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參數(shù)資料
| 型號(hào)： | AD9742 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | 12-Bit, 165 MSPS TxDAC D/A Converter |
| 中文描述： | 12位，165 MSPS的TxDAC系列D / A轉(zhuǎn)換 |
| 文件頁數(shù)： | 10/20頁 |
| 文件大?。?/td> | 783K |
| 代理商： | AD9742 |
REV. 0
AD9742
–10–
REFERENCE CONTROL AMPLIFIER
The AD9742 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
A and 625
m
A. The wide adjustment span of I
OUTFS
provides
several benefits. The first relates directly to the power dissipation
of the AD9742, 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 AD9742 provide complementary current
outputs, IOUTA and IOUTB. IOUTA will provide a near full-
scale current output, I
OUTFS
, when all bits are high (i.e., DAC
CODE = 4095), 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:
IOUTA
DAC CODE
=
(
–
4095
where
DAC CODE
= 0 to 4095 (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:
DAC CODE
I
OUTFS
=
￥
(
/
)
4096
(1)
IOUTB
I
OUTFS
￥
)/
4096
(2)
I
I
OUTFS
REF
=
￥
32
(3)
where
I
V
R
REF
REFIO
SET
=
/
(4)
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,
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;
V
IOUTA
R
OUTA
LOAD
=
￥
(5)
V
IOUTB
R
OUTB
LOAD
=
￥
(6)
Note the full-scale value of
V
OUTA
and
V
OUTB
should not exceed
the specified output compliance range to maintain specified
distortion and linearity performance.
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
4095
4096
(8)
These last two equations highlight some of the advantages of
operating the AD9742 differentially. First, the differential
operation will help cancel common-mode error sources associ-
ated 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, the gain drift temperature performance for a single-ended
(V
OUTA
and V
OUTB
) or differential output (V
DIFF
) of the AD9742
can be enhanced by selecting temperature tracking resistors for
R
LOAD
and R
SET
due to their ratiometric relationship 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 Function
section by Equations 5 through 8. The differential voltage, V
DIFF
,
existing between V
OUTA
and V
OUTB
can also be converted to a
single-ended voltage via a transformer or differential amplifier
configuration. The ac performance of the AD9742 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 AD9742 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 (i.e., 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 AD9742 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
associated 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 optimum dc linearity. Note the INL/DNL specifications
for the AD9742 are measured with IOUTA maintained at a
virtual ground via an op amp.
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