MAX521ACAG 데이터 시트보기 (PDF) - Maxim Integrated

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MAX521ACAG

Quad/Octal, 2-Wire Serial 8-Bit DACs with Rail-to-Rail Outputs

Maxim Integrated 

Quad/Octal, 2-Wire Serial 8-Bit DACs

with Rail-to-Rail Outputs

R

2R 2R

D0

OUT_

(MAX521)

R

R

OUT_

2R

2R

2R

(MAX520)

D5

D6

D7

REF_

AGND

SHOWN FOR ALL 1s ON DAC

Figure 14. DAC Simplified Circuit Diagram

(RIN) is code dependent, it must be driven by a circuit

with low output impedance (no more than RIN ÷ 2000) to

maintain output linearity. The REF input capacitance is

also code dependent, with the maximum value occur-

ring at code FF hex (typically 30pF for the MAX520/

MAX521’s REF0–REF3, and 120pF for the MAX521’s

REF4). The output voltage for any DAC can be repre-

sented by a digitally programmable voltage source as:

VOUT = (N x VREF) / 256, where N is the numerical value

of the DAC’s binary input code. Table 1 shows the

unipolar code.

Table 1. Unipolar Code Table

DAC CONTENTS

11111111

10000001

10000000

01111111

00000001

00000000

ANALOG OUTPUT

( ) 255

+ VREF

———

256

( ) 129

+ VREF

———

256

( ) + VREF

128

———

256

= V—RE—F

2

( ) 127

+ VREF

———

256

+

VREF

(——1 —)

256

0V

MAX520 Unbuffered DAC Outputs

The unbuffered DAC outputs (OUT0–OUT3) connect

directly to the internal 16kΩ R-2R network. The outputs

swing from 0V to VDD.

The MAX520 has no output buffer amplifiers, giving it

very low supply current. The output-offset voltage is

lower without the output buffer, and the output can also

slew and settle faster if capacitive loading is minimized.

Resistive loading should be very light for highest accu-

racy. Any output loading generates some gain error,

increasing full-scale error. The R-2R ladder’s output

resistance is 16kΩ, so a 1µA output current creates a

16mV error. Linearity is not affected because the ladder

output resistance does not change with DAC code.

Ladder-resistance changes with temperature are also

very small.

DACs are often used in trimming applications to

replace hardware potentiometers. Figure 15a shows a

typical application, which requires a buffered output so

that a precise current can be injected into the summing

node through precision resistor RT. For this application,

the MAX520A features a precise ±1% (TA = +25°C,

±2.5% over temperature) factory-trimmed output resis-

tance. Because the MAX520A’s output resistance is

precisely trimmed, there is no need for an internal

buffer or external precision resistor (Figure 15b). For

applications where the output resistance value is not

critical, use the MAX520B.

All DACs exhibit output glitches during code transitions.

An output filter is sometimes used to reduce these

glitches in sensitive applications. The MAX520 simpli-

fies output filtering because its internal resistive ladder

network serves as the “R” in an RC filter. Simply con-

nect a small capacitor from the DAC output to ground.

See the Typical Operating Characteristics for oscillo-

scope photos of the worst-case 1LSB step change both

without and with 25pF of capacitance on the MAX520’s

output.

MAX521 Output Buffer Amplifiers

The MAX521 voltage outputs (OUT0–OUT7) are inter-

nally buffered precision unity-gain followers that slew

up to 1V/µs. The outputs can swing from 0V to VDD.

With a 0V to 4V (or 4V to 0V) output transition, the

amplifier outputs typically settle to 1/2LSB in 6µs when

loaded with 10kΩ in parallel with 100pF. The buffer

amplifiers are stable with any combination of resistive

loads ≥2kΩ and capacitive loads ≤300pF.

______________________________________________________________________________________ 15

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