AD9760 데이터 시트보기 (PDF) - Analog Devices

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AD9760

10-Bit, 125 MSPS TxDAC® D/A Converter

Analog Devices 

AD9760

FUNCTIONAL DESCRIPTION

Figure 39 shows a simplified block diagram of the AD9760.

The AD9760 consists of a large PMOS current source array that

is capable of providing up to 20 mA of total current. The array

is divided into 31 equal currents that make up the 5 most sig-

nificant bits (MSBs). The next 4 bits or middle bits consist

of 15 equal current sources whose value is 1/16th of an MSB

current source. The remaining LSBs is a binary weighted frac-

tion of the middle-bits current sources. 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 DAC’s high output

impedance (i.e., >100 kΩ).

All of these current sources are switched to one or the other of

the two output nodes (i.e., IOUTA or IOUTB) via PMOS differen-

tial current switches. The switches are based on a new architec-

ture that drastically improves distortion performance. This new

switch architecture reduces various timing errors and provides

matching complementary drive signals to the inputs of the dif-

ferential current switches.

The analog and digital sections of the AD9760 have separate

power supply inputs (i.e., AVDD and DVDD) that can operate

independently over a 2.7 volt to 5.5 volt range. The digital

section, which is capable of operating up to a 125 MSPS clock

rate, consists of edge-triggered latches and segment decoding

logic circuitry. The analog section includes the PMOS current

sources, the associated differential switches, a 1.20 V bandgap

voltage reference and a reference control amplifier.

The full-scale output current is regulated by the reference con-

trol amplifier and can be set from 2 mA to 20 mA via an exter-

nal resistor, RSET. The external resistor, in combination with

both the reference control amplifier and voltage reference

VREFIO, sets the reference current IREF, which is mirrored over to

the segmented current sources with the proper scaling factor.

The full-scale current, IOUTFS, is thirty-two times the value of IREF.

DAC TRANSFER FUNCTION

The AD9760 provides complementary current outputs, IOUTA

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

IOUTFS, when all bits are high (i.e., DAC CODE = 1023) 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/1024) × IOUTFS

(1)

IOUTB = (1023 – DAC CODE)/1024 × IOUTFS

(2)

where DAC CODE = 0 to 1023 (i.e., Decimal Representation).

As mentioned previously, IOUTFS is a function of the reference

current IREF, which is nominally set by a reference voltage,

VREFIO and external resistor RSET. It can be expressed as:

IOUTFS = 32 × IREF

(3)

where IREF = VREFIO/RSET

(4)

The two current outputs will 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,

RLOAD may 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 the full-scale value of VOUTA and VOUTB should not exceed

the specified output compliance range to maintain specified

distortion and linearity performance.

+5V

0.1␮F

0.1␮F

VREFIO

RSET

2k⍀

CLOCK

IREF

+5V

REFLO

COMP1

AVDD

ACOM

+1.20V REF

REFIO

FS ADJ

50pF

PMOS

CURRENT SOURCE

ARRAY

DVDD

DCOM

SEGMENTED SWITCHES

FOR DB9–DB1

LSB

SWITCH

CLOCK

SLEEP

LATCHES

AD9760

COMP2

IOUTA

IOUTB

0.1␮F

IOUTB

VDIFF = VOUTA – VOUTB

IOUTA

VOUTB

RLOAD

50⍀

VOUTA

RLOAD

50⍀

DIGITAL DATA INPUTS (DB9–DB0)

Figure 39. Functional Block Diagram

REV. B

–11–

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