Datasheet

AD9709
Rev. B | Page 13 of 32
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
REFIO
) sets the reference current (I
REF
), which is replicated to
the segmented current sources with the proper scaling factor.
The full-scale current (I
OUTFS
) is 32 × I
REF
.
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.
AD9709
REFERENCE
SECTION
AVDDGAINCTRL
REFIO
FSADJ1/
FSADJ2
ACOM
CURRENT
SOURCE
ARRAY
1.2V
REF
I
REF
0.1µF
OPTIONAL
EXTERNAL
REFERENCE
BUFFER
ADDITIONAL
EXTERNAL
LOAD
R
SET
256Ω
22nF
00606-023
Figure 23. Internal Reference Configuration
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.
AD9709
REFERENCE
SECTION
AVDDGAINCTRL
REFIO
FSADJ1/
FSADJ2
ACOM
AVDD
CURRENT
SOURCE
ARRAY
EXTERNAL
REFERENCE
1.2V
REF
0
0606-024
I
REF
R
SET
256Ω
22nF
Figure 24. External Reference Configuration
GAIN CONTROL MODE
The AD9709 allows the gain of each channel to be set
independently by connecting one R
SET
resistor to FSADJ1 and
another R
SET
resistor to FSADJ2. To add flexibility and reduce
system cost, a single R
SET
resistor can be used to set the gain of
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
OUTFS
). The
control amplifier is configured as a V-I converter, as shown in
Figure 23, so that its current output (I
REF
) is determined by the ratio
of the V
REFIO
and an external resistor, R
SET
.
SET
REFIO
REF
R
V
I =
The DAC full-scale current, I
OUTFS
, is an output current 32 times
larger than the reference current, I
REF
.
REF
OUTFS
II
×
=
32
The control amplifier allows a wide (10:1) adjustment span of
I
OUTFS
from 2 mA to 20 mA by setting I
REF
between 62.5 A and
625 A. The wide adjustment range of I
OUTFS
provides several
benefits. The first relates directly to the power dissipation of
the AD9709, which is proportional to I
OUTFS
(refer to the Power
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
SET
resistors are 2 kΩ or less,
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
SET
resistors alone. For R
SET
values greater than 2 kΩ, the
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
SET
, however, if the R
SET
resistor is
located more than ~10 cm away from the pin, use of the 22 nF
capacitor and 256 Ω resistor is recommended.