Full-Scale Current Generation; Internal Reference - Analog Devices AD9776A Manual

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 75. 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. Internal current mirrors
provide a current-gain scaling, where I DAC or Q DAC gain is a
10-bit word in the SPI port register (Register 0x0B, Register 0x0C,
Register 0x0F, and Register 0x10). The default value for the DAC
gain registers gives an I
1.2 V
= ×
I
FS
R
Downloaded from Elcodis.com electronic components distributor
in the resistor of
REFERENCE
of approximately 20 mA. I
FS
⎛ ⎞
⎛ ⎞
27 6
+ × ×
⎜ ⎟
⎜ ⎟
DAC Gain 32
⎝ ⎠
⎝ ⎠
12 1024
VREF
0.1µF
35
30
25
is equal to
FS
20
15
10
Rev. A | Page 39 of 60
AD9776A/AD9778A/AD9779A
AD9776A/AD9778A/AD9779A
I DAC GAIN
1.2V BAND GAP
CURRENT
I120
SCALING
10kΩ
Q DAC GAIN
Figure 75. Reference Circuitry
5
0
0 200 400 600
DAC GAIN CODE
Figure 76. I vs. DAC Gain Code
FS
I DAC
DAC FULL-SCALE
REFERENCE
CURRENT
Q DAC
800 1000