AD9740 Analog Devices, AD9740 Datasheet - Page 14

• Manufacturer Part Number: AD9740
• Manufacturer: Analog Devices

Specifications of AD9740

• Resolution (bits): 10bit
• Dac Update Rate: 210MSPS
• Dac Settling Time: 11ns
• Max Pos Supply (v): +3.6V
• Single-supply: Yes
• Dac Type: Current Out
• Dac Input Format: Par

AD9740
REFERENCE CONTROL AMPLIFIER
The AD9740 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
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
the Power Dissipation section). The second relates to a 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
The AD9740 provides complementary current outputs, IOUTA
and IOUTB. IOUTA provides a near full-scale current output,
I
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
where DAC CODE = 0 to 1023 (that is, decimal representation).
As mentioned previously, I
current I
V
where
OUTFS
OUTFS
REFIO
OUTFS
IOUTA = (DAC CODE/1023) × I
IOUTB = (1023 − DAC CODE)/1024 × I
I
I
, when all bits are high (that is, DAC CODE = 1023), while
OUTFS
REF
, and external resistor, R
over a 2 mA to 20 mA range by setting I
= V
, as stated in Equation 3.
REF
= 32 × I
, which is nominally set by a reference voltage,
REFIO
Figure 25. External Reference Configuration
REFIO
FS ADJ
/R
REF
1.2V REF
AD9740
REF
SET
, is determined by the ratio of the V
REFLO
SET
, as stated in Equation 4. I
OUTFS
OUTFS
150pF
SET
OUTFS
and can be expressed as:
is a function of the reference
. It can be expressed as:
REFERENCE
CONTROL
AMPLIFIER
. The control amplifier is
OUTFS
CURRENT
SOURCE
ARRAY
3.3V
AVDD
OUTFS
REF
between
REF
OUTFS
is copied
OUTFS
REFIO
(see
and
Rev. B | Page 14 of 32
(1)
(2)
(3)
(4)
The two current outputs typically drive a resistive load directly
or via a transformer. If dc coupling is required, then 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 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:
Equation 7 and Equation 8 highlight some of the advantages of
operating the AD9740 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
voltage output (that is, V
signal power to the load.
Note that the gain drift temperature performance for a single-
ended (V
AD9740 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 can be configured for single-ended or differential
operation. IOUTA and IOUTB can be converted into
complementary single-ended voltage outputs, V
via a load resistor, R
Function section by Equation 5 through Equation 8. The
differential voltage, V
also be converted to a single-ended 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
LOAD
V
V
V
V
(32 × R
can represent the equivalent load resistance seen by
OUTA
OUTB
DIFF
DIFF
LOAD
OUTA
= (IOUTA − IOUTB) × R
= {(2 × DAC CODE − 1023)/1024}
= IOUTB × R
= IOUTA × R
, that are tied to analog common, ACOM. Note that
LOAD
and V
LOAD
/R
SET
and R
OUTB
LOAD
) × V
DIFF
DIFF
OUTA
) or differential output (V
B
, as described in the DAC Transfer
LOAD
SET
LOAD
, is twice the value of the single-ended
, existing between V
REFIO
due to their ratiometric relationship,
or V
OUTB
OUTA
LOAD
), thus providing twice the
and V
REF
OUTB
OUTA
, and V
OUTA
should not
DIFF
and V
and V
) of the
DIFF
OUTB
can be
OUTB
, can
(5)
(6)
(7)
(8)
,