AD9709-EBZ Analog Devices Inc, AD9709-EBZ Datasheet - Page 13

• Manufacturer Part Number: AD9709-EBZ
• Description: 8-BIT 125 MSPS Dual TxDAC+ D/A Converter
• Manufacturer: Analog Devices Inc
• Series: TxDAC®r

Specifications of AD9709-EBZ

• Number Of Dac's: 2
• Number Of Bits: 8
• Outputs And Type: 2, Differential
• Sampling Rate (per Second): 125M
• Data Interface: Parallel
• Settling Time: 35ns
• Dac Type: Current
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD9709
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The full-scale output current of each DAC is regulated by
separate reference control amplifiers and can be set from 2 mA
to 20 mA via an external network connected to the full-scale
adjust (FSADJ) pin. The external network in combination with
both the reference control amplifier and voltage reference
(V
the segmented current sources with the proper scaling factor.
The full-scale current (I
REFERENCE OPERATION
The AD9709 contains an internal 1.20 V band gap reference.
This can easily be overridden by a low noise external reference
with no effect on performance. REFIO serves as either an input
or output depending on whether the internal or an external
reference is used. To use the internal reference, simply decouple
the REFIO pin to ACOM with a 0.1 μF capacitor. The internal
reference voltage will be present at REFIO. If the voltage at
REFIO is to be used elsewhere in the circuit, an external buffer
amplifier with an input bias current of less than 100 nA should
be used. An example of the use of the internal reference is
shown in Figure 23.
ADDITIONAL
An external reference can be applied to REFIO as shown in
Figure 24. The external reference can provide either a fixed
reference voltage to enhance accuracy and drift performance or
a varying reference voltage for gain control. Note that the 0.1 μF
compensation capacitor is not required because the internal
reference is overridden and the relatively high input impedance
of REFIO minimizes any loading of the external reference.
EXTERNAL
REFIO
REFERENCE
LOAD
EXTERNAL
REFERENCE
EXTERNAL
) sets the reference current (I
OPTIONAL
BUFFER
AVDD
I
REF
R
SET
Figure 24. External Reference Configuration
Figure 23. Internal Reference Configuration
0.1µF
I
REF
R
256Ω
SET
22nF
OUTFS
REFIO
FSADJ1/
FSADJ2
GAINCTRL
256Ω
) is 32 × I
22nF
1.2V
REF
GAINCTRL
FSADJ1/
FSADJ2
REFIO
1.2V
REF
REF
REF
), which is replicated to
REFERENCE
AD9709
SECTION
.
REFERENCE
AD9709
SECTION
CURRENT
SOURCE
ARRAY
AVDD
CURRENT
SOURCE
ARRAY
ACOM
AVDD
ACOM
Rev. B | Page 13 of 32
GAIN CONTROL MODE
The AD9709 allows the gain of each channel to be set
independently by connecting one R
another R
system cost, a single R
both channels simultaneously.
When GAINCTRL is low (that is, connected to analog ground),
the independent channel gain control mode using two resistors
is enabled. In this mode, individual RSET resistors should be
connected to FSADJ1 and FSADJ2. When GAINCTRL is high
(that is, connected to AVDD), the master/slave channel gain
control mode using one network is enabled. In this mode, a
single network is connected to FSADJ1, and the FSADJ2 pin
must be left unconnected.
Note that only parts with a date code of 9930 or later have the
master/slave gain control function. For parts with a date code
before 9930, Pin 42 must be connected to AGND, and the part
operates in the two-resistor, independent gain control mode.
SETTING THE FULL-SCALE CURRENT
Both of the DACs in the AD9709 contain a control amplifier
that is used to regulate the full-scale output current (I
control amplifier is configured as a V-I converter, as shown in
Figure 23, so that its current output (I
of the V
The DAC full-scale current, I
larger than the reference current, I
The control amplifier allows a wide (10:1) adjustment span of
I
625 μA. The wide adjustment range of I
benefits. The first relates directly to the power dissipation of
the AD9709, which is proportional to I
Dissipation section). The second relates to the 20 dB adjustment,
which is useful for system gain control purposes.
It should be noted that when the R
the 22 nF capacitor and 256 Ω resistor shown in Figure 23 and
Figure 24 are not required and the reference current can be set
by the R
22 nF capacitor and 256 Ω resistor networks are required to
ensure the stability of the reference control amplifier(s).
Regardless of the value of R
located more than ~10 cm away from the pin, use of the 22 nF
capacitor and 256 Ω resistor is recommended.
OUTFS
I
I
from 2 mA to 20 mA by setting I
OUTFS
REF
REFIO
SET
SET
=
resistors alone. For R
and an external resistor, R
= 32
V
resistor to FSADJ2. To add flexibility and reduce
R
REFIO
SET
×
I
REF
SET
resistor can be used to set the gain of
SET
OUTFS
, however, if the R
SET
, is an output current 32 times
REF
SET
values greater than 2 kΩ, the
SET
REF
SET
.
OUTFS
resistors are 2 kΩ or less,
) is determined by the ratio
resistor to FSADJ1 and
REF
.
OUTFS
between 62.5 μA and
(refer to the Power
provides several
SET
resistor is
AD9709
OUTFS
). The