Datasheet

Data Sheet AD9114/AD9115/AD9116/AD9117
Rev. C | Page 43 of 52
REFERENCE OPERATION
The AD9114/AD9115/AD9116/AD9117 contains 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 μF 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
x32
AD9114/AD9115/
AD9116/AD9117
I DAC
OR
Q DAC
07466-218
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
reference
Register 0x01, Bit 0 = 1
(for power saving)
An external reference can be used in applications requiring tighter
gain tolerances or lower temperature drift. In addition, 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 µF compensation capacitor on REFIO is not
required unless specified by the external voltage reference
manufacturer. The input impedance of REFIO is 10 kΩ when
the internal reference is powered up and 1 MΩ when it is
powered down.
REFERENCE CONTROL AMPLIFIER
The AD9114/AD9115/AD9116/AD9117 contains 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 (see the DAC Transfer Function
section).
The control amplifier allows a 10:1 adjustment span of I
xOUTFS
from 2 mA to 20 mA by setting I
xREF
between 62.5 µA and 625 µA
(xR
SET
between 1.6 kΩ and 16 kΩ). When using a resistor larger
than 4 kΩ, split the resistor with 4 kΩ plus the additional
resistance needed, for example, 16 kΩ made of a 4 kΩ + 12 kΩ
combination, and add a 1 µF capacitor from 4 kΩ to ground.
The wide adjustment span of I
xOUTFS
provides several benefits.
The first relates directly to the power dissipation of the
AD9114/AD9115/AD9116/AD9117, 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.
DAC TRANSFER FUNCTION
The AD9114/AD9115/AD9116/AD9117 provides two differential
current outputs, IOUTP/IOUTN and QOUTP/ QOUTN. IOUTP
and QOUTP provide a near full-scale current output, I
xOUTFS
,
when all bits are high (that is, DAC CODE = 2
N
− 1, where N = 8,
10, 12, or 14 for the AD9114, AD9115, AD9116, and AD9117,
respectively), while IOUTN and QOUTN, the complementary
outputs, provide no current. The current outputs appearing at the
positive DAC outputs, IOUTP and QOUTP, and at the negative
DAC outputs, IOUTN and QOUTN, are a function of both the
input code and I
xOUTFS
and can be expressed as follows:
IOUTP = (IDAC CODE/2
N
) × I
IOUTFS
(1)
QOUTP = (QDAC CODE/2
N
) × I
QOUTFS
IOUTN = ((2
N
− 1) − IDAC CODE)/2
N
× I
IOUTFS
(2)
QOUTN = ((2
N
− 1) − QDAC CODE)/2
N
× I
QOUTFS
where:
IDAC CODE and QDAC CODE = 0 to 2
N
− 1 (that is, decimal
representation).
I
IOUTFS
and I
QOUTFS
are functions of the reference currents, I
IREF
and I
QREF
, respectively, which are nominally set by a reference
voltage, V
REFIO
, and external resistors, IR
SET
and QR
SET,
respectively.
I
IOUTFS
and I
QOUTFS
can be expressed as follows:
I
IOUTFS
= 32 × I
IREF
(3)
I
QOUTFS
= 32 × I
QREF