Full-Scale Current Generation - Analog Devices AD9776 Instruction Manual

AD9776/AD9778/AD9779
60
56
52
48
44
40
36
32
28
24
20
16
12
8
4
0
Figure 73. Typical PLL Band Select vs. Frequency over Temperature
The AD977x has an autosearch feature that can be used to
determine the optimal settings for the PLL. To enable the
autosearch mode, set Reg. 0x08, Bits <7:2> to 11111b, and read
back the value from Reg. 0x08, Bits <7:2>. Autosearch mode is
intended to find the optimal PLL settings only, after which the
same settings should be applied in manual mode. It is not
recommended that the PLL be set to autosearch mode during
regular operation.

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 74. The recommended value for the
external resistor is 10 kΩ, which sets up an I
resistor of 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
(Reg. 0x0A, 0x0B, 0x0E, and 0x0F). The default value for the
DAC gain registers gives an I
I is equal to
FS
⎛
⎛
1.2 V 27 6
× + ×
⎜
⎜
DAC gain
⎝
⎝
R 12 1024
AD9779
1.2V BAND GAP
VREF
I120
0.1μF
10kΩ
Figure 74. Reference Circuitry
F (MHz)
VCO
in the
REFERENCE
of approximately 20 mA, where
FS
⎞
⎞
× ⎟
⎟
32
⎠
⎠
I DAC GAIN
I DAC
DAC FULL-SCALE
CURRENT
REFERENCE
SCALING
CURRENT
Q DAC
Q DAC GAIN
35
30
25
20
15
10
5
0
0
Auxiliary DACS
Two auxiliary DACs are provided on the AD977x. The full-scale
output current on these DACs is derived from the 1.2 V band
gap reference and external resistor. The gain scale from the
reference amplifier current I
ence current is 16.67 with the auxiliary DAC gain set to full
scale (10-bit values, SPI Reg. 0x0C, 0x0D, 0x10, and 0x11), this
gives a full-scale current of approximately 2 mA for auxiliary
DAC1 and auxiliary DAC2. The auxiliary DAC outputs are not
differential. Only one side of the auxiliary DAC (P or N) is
active at one time. The inactive side goes into a high impedance
state (>100 kΩ). In addition, the P or N outputs can act as
current sources or sinks. The control of the P and N side
for both auxiliary DACs is via Reg. 0x0E and 0x10,
Bits <7:6>. When sourcing current, the output compliance
voltage is 0 V to 1.6 V; when sinking current, the output
compliance voltage is 0.8 V to 1.6 V.
The auxiliary DACs can be used for local oscillator (LO) cancel-
lation when the DAC output is followed by a quadrature modu-
lator. A typical DAC-to-quadrature modulator interface is shown
in Figure 76. Often, the input common-mode voltage for the
modulator is much higher than the output compliance range of
the DAC, so that ac coupling is necessary. If the required com-
mon-mode input voltage on the quadrature modulator matches
that of the DAC, then the ac coupling capacitors can be removed.
The input referred dc offset voltage of the quadrature modulator
(and the DAC output offset voltage mismatch) can result in LO
feedthrough on the modulator output, thus degrading system
performance. If the configuration of Figure 76 is used, the auxi-
liary DACs can be used to compensate for this dc offset, thus
reducing LO feedthrough. A low-pass or band-pass filter is
recommended when spurious signals from the DAC (distortion
and DAC images) at the quadrature modulator inputs can affect
the system performance. This filter should be placed at the
quadrature modulator inputs.
Rev. 0 | Page 36 of 56
200 400 600 800
DAC GAIN CODE
Figure 75. I vs. DAC Gain Code
FS
to the auxiliary DAC refer-
REFERENCE
1000