AD9148 Analog Devices, AD9148 Datasheet - Page 59

• Manufacturer Part Number: AD9148
• Manufacturer: Analog Devices

Specifications of AD9148

• Resolution (bits): 16bit
• Dac Update Rate: 1GSPS
• Dac Settling Time: n/a
• Max Pos Supply (v): +3.47V
• Single-supply: No
• Dac Type: Current Out
• Dac Input Format: LVDS,Par

Data Sheet
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
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
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
output amplitude.
The full-scale current can be calculated by
where DAC gain is set individually for the I and Q DACs in
Register 0x30, Register 0x31, Register 0x34, and Register 0x35,
respectively.
I
OUTFS
=
V
R
SET
REF
×



72
+



SET
16
3
, must be connected from
×
DAC gain
SET
0.1µF
is reflected in the full-scale
RSET
10kΩ






VREF
OUTFS
I120
Figure 77. Simplified Block Diagram of the DAC Core
) is nominally
1.2V
5kΩ
Rev. B | Page 59 of 72
Q DAC GAIN
I DAC GAIN
CURRENT
SCALING
For nominal values of VREF (1.2 V), R
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.
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
where DACCODE = 0 to 2
Q DAC
I DAC
I
I
OUT_N
OUT
35
30
25
20
15
10
5
0
0
_
Figure 76. DAC Full-Scale Current vs. DAC Gain Code
P
= I
=
OUTFS

 
IOUT1_P/IOUT3_P
IOUT1_N/IOUT3_N
IOUT2_N/IOUT4_N
IOUT2_P/IOUT4_P
DACCODE
200
– I
2
OUT_P
N
N
DAC GAIN CODE
400

 
− 1.
×
I
OUTFS
600
SET
(10 kΩ), and DAC
800
AD9148
1000
(1)
(2)