Since the output currents of IOUTA and IOUTB are comple-
mentary, they become additive when processed differentially. A
properly selected transformer will allow the AD9744 to provide
the required power and voltage levels to different loads.
The output impedance of IOUTA and IOUTB is determined by
the equivalent parallel combination of the PMOS switches asso-
ciated with the current sources and is typically 100 k
W
in parallel
with 5 pF. It is also slightly dependent on the output voltage
(i.e., V
OUTA
and V
OUTB
) due to the nature of a PMOS device.
As a result, maintaining IOUTA and/or IOUTB at a virtual
ground via an I-V op amp configuration will result in the opti-
mum dc linearity. Note that the INL/DNL specifications for the
AD9744 are measured with IOUTA maintained at a virtual
ground via an op amp.
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