AD9146 Datasheet, PDF (38/52 Pages) Analog Devices – Dual, 16-Bit, 1230 MSPS, TxDAC+ Digital-to-Analog Converter

AD9146
ANALOG OUTPUTS
TRANSMIT DAC OPERATION
Figure 50 shows a simplified block diagram of the transmit path
DACs. The DAC core consists of a current source array, a switch
core, digital control logic, and full-scale output current control.
The DAC full-scale output current (IFS) is nominally 20 mA.
The output currents from the IOUT1P/IOUT2P and IOUT1N/
IOUT2N pins are complementary, meaning that the sum of the
two currents always equals the full-scale current of the DAC.
The digital input code to the DAC determines the effective
differential current delivered to the load.
0.1µF
REFIO
FSADJ
10kΩ
R_SET
1.2V
5kΩ
I DAC FS ADJUST
REGISTER 0x40
I DAC
CURRENT
SCALING
Q DAC
Q DAC FS ADJUST
REGISTER 0x44
Figure 50. Simplified Block Diagram of DAC Core
IOUT1P
IOUT1N
IOUT2N
IOUT2P
The DAC has a 1.2 V band gap reference with an output imped-
ance of 5 kΩ. The reference output voltage appears on the REFIO
pin. When using the internal reference, decouple the REFIO pin
to AVSS with a 0.1 μF capacitor. Use the internal reference only for
external circuits that draw dc currents of 2 μA or less. For dynamic
loads or static loads greater than 2 μA, buffer the REFIO pin. If
desired, the internal reference can be overdriven by applying an
external reference (from 1.10 V to 1.30 V) to the REFIO pin.
A 10 kΩ external resistor, RSET, must be connected from the
FSADJ pin to AVSS. This resistor, along with the reference
control amplifier, sets up the correct internal bias currents for
the DAC. Because the full-scale current is inversely proportional
to this resistor, the tolerance of RSET is reflected in the full-scale
output amplitude.
The full-scale current equation, where the DAC gain is set individ-
ually for the I and Q DACs in Register 0x40 and Register 0x44,
respectively, is as follows:
I _FS
=
V_REF
R_SET
×
⎜⎛ 72
⎝
+
⎜⎝⎛
3
16
×
DAC
gain _{⎟⎠⎞} ⎟⎠⎞
For the nominal values of VREF (1.2 V), RSET (10 kΩ), and
DAC gain (512), the full-scale current of the DAC is typically
20.16 mA. The DAC full-scale current can be adjusted from
8.64 mA to 31.68 mA by setting the DAC gain parameter, as
shown in Figure 51.
35
30
25
20
15
10
5
0
0
200
400
600
800
1000
DAC GAIN CODE
Figure 51. DAC Full-Scale Current vs. DAC Gain Code
Transmit DAC Transfer Function
The output currents from the IOUT1P/IOUT2P and IOUT1N/
IOUT2N pins are complementary, meaning that the sum of the
two currents always equals the full-scale current of the DAC. The
digital input code to the DAC determines the effective differential
current delivered to the load. IOUT1P/IOUT2P provide maxi-
mum output current when all bits are high. The output currents
vs. DACCODE for the DAC outputs are expressed as
I _OUTxP
=
⎡ ^DACCODE ⎤
⎢⎣ ₂^N ⎥⎦
×
I _FS
(1)
I _OUTxN = I _FS − I _OUTxP
(2)
where DACCODE = 0 to 2^N − 1.
Transmit DAC Output Configurations
The optimum noise and distortion performance of the AD9146
is realized when it is configured for differential operation. The
common-mode error sources of the DAC outputs are 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 waveform increases and/or its ampli-
tude increases. This is due to the first-order cancellation of various
dynamic common-mode distortion mechanisms, digital feed-
through, and noise.
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