AD9742 Analog Devices, AD9742 Datasheet - Page 13

• Manufacturer Part Number: AD9742
• Manufacturer: Analog Devices

Specifications of AD9742

• Resolution (bits): 12bit
• Dac Update Rate: 210MSPS
• Dac Settling Time: 11ns
• Max Pos Supply (v): +3.6V
• Single-supply: No
• Dac Type: Current Out
• Dac Input Format: Par

REFERENCE CONTROL AMPLIFIER
The AD9742 contains a control amplifier that is used to regulate
the full-scale output current, I
configured as a V-I converter, as shown in Figure 24, so that its
current output, I
an external resistor, R
the segmented current sources with the proper scale factor to
set I
The control amplifier allows a wide (10:1) adjustment span of
IOUTFS over a 2 mA to 20 mA range by setting IREF between
62.5 µA and 625 µA. The wide adjustment span of IOUTFS
provides several benefits. The first relates directly to the power
dissipation of the AD9742, which is proportional to IOUTFS
(see 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 provides a near full-scale
current output, I
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
expressed as:
where DAC CODE = 0 to 4095 (i.e., decimal representation).
As mentioned previously, I
current I
and external resistor, R
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
R
IOUTA or IOUTB as would be the case in a doubly terminated
50 Ω or 75 Ω cable. The single-ended voltage output appearing
at the IOUTA and IOUTB nodes is simply
LOAD
OUTFS
IOUTA
IOUTB
I
I
may represent the equivalent load resistance seen by
OUTFS
REF
LOAD
, as stated in Equation 3.
REF
=
, which is nominally set by a reference voltage, V
, that are tied to analog common, ACOM. Note that
= 32
V
=
=
REFIO
(
(
4095
DAC
×
REF
OUTFS
I
/
, is determined by the ratio of the V
REF
R
SET
−
, when all bits are high (i.e., DAC CODE =
SET
CODE
DAC
SET
, as stated in Equation 4. I
. It can be expressed as:
OUTFS
/
CODE
4096
OUTFS
is a function of the reference
. The control amplifier is
)
)
×
/4096
I
OUTFS
OUTFS
×
I
and can be
OUTFS
REF
is copied to
REFIO
and
REFIO
(1)
(2)
(3)
(4)
Rev. B | Page 13 of 32
,
Note that the full-scale value of V
exceed the specified output compliance range to maintain
specified distortion and linearity performance.
Substituting the values of IOUTA, IOUTB, I
expressed as:
Equations 7 and 8 highlight some of the advantages of operat-
ing the AD9742 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
output (i.e., V
power to the load.
Note that the gain drift temperature performance for a single-
ended (V
AD9742 can be enhanced by selecting temperature tracking
resistors for R
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
load resistor, R
section by Equations 5 through 8. The differential voltage, V
existing between V
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
significant as the frequency content of the reconstructed wave-
form 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.
V
V
V
V
(
32
OUTA
OUTB
DIFF
DIFF
×
OUTA
DIFF
R
=
=
=
=
LOAD
(
, is twice the value of the single-ended voltage
{
IOUTB
OUTA
IOUTA
IOUTA
(
and V
LOAD
2
LOAD
×
/
DAC
R
or V
and R
, as described in the DAC Transfer Function
OUTA
SET
OUTB
×
×
−
)
OUTB
R
CODE
R
×
IOUTB
and V
) or differential output (V
SET
LOAD
LOAD
V
REFIO
), thus providing twice the signal
due to their ratiometric relationship,
OUTB
−
)
4095
×
, can also be converted to a
OUTA
R
LOAD
)
/
and V
4096
OUTA
REF
}
OUTB
and V
, and V
should not
DIFF
OUTB
AD9742
) of the
DIFF
, via a
can be
DIFF
(5)
(6)
(7)
(8)
,