AD8021 datasheet(9/24 Pages) AD

9 / 24 page

Data Sheet

AD5545/AD5555

Rev. G | Page 9 of 24

THEORY OF OPERATION

The AD5545/AD5555 contain a 16-/14-bit, current-output,

digital-to-analog converter, a serial-input register, and a DAC

interface with an additional LDAC for dual channel simultaneous

update.

DIGITAL-TO-ANALOG CONVERTER

The DAC architecture uses a current-steering R-2R ladder

design. Figure 17 shows the typical equivalent DAC. The DAC

contains a matching feedback resistor for use with an external

I-to-V converter amplifier. The RFB pin is connected to the

output of the external amplifier. The IOUT terminal is connected

to the inverting input of the external amplifier. These DACs are

designed to operate with either negative or positive reference

voltages. The VDD power pin is used only by the logic to drive

the DAC switches on and off. Note that a matching switch is

used in series with the internal 5 kΩ feedback resistor. If users

attempt to measure the RFB value, power must be applied to VDD

to achieve continuity. The VREF input voltage and the digital data

(D) loaded into the corresponding DAC register, according to

Equation 1 and Equation 2, determine the DAC output voltage.

536

–

REF

OUT

(1)

384

–

REF

OUT

(2)

Note that the output full-scale polarity is the opposite of the

VREF polarity for dc reference voltages.

VREF

DIGITAL INTERFACE CONNECTIONS OMITTED FOR CLARITY:

SWITCHES S1 AND S2 ARE CLOSED, VDD MUST BE POWERED

Ω

VDD

RFB

IOUT

GND

02918- 0- 005

Figure 17. Equivalent R-2R DAC Circuit

These DACs are also designed to accommodate ac reference input

signals. The AD5545/AD5555 accommodate input reference

voltages in the range of –12 V to +12 V. The reference voltage

inputs exhibit a constant nominal input-resistance value of

5 kΩ, ±30%. The DAC output (IOUT) is code dependent, pro-

ducing various output resistances and capacitances. When

choosing an external amplifier, the user should take into

account the variation in impedance generated by the AD5545/

AD5555 on the amplifiers inverting input node. The feedback

resistance in parallel with the DAC ladder resistance dominates

output voltage noise.

VREFA

DIGITAL INTERFACE CONNECTIONS OMITTED FOR CLARITY:

SWITCHES S1 AND S2 ARE CLOSED, VDD MUST BE POWERED

Ω

+3V

–3V

VOUT

VIN

VDD

2.500V

RFBA

IOUTA

AGNDA

GND

02918- 0- 006

AD5545/AD5555

ADR03

AD8628

LOAD

VOUT

VEE

VCC

Figure 18. Recommended System Connections

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