AD9709 Datasheet, PDF (10/27 Pages) Analog Devices – 8-Bit, 125 MSPS Dual TxDAC D/A Converter

AD9709
REFERENCE CONTROL AMPLIFIER
Both of the DACs in the AD9709 contain a control amplifier
that is used to regulate the full-scale output current, I_OUTFS. The
control amplifier is configured as a V-I converter as shown in
Figure 21, 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.
OPTIONAL
EXTERNAL
REFERENCE
BUFFER
ADDITIONAL
EXTERNAL
LOAD
GAINCTRL
AVDD
0.1␮F
+1.2V
REF
REFIO
FSADJ
AD9709
REFERENCE
SECTION
CURRENT
SOURCE
ARRAY
I_REF 2k⍀
ACOM
Figure 21. Internal Reference Configuration
AVDD
EXTERNAL
REFERENCE
I_REF
GAINCTRL
AVDD
+1.2V
REF
REFIO
FSADJ
AD9709
REFERENCE
SECTION
CURRENT
SOURCE
ARRAY
2k⍀
ACOM
Figure 22. External Reference Configuration
The control amplifier allows a wide (10:1) adjustment span of
I_OUTFS from 2 mA to 20 mA by setting I_REF between 62.5 µA
and 625 µA. The wide adjustment range of I_OUTFS provides
several benefits. The first relates directly to the power dissipa-
tion of the AD9709, 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 AD9709 provide complementary current out-
puts, I_OUTA and I_OUTB. I_OUTA will provide a near full-scale current
output, I_OUTFS, when all bits are high (i.e., DAC CODE = 1023)
while I_OUTB, the complementary output, provides no current. The
current output appearing at I_OUTA and I_OUTB is a function of both
the input code and I_OUTFS and can be expressed as:
I_OUTA = (DAC CODE/256) × I_OUTFS
(1)
I_OUTB = (255 – DAC CODE)/256 × I_OUTFS
(2)
where DAC CODE = 0 to 255 (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 = 32 × I_REF
(3)
where
I_REF = V_REFIO /R_SET
(4)
The two current outputs will typically drive a resistive load
directly or via a transformer. If dc coupling is required, I_OUTA
and I_OUTB 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
I_OUTA or I_OUTB as would be the case in a doubly terminated 50 Ω
or 75 Ω cable. The single-ended voltage output appearing at the
I_OUTA and I_OUTB nodes is simply :
V_OUTA = I_OUTA × R_LOAD
(5)
V_OUTB = I_OUTB × R_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_DIFF = (I_OUTA – I_OUTB) × R_LOAD
(7)
Substituting the values of I_OUTA, I_OUTB and I_REF; V_DIFF can be
expressed as:
V_DIFF = {(2 × DAC CODE – 255)/256} ×
(32 × R_LOAD/R_SET) × V_REFIO
(8)
These last two equations highlight some of the advantages of
operating the AD9709 differentially. First, the differential
operation will help cancel common-mode error sources associ-
ated with I_OUTA and I_OUTB 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 AD9709
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, I_OUTA and
I_OUTB, may be configured for single-ended or differential opera-
tion. I_OUTA and I_OUTB can be converted into complementary
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 AD9709 is optimum
and specified using a differential transformer coupled output in
which the voltage swing at I_OUTA and I_OUTB is limited to ± 0.5 V.
If a single-ended unipolar output is desirable, I_OUTA should be
selected.
The distortion and noise performance of the AD9709 can be
enhanced when it is configured for differential operation. The
common-mode error sources of both I_OUTA and I_OUTB 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. This is due to the first order cancellation of various
dynamic common-mode distortion mechanisms, digital feed-
through and noise.
–10–
REV. 0