AD9705-EB datasheet(34/52 Pages) AD

34 / 52 page

AD9704/AD9705/AD9706/AD9707

Rev. 0 | Page 34 of 52

REFERENCE OPERATION

The AD970x contains an internal 1.0 V band gap reference.

The internal reference can be disabled by writing a Logic 1 to

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 should be used. An example of the

use of the internal reference is shown in Figure 79.

CURRENT

SCALING

x32

AD9704/AD9705/

AD9706/AD9707

DAC

IOUTFS

RSET

0.1µF

REFIO

IREF

AVSS

FSADJ

VBG

1.0V

–

Figure 79. Internal Reference Configuration

REFIO serves as either an input or an output, depending on

whether the internal or an external reference is used. Table 23

summarizes the reference operation.

Table 23. Reference Operation

Reference

Mode

REFIO Pin

Internal

Connect 0.1 μF capacitor

(default)

External

Apply external reference

(for power saving)

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 AD970x contains a control amplifier that regulates the full-

scale output current, IOUTFS. The control amplifier is configured

as a V-I converter, as shown in Figure 79. The output current,

IREF, is determined by the ratio of the VREFIO and an external

resistor, RSET, as stated in Equation 4. IREF is mirrored to the

segmented current sources with the proper scale factor to set

IOUTFS, as stated in Equation 3.

The control amplifier allows a 5:1 adjustment span of IOUTFS

from 1 mA to 5 mA by setting IREF between 31.25 μA and

156.25 μA (RSETbetween 6.4 kΩ and 32 kΩ). The wide adjustment

span of IOUTFS provides several benefits. The first relates directly

to the power dissipation of the AD970x, which is proportional

to IOUTFS (see the Power Dissipation 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 AD970x provides complementary current outputs, IOUTA

and IOUTB. IOUTA provides a near full-scale current output,

IOUTFS, when all bits are high (that is, DAC CODE = 2N − 1,

where N = 8, 10, 12, or 14 for the 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 IOUTFS

and can be expressed as

IOUTA = (DAC CODE/2N) × IOUTFS

(1)

IOUTB = ((2N − 1) − DAC CODE)/2N × IOUTFS

(2)

where DAC CODE = 0 to 2N − 1 (that is, decimal representation).

IOUTFS is a function of the reference current, IREF, which is

nominally set by a reference voltage, VREFIO, and an external

resistor, RSET. It can be expressed as

IOUTFS = 32 × IREF

(3)

where

IREF = VREFIO/RSET

(4)

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, RLOAD,

that are tied to analog common, ACOM. The single-ended

voltage output appearing at the IOUTA and IOUTB nodes is

VIOUTA = IOUTA × RLOAD

(5)

VIOUTB = IOUTB × RLOAD

(6)

Note: To achieve the maximum output compliance of 1 V at the

nominal 2 mA output current, RLOAD must be set to 500 Ω.

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