Datasheet

Data Sheet AD9148
Rev. B | Page 59 of 72
ANALOG OUTPUTS
TRANSMIT DAC OPERATION
Figure 77 shows a simplified block diagram of one pair of the
transmit path DACs. The DAC core consists of a current source
array, switch core, digital control logic, and full-scale output current
control. The DAC full-scale output current (I
OUTFS
) is nominally
20 mA. The output currents from the IOUTx_P and IOUTx_N
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.
The DAC has a 1.2 V band gap reference with an output impedance
of 5 kΩ. The reference output voltage appears on the VREF pin.
When using the internal reference, the VREF pin should be
decoupled to AVSS with a 0.1 µF capacitor. The internal reference
should only be used for external circuits that draw dc currents
of 2 µA or less. For dynamic loads or static loads greater than
2 µA, the VREF pin should be buffered. If desired, an external
reference (between 1.10 V to 1.30 V) can be applied to the
VREF pin.
A 10 kΩ external resistor, R
SET
, must be connected from
the
RESET
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 R
SET
is reflected in the full-scale
output amplitude.
The full-scale current can be calculated by












×+×= DAC gain
R
I
SET
OUTFS
16
3
72
V
REF
where DAC gain is set individually for the I and Q DACs in
Register 0x30, Register 0x31, Register 0x34, and Register 0x35,
respectively.
For nominal values of VREF (1.2 V), R
SET
(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 8.66 mA to
31.66 mA by setting the DAC gain parameter setting as shown
in Figure 76.
35
0
0
1000
DAC GAIN CODE
I
OUTFS
(mA)
30
25
20
15
10
5
200 400 600 800
08910-074
Figure 76. DAC Full-Scale Current vs. DAC Gain Code
Transmit DAC Transfer Function
The output currents from the IOUTx_P and IOUTx_N 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. IOUTx_P provides the maximum output current
when all bits are high. The output currents vs. DACCODE for
the DAC outputs are expressed as
OUTFS
N
POUT
I
DACCODE
I ×






=
2
_
(1)
I
OUT_N
= I
OUTFS
– I
OUT_P
(2)
where DACCODE = 0 to 2
N
− 1.
I DAC
IOUT1_P/IOUT3_P
IOUT1_N/IOUT3_N
Q DAC
IOUT2_N/IOUT4_N
IOUT2_P/IOUT4_P
CURRENT
SCALING
I DAC GAIN
Q DAC GAIN
0.1µF
10kΩ
I120
VREF
RSET
5kΩ
1.2V
08910-075
Figure 77. Simplified Block Diagram of the DAC Core