AD9776A Analog Devices, AD9776A Datasheet - Page 43

• Manufacturer Part Number: AD9776A
• Description: Dual 12-Bit, 1 GSPS, Digital-to-Analog Converter
• Manufacturer: Analog Devices

Specifications of AD9776A

• Resolution (bits): 12bit
• 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: Par

FULL-SCALE CURRENT GENERATION
INTERNAL REFERENCE
Full-scale current on the I DAC and Q DAC can be set from
8.66 mA to 31.66 mA. Initially, the 1.2 V band gap reference is
used to set up a current in an external resistor connected to I120
(Pin 75). A simplified block diagram of the reference circuitry is
shown in Figure 76. The recommended value for the external
resistor is 10 kΩ, which sets up an I
120 μA, which in turn provides a DAC output full-scale current
of 20 mA. Because the gain error is a linear function of this resistor,
a high precision resistor improves gain matching to the internal
matching specification of the devices. Gain drift over temperature
is also affected by this resistor. A resistor with a low temperature
coefficient is recommended in applications requiring good gain
stability.
Internal current mirrors provide a current-gain scaling, where
the I DAC or Q DAC gain is a 10-bit word in the 3-wire interface
port register (Register 0x0B, Register 0x0C, Register 0x0F, and
Register 0x10). The default value for the DAC gain registers
gives an I
I
FS
=
FS
1.2
of approximately 20 mA. I
R
V
×
⎛
⎜
⎝
27
12
+
⎛
⎜
⎝
1024
6
×
DAC Gain
REFERENCE
FS
is equal to
in the resistor of
⎞
⎟
⎠
⎞
⎟
⎠
×
32
Rev. B | Page 43 of 56
0.1µF
35
30
25
20
15
10
VREF
5
0
10kΩ
I120
0
AD9776A/AD9778A/AD9779A
200
AD9776A/AD9778A/AD9779A
1.2V BAND GAP
Figure 77. I
Figure 76. Reference Circuitry
DAC GAIN CODE
400
FS
vs. DAC Gain Code
Q DAC GAIN
I DAC GAIN
CURRENT
SCALING
600
Q DAC
I DAC
800
DAC FULL-SCALE
REFERENCE
CURRENT
1000