AD9772A-EB datasheet(22/40 Pages) AD

22 / 40 page

AD9772A

Rev. C | Page 22 of 40

DAC OPERATION

The 14-bit DAC, along with the 1.2 V reference and reference

control amplifier, is shown in Figure 37. The DAC consists of a

large PMOS current source array capable of providing up to

20 mA of full-scale current, IOUTFS. The array is divided into 31

equal currents that make up the five most significant bits

(MSBs). The next four bits, or middle bits, consist of 15 equal

current sources whose values are 1/16th of an MSB current

source. The remaining LSBs are binary-weighted fractions of

the middle bits’ current sources. All of these current sources are

switched to one of the two output nodes (that is, IOUTA or IOUTB) via

the PMOS differential current switches. Implementing the middle

and lower bits with current sources instead of an R-2R ladder

enhances its dynamic performance for multitone or low amplitude

signals and helps maintain the high output impedance of the DAC.

REFIO

FSADJ

250pF

REFLO

AVDD

AD9772A

RSET

2kΩ

0.1µF

ACOM

CURRENT

SOURCE

ARRAY

IOUTA

IOUTB

INTERPOLATED

DIGITAL DATA

RLOAD

VDIFF = VOUTA – VOUTB

IOUTA

IOUTB

SEGMENTED

SWITCHES

LSB

SWITCHES

1.2V REF

2.7V TO 3.6V

IREF

Figure 37. Block Diagram of Internal DAC, 1.2 V Reference, and Reference

Control Circuits

The full-scale output current is regulated by the reference

control amplifier and can be set from 2 mA to 20 mA via an

external resistor, RSET, as shown in Figure 37. RSET, in

combination with both the reference control amplifier and

voltage reference, REFIO, sets the reference current, IREF, which

is mirrored to the segmented current sources with the proper

scaling factor. The full-scale current, IOUTFS, is exactly 32 times

the value of IREF.

DAC TRANSFER FUNCTION

The AD9772A provides complementary current outputs, IOUTA

and IOUTB. I

OUTA

provides a near full-scale current output, IOUTFS,

when all bits are high (that is, DAC CODE = 16,383), whereas

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

OUTFS

and can be expressed as

IOUTA = (DAC CODE/16,384) × IOUTFS

(1)

IOUTB = (16,383 − DAC CODE)/16,384 × IOUTFS

(2)

where DAC CODE = 0 to 16,383 (that is, decimal representation).

As previously mentioned, 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 directly connected to matching resistive loads, RLOAD,

that are tied to analog common, ACOM. Note that RLOAD can

represent the equivalent load resistance seen by IOUTA or IOUTB, as

would be the case in a doubly terminated 50 Ω or 75 Ω cable.

The single-ended voltage output appearing at the IOUTA and IOUTB

nodes is simply

VOUTA = IOUTA × RLOAD

(5)

VOUTB = IOUTB × RLOAD

(6)

Note that the full-scale value of VOUTA and VOUTB should not

exceed the specified output compliance range of 1.25 V to

prevent signal compression. To maintain optimum distortion

and linearity performance, the maximum voltages at VOUTA and

VOUTB should not exceed ±500 mV p-p.

The differential voltage, VDIFF, appearing across IOUTA and IOUTB is

VDIFF = (IOUTA − IOUTB) × RLOAD

(7)

Substituting the values of IOUTA, IOUTB, and I

REF

, VDIFF can be

expressed as

VDIFF = [(2 DAC CODE − 16,383)/16,384] ×

(32 × RLOAD/RSET) × VREFIO

(8)

The last two equations highlight some of the advantages of

operating the AD9772A differentially. First, the differential

operation helps cancel common-mode error sources, such as

noise, distortion, and dc offsets, associated with IOUTA and IOUTB.

Second, the differential code-dependent current and

subsequent voltage, VDIFF, is twice the value of the single- ended

voltage output (that is, VOUTA or VOUTB), thus providing twice the

signal power to the load.

Note that the gain drift temperature performance for a single-

ended (VOUTA and VOUTB) or differential output (V

DIFF

) of the

AD9772A can be enhanced by selecting temperature tracking

resistors for RLOAD and RSET due to their ratiometric relationship,

as shown in Equation 8.

REFERENCE OPERATION

The AD9772A contains an internal 1.20 V band gap reference

that can easily be disabled and overridden by an external

reference. REFIO serves as either an output or input, depend-

ing on whether the internal or external reference is selected. If

REFLO is tied to ACOM, as shown in Figure 38, the internal

reference is activated, and REFIO provides a 1.20 V output. In

this case, the internal reference must be compensated externally

with a ceramic chip capacitor of 0.1 μF or greater from REFIO

to REFLO. If any additional loading is required, REFIO should

be buffered with an external amplifier having an input bias

current less than 100 nA.

AD9772A-EB Datasheet(PDF) 22 Page - Analog Devices

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