AD9709 datasheet(13/32 Pages) AD

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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

(VREFIO) sets the reference current (IREF), which is replicated to

the segmented current sources with the proper scaling factor.

The full-scale current (IOUTFS) is 32 × IREF.

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

AVDD

GAINCTRL

REFIO

FSADJ1/

FSADJ2

ACOM

CURRENT

SOURCE

ARRAY

1.2V

REF

IREF

0.1µF

OPTIONAL

EXTERNAL

REFERENCE

BUFFER

ADDITIONAL

EXTERNAL

LOAD

RSET

256

Ω

22nF

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

AVDD

GAINCTRL

REFIO

FSADJ1/

FSADJ2

ACOM

AVDD

CURRENT

SOURCE

ARRAY

EXTERNAL

REFERENCE

1.2V

REF

IREF

RSET

256

Ω

22nF

Figure 24. External Reference Configuration

GAIN CONTROL MODE

The AD9709 allows the gain of each channel to be set

independently by connecting one RSET resistor to FSADJ1 and

another RSET resistor to FSADJ2. To add flexibility and reduce

system cost, a single RSET 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 (IOUTFS). The

control amplifier is configured as a V-I converter, as shown in

Figure 23, so that its current output (IREF) is determined by the ratio

of the VREFIO and an external resistor, RSET.

SET

REFIO

REF

The DAC full-scale current, IOUTFS, is an output current 32 times

larger than the reference current, IREF.

REF

OUTFS

= 32

The control amplifier allows a wide (10:1) adjustment span of

IOUTFS from 2 mA to 20 mA by setting IREF between 62.5 μA and

625 μA. The wide adjustment range of IOUTFS provides several

benefits. The first relates directly to the power dissipation of

the AD9709, which is proportional to IOUTFS (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 RSET 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 RSET resistors alone. For RSET 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 RSET, however, if the RSET resistor is

located more than ~10 cm away from the pin, use of the 22 nF

capacitor and 256 Ω resistor is recommended.

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