AD9704/AD9705/AD9706/AD9707 Data Sheet
Rev. D | Page 34 of 42
Table 20. MEMRDWR—Register 0x0F
Mnemonic Bit No. Direction (I/O) Default Description
CALSTAT 7 O 0 1 = calibration cycle complete
CALEN 6 I 0 1 = initiates device self-calibration
SMEMWR 3 I 0 1 = writes to static memory (calibration coefficients)
SMEMRD 2 I 0 1 = reads from static memory (calibration coefficients)
UNCAL 0 I 0 1 = resets calibration coefficients to default (uncalibrated)
Table 21. MEMADDR—Register 0x10
Mnemonic Bit No. Direction (I/O) Default Description
MEMADDR[5:0] [5:0] I/O 000000 Address of static memory to be accessed
Table 22. MEMDATA—Register 0x11
Mnemonic Bit No. Direction (I/O) Default Description
MEMDATA[5:0] [5:0] I/O 111111 Data for static memory access
REFERENCE OPERATION
The
AD9704/AD9705/AD9706/AD9707 contain an internal 1.0 V
band gap reference. The internal reference can be disabled by
writing a Logic 1 to Register 0x00, Bit 0 (EXREF) in the SPI.
The internal 1.0 V band gap reference may on occasion power
up in a state that leaves the DAC output nonfunctional. To clear
this state, power up again, and check that the voltage on the
REFIO pin is within the reference output specifications shown
in Table 1 or Table 4. After the internal reference is powered up
correctly, it does not fail as long as power is applied.
To use the internal reference, decouple the REFIO pin to ACOM
with a 0.1 μF capacitor, enable the internal reference by writing
a Logic 0 to Register 0x00, Bit 0 in the SPI. (Note that this is the
default configuration.) The internal reference voltage is present
at REFIO. If the voltage at REFIO is to be used anywhere else in
the circuit, an external buffer amplifier with an input bias current of
less than 100 nA must be used to avoid loading the reference. An
example of the use of the internal reference is shown in Figure 78.
CURRENT
SCALING
x32
AD9704/AD9705/
AD9706/AD9707
DAC
I
OUTFS
R
SET
0.1µF
REFIO
I
REF
AVSS
FS ADJ
V
BG
1.0V
+
–
05926-094
Figure 78. Internal Reference Configuration
REFIO serves as either an input or an output, depending on
whether the internal or an external reference is used. Table 23
summarizes the reference operation.
Table 23. Reference Operation
Reference
Mode REFIO Pin Register Setting
Internal Connect 0.1 µF capacitor
Register 0x00, Bit 0 = 0
(default)
External Apply external reference
Register 0x00, Bit 0 = 1
(for power saving)
An external reference can be used in applications requiring
tighter gain tolerances or lower temperature drift. Also, a variable
external voltage reference can be used to implement a method
for gain control of the DAC output. The external reference is
applied to the REFIO pin. Note that the 0.1 μF compensation
capacitor is not required. The internal reference can be directly
overdriven by the external reference, or the internal reference
can be powered down. The input impedance of REFIO is 10 kΩ
when powered up and 1 MΩ when powered down.
REFERENCE CONTROL AMPLIFIER
The
AD9704/AD9705/AD9706/AD9707 contain a control
amplifier that regulates the full-scale output current, I
OUTFS
. The
control amplifier is configured as a V-I converter, as shown in
Figure 78. The output current, I
REF
, is determined by the ratio of
the V
REFIO
and an external resistor, R
SET
, as stated in Equation 4.
I
REF
is mirrored to the segmented current sources with the
proper scale factor to set I
OUTFS
, as stated in Equation 3.
The control amplifier allows a 5:1 adjustment span of I
OUTFS
from
1 mA to 5 mA by setting I
REF
between 31.25 μA and 156.25 μA
(R
SET
between 6.4 kΩ and 32 kΩ). The wide adjustment span of
I
OUTFS
provides several benefits. The first relates directly to the
power dissipation of the
AD9704/AD9705/AD9706/AD9707,
which is proportional to I
OUTFS
(see the Power Dissipation section).
The second benefit relates to the ability to adjust the output over a
14 dB range, which is useful for controlling the transmitted power.
