AD9740-EB Analog Devices, AD9740-EB Datasheet - Page 10

• Manufacturer Part Number: AD9740-EB
• Description: 10-Bit/ 165 MSPS TxDAC D/A Converter
• Manufacturer: Analog Devices

AD9740
REFERENCE CONTROL AMPLIFIER
The AD9740 contains a control amplifier that is used to regu-
late the full-scale output current, I
is configured as a V-I converter as shown in Figure 4, so that its
current output, I
and an external resistor, R
copied to the segmented current sources with the proper scale
factor to set I
The control amplifier allows a wide (10:1) adjustment span of
I
62.5 µA and 625 µA. The wide adjustment span of I
provides several benefits. The first relates directly to the power
dissipation of the AD9740, which is proportional to I
(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 AD9740 provide complementary current
outputs, IOUTA and IOUTB. IOUTA will provide a near full-
scale current output, I
CODE = 1023) 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
can be expressed as:
where DAC CODE = 0 to 1023 (i.e., decimal representation).
As mentioned previously, 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
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:
Note the full-scale value of V
the specified output compliance range to maintain specified
distortion and linearity performance.
OUTFS
REFIO
LOAD
V
V
V
IOUTA
IOUTB
I
I
OUTFS
REF
OUTA
OUTB
DIFF
, and external resistor, R
may represent the equivalent load resistance seen by
over a 2 mA to 20 mA range by setting I
=
LOAD
=
REF
V
=
=
=
(
REFIO
=
=
IOUTA R
IOUTB
IOUTA IOUTB
32
, which is nominally set by a reference voltage,
, that are tied to analog common, ACOM. Note,
OUTFS
(
(
1023
DAC CODE
×
REF
/
I
REF
R
as stated in Equation 3.
SET
, is determined by the ratio of the V
–
×
×
–
DAC CODE
OUTFS
R
LOAD
LOAD
SET
OUTFS
, when all bits are high (i.e., DAC
/
1024
OUTA
, as stated in Equation 4. I
)
SET
×
is a function of the reference
R
. It can be expressed as:
OUTFS
and V
)
) /
LOAD
×
1024
I
OUTFS
. The control amplifier
OUTB
×
I
should not exceed
OUTFS
REF
between
OUTFS
OUTFS
REFIO
OUTFS
REF
is
and
(1)
(2)
(3)
(4)
(5)
(6)
(7)
–10–
Substituting the values of IOUTA, IOUTB, I
be expressed as:
These last two equations highlight some of the advantages of
operating the AD9740 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
voltage output (i.e., V
signal power to the load.
Note, the gain drift temperature performance for a single-ended
(V
can be enhanced by selecting temperature tracking resistors for
R
Equation 8.
ANALOG OUTPUTS
The complementary current outputs in each DAC, IOUTA and
IOUTB, may be configured for single-ended or differential opera-
tion. IOUTA and IOUTB can be converted into complementary
single-ended voltage outputs, V
tor, R
Equations 5 through 8. The differential voltage, V
between V
voltage via a transformer or differential amplifier configuration.
The ac performance of the AD9740 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 AD9740 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 AD9740 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 kΩ in
parallel with 5 pF. It is also slightly dependent on the output
voltage (i.e., V
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 AD9740 are measured with IOUTA maintained at a
virtual ground via an op amp.
LOAD
OUTA
V
(
32
DIFF
LOAD
and R
×
and V
R
=
OUTA
, as described in the DAC Transfer Function section by
LOAD
{
(
SET
2
OUTB
OUTA
×
and V
/
due to their ratiometric relationship as shown in
DAC CODE
R
) or differential output (V
SET
and V
OUTB
DIFF
OUTA
)
×
V
, is twice the value of the single-ended
can also be converted to a single-ended
OUTB
REFIO
or V
–
OUTA
) due to the nature of a PMOS
OUTB
1023 1024
), thus providing twice the
and V
) /
OUTB
DIFF
REF
}
) of the AD9740
, via a load resis-
, and V
DIFF
, existing
DIFF
REV. 0
can
(8)