AD9706-EBZ Analog Devices Inc, AD9706-EBZ Datasheet - Page 34

• Manufacturer Part Number: AD9706-EBZ
• Description: 12-BIT Low Power TxDAC DAC IC
• Manufacturer: Analog Devices Inc
• Series: TxDAC®r

Specifications of AD9706-EBZ

• Number Of Dac's: 1
• Number Of Bits: 12
• Outputs And Type: 1, Differential
• Sampling Rate (per Second): 175M
• Data Interface: Serial
• Settling Time: 11ns
• Dac Type: Current
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD9706
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD9704/AD9705/AD9706/AD9707
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.
To use the internal reference, decouple the REFIO pin to ACOM
with a 0.1 μF capacitor, enable the internal reference, and write
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.
REFIO serves as either an input or an output, depending on
whether the internal or an external reference is used. Table 24
summarizes the reference operation.
Table 24. Reference Operation
Reference
Mode
Internal
External
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
control amplifier is configured as a V-I converter, as shown in
Figure 78. The output current, I
the V
I
proper scale factor to set I
REF
is mirrored to the segmented current sources with the
0.1µF
REFIO
AVSS
and an external resistor, R
R
FS ADJ
REFIO Pin
Connect 0.1 μF capacitor
Apply external reference
SET
Figure 78. Internal Reference Configuration
REFIO
I
REF
V
1.0V
BG
AD9704/AD9705/
AD9706/AD9707
–
+
OUTFS
, as stated in Equation 3.
REF
, is determined by the ratio of
SET
CURRENT
SCALING
, as stated in Equation 4.
x32
Register Setting
Register 0x00, Bit 0 = 0
(default)
Register 0x00, Bit 0 = 1
(for power saving)
I
OUTFS
DAC
OUTFS
. The
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The control amplifier allows a 5:1 adjustment span of I
from 1 mA to 5 mA by setting I
156.25 μA (R
span of I
to the power dissipation of the AD9704/AD9705/AD9706/
AD9707, which is proportional to I
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.
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 AD9704/AD9705/AD9706/AD9707 provide complemen-
tary current outputs, IOUTA and IOUTB. IOUTA provides a
near full-scale current output, I
is, DAC CODE = 2
AD9704, AD9705, AD9706, and AD9707, respectively), while
IOUTB, the complementary output, provides no current. The
current output appearing at IOUTA and IOUTB is a function of
both the input code and I
where DAC CODE = 0 to 2
I
nominally set by a reference voltage, V
resistor, R
where
The two current outputs typically drive a resistive load directly
or via a transformer. If dc coupling is required, IOUTA and
IOUTB should be connected to matching resistive loads, R
that are tied to analog common, ACOM. The single-ended
voltage output appearing at the IOUTA and IOUTB nodes is
Note: To achieve the maximum output compliance of 1 V at the
nominal 2 mA output current, R
Also, note that the full-scale value of V
exceed the specified output compliance range to maintain
specified distortion and linearity performance.
Substituting the values of IOUTA, IOUTB, I
expressed as
OUTFS
IOUTA = (DAC CODE/2
IOUTB = ((2
I
I
V
V
V
V
OUTFS
REF
is a function of the reference current, I
DIFF
DIFF
IOUTA
IOUTB
OUTFS
= V
SET
= (IOUTA – IOUTB) × R
= {(2 × DAC CODE – (2
= 32 × I
= IOUTB × R
(32 × V
= IOUTA × R
. It can be expressed as
REFIO
SET
provides several benefits. The first relates directly
between 6.4 kΩ and 32 kΩ). The wide adjustment
/R
N
REF
SET
N
REFIO
− 1) − DAC CODE)/2
− 1, where N = 8, 10, 12, or 14 for the
/R
OUTFS
LOAD
LOAD
SET
N
− 1 (that is, decimal representation).
) × R
and can be expressed as
N
) × I
OUTFS
REF
LOAD
LOAD
OUTFS
N
between 31.25 μA and
LOAD
OUTFS
, when all bits are high (that
− 1))/2
must be set to 500 Ω.
(see the Power Dissipation
REFIO
IOUTA
N
× I
, and an external
N
and V
REF
} ×
REF
OUTFS
, and V
, which is
IOUTB
DIFF
OUTFS
must not
LOAD
can be
,
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)