Datasheet

AD9714/AD9715/AD9716/AD9717
Rev. A | Page 43 of 80
REFERENCE OPERATION
The AD9714/AD9715/AD9716/AD9717 contain an internal
1.0 V band gap reference. The internal reference can be disabled
by setting Bit 0 (EXTREF) of the power-down register (Address
0x01) through the SPI interface. To use the internal reference,
decouple the REFIO pin to AVSS with a 0.1 F capacitor, enable
the internal reference, and clear Bit 0 of the power-down register
(Address 0x01) through the SPI interface. 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 96.
CURRENT
SCALING
×32
AD9714/AD9715/
AD9716/AD9717
I DAC
OR
Q DAC
07265-218
I
xOUTFS
xR
SET
0.1µF
REFIO
I
xREF
AVSS
FSADJx
V
BG
1.0V
+
–
Figure 96. Internal Reference Configuration
REFIO serves as either an input or an output, depending on
whether the internal or an external reference is used. Table 17
summarizes the reference operation.
Table 17. Reference Operation
Reference Mode REFIO Pin Register Setting
Internal
Connect 0.1 µF
capacitor
Register 0x01, Bit 0 = 0
(default)
External
Apply external
capacitor
Register 0x01, 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.
Recommendations When Using an External Reference
Apply the external reference to the REFIO pin. The internal
reference can be directly overdriven by the external reference,
or the internal reference can be powered down to save power
consumption
The external 0.1 F compensation capacitor on REFIO is not
required unless specified by the external voltage reference
manufacturer. The input impedance of REFIO is 10 k when
the internal reference is powered up and 1 M when it is
powered down.
REFERENCE CONTROL AMPLIFIER
The AD9714/AD9715/AD9716/AD9717 contain a control
amplifier that regulates the full-scale output current, I
xOUTFS
.
The control amplifier is configured as a V-I converter, as shown
in Figure 96. The output current, I
xREF
, is determined by the
ratio of the V
REFIO
and an external resistor, xR
SET
, as stated in
Equation 4 (see the DAC Transfer Function section). I
xREF
, is
mirrored to the segmented current sources with the proper scale
factor to set I
xOUTFS
, as stated in Equation 3.
The control amplifier allows a 2.5:1 adjustment span of I
xOUTFS
from 1 mA to 4 mA by setting I
xREF
between 125 A and 31.25 A
(set xR
SET
between 8 k and 32 k). The wide adjustment span
of I
xOUTFS
provides several benefits. The first relates directly to
the power dissipation of the AD9714/AD9715/AD9716/AD9717,
which is proportional to I
xOUTFS
(see the DAC Transfer Function
section). The second benefit relates to the ability to adjust the
output over a 8 dB range with 0.25 dB steps, which is useful for
controlling the transmitted power. The small signal bandwidth
of the reference control amplifier is approximately 500 kHz.
This allows the device to be used for low frequency, small signal
multiplying applications.
When an external resistor greater than 16 k is used on the
FSADJx pins, care must be taken to maintain the high frequency
equivalent circuit to an impedance lower than 16 k by
splitting the resistor into two resistors in series with a 10 nF
capacitor in parallel with the resistor to AVSS (see Figure 97).
AD9714/AD9715/
AD9716/AD9717
07265-219
xR
SET
0.1µF
R < 16kΩ
REFIO
AVSS
FSADJx
10nF
Figure 97. xR
SET
Configuration for Values > 16 kΩ