+3V/+5V, 12-Bit, Serial Voltage-Output DACs

with Internal Reference

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9

_______________Detailed Description

The MAX5120/MAX5121 12-bit, voltage-output DACs

are easily configured with a 3-wire serial interface. They

include a 16-bit data-in/data-out shift register and have

a double-buffered input consisting of an input register

and a DAC register. In addition, these devices employ

precision bandgap references and trimmed internal

resistors to produce a gain of 1.6384V/V, maximizing

the output voltage swing (Figure 1). The MAX5120/

MAX5121 output amplifier’s offset-adjust pin allows for

a DC shift in the DAC outputs. The full-scale output volt-

age is +4.095V for the MAX5120 and +2.0475V for the

MAX5121. These DACs are designed with an inverted

R-2R ladder network (Figure 2) that produces a weight-

ed output voltage proportional to the digital input code.

Internal Reference

Both the MAX5120 and MAX5121 use an on-board pre-

cision bandgap reference to generate an output volt-

age of +2.5V (MAX5120) or +1.25V (MAX5121). With a

low temperature coefficient of only 10ppm/°C (max),

the REF pin can source up to 100µA and may become

unstable with capacitive loads exceeding 100pF.

REFADJ can be used for minor adjustments (1%) to the

reference voltage. Use the circuits shown in Figure 3a

MAX5120

MAX5121

SR

CONTROL

16-BIT

SHIFT REGISTER

DECODE

CONTROL

INPUT

REGISTER

BANDGAP

REFERENCE

REFERENCE

BUFFER

DAC

REGISTER

DAC

2X

(1X)

DOUT

UPO

OUT

OS

R

0.6384R

GAIN = 1.6384X

4k

1.25V

AGND DGND

DIN SCLK

CS

2.5V, (1.25V)

LOGIC

OUTPUT

CLR

PDL

RSTVAL

PD

12

REFADJ

REF

( ) FOR MAX5121 ONLY

Figure 1. Simplified Functional Diagram

OUT

OS

R

0.6384R

SHOWN FOR ALL 1s ON DAC

*INTERNAL 2.5V (MAX5120) AND 1.25V (MAX5121)

OR EXTERNAL REFERENCE.

D0

D9

D10

D11

2R

2R

2R

2R

2R

RR

R

REF*

AGND

Figure 2. Simplified Inverted R-2R DAC Structure