________________Detailed Description

The MAX503 consists of a parallel-input logic interface, a

10-bit R-2R ladder, a reference, and an op amp. The

Functional Diagram shows the control lines and signal

flow through the input data latch to the DAC latch, as well

as the 2.048V reference and output op amp. Total supply

current is typically 250µA with a single +5V supply. This

circuit is ideal for battery-powered, microprocessor-con-

trolled applications where high accuracy, no adjustments,

and minimum component count are key requirements.

R-2R Ladder

The MAX503 uses an “inverted” R-2R ladder network with

a BiCMOS op amp to convert 10-bit digital data to analog

voltage levels. Figure 1 shows a simplified diagram of the

R-2R DAC and op amp. Unlike a standard DAC, the

MAX503 uses an “inverted” ladder network. Normally, the

REFIN pin is the current output of a standard DAC and

would be connected to the summing junction, or virtual

ground, of an op amp. In this standard DAC configura-

tion, however, the output voltage would be the inverse of

5V, Low-Power, Parallel-Input,

Voltage-Output, 10-Bit DAC

8

_______________________________________________________________________________________

2R

2R

2R

2R

2R

RR

R

MSB

OUTPUT

BUFFER

VOUT

RFB

ROFS

MAX503

2R

2R

REFIN

AGND

DAC LATCH

R

= 80kΩ

LSB

NBL

INPUT

LATCH

NBH

INPUT

LATCH

NBM

INPUT

LATCH

D6/S0

D7/S1

D8/D0

D2

D9/D1

D4

D3

D5

2.048V

REFOUT

REFGND

*

LSB

MSB

CLR

Figure 1. Simplified MAX503 DAC Circuit

the reference voltage. The MAX503’s topology makes the

ladder output voltage the same polarity as the reference

input, making the device suitable for single-supply oper-

ation. The BiCMOS op amp is then used to buffer, invert,

or amplify the ladder signal.

Ladder resistors are nominally 80k

Ω to conserve power

and are laser trimmed for gain and linearity. The input

impedance at REFIN is code dependent. When the DAC

register is all 0s, all rungs of the ladder are grounded

and REFIN is open or no load. Maximum loading (mini-

mum REFIN impedance) occurs at code 010101....

Minimum reference input impedance at this code is guar-

anteed to be not less than 40k

Ω.

The REFIN and REFOUT pins allow the user to choose

between driving the R-2R ladder with the on-chip refer-

ence or an external reference. REFIN may be below ana-

log ground when using dual supplies. See the

External

Reference and Four-Quadrant Multiplication sections for

more information.

Internal Reference

The on-chip reference is laser trimmed to generate

2.048V at REFOUT. The output stage can source and

sink current so REFOUT can settle to the correct volt-

age quickly in response to code-dependent loading

changes. Typically, source current is 5mA and sink

current is 100µA.

REFOUT connects the internal reference to the R-2R

DAC ladder at REFIN. The R-2R ladder draws 50µA

maximum load current. If any other connection is made

to REFOUT, ensure that the total load current is less

than 100µA to avoid gain errors.

A separate REFGND pin is provided to isolate refer-

ence currents from other analog and digital ground

currents. To achieve specified noise performance, con-

nect a 33µF capacitor from REFOUT to REFGND (see

Figure 2). Using smaller capacitance values increases

noise, and values less than 3.3µF may compromise the

reference’s stability. For applications requiring the low-

est noise, insert a buffered RC filter between REFOUT

and REFIN. When using the internal reference,

REFGND must be connected to AGND. In applications

not requiring the internal reference, connect REFGND