Low-Power, 12-Bit Voltage-Output DAC

with Serial Interface

6

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Detailed Description

The MAX5302 contains a voltage-output digital-to-analog

converter (DAC) that is easily addressed using a simple

3-wire serial interface. The IC includes a 16-bit shift

register, and has a double-buffered input composed of

Diagram). In addition to the voltage output, the amplifier’s

negative input is available to the user.

The DAC is an inverted R-2R ladder network that converts

a digital input (12 data bits plus 1 sub-bit) into an

equivalent analog output voltage in proportion to the

applied reference voltage. Figure 1 shows a simplified

circuit diagram of the DAC.

Reference Inputs

The reference input accepts positive DC and AC signals.

The voltage at the reference input sets the full-scale out-

put voltage for the DAC. The reference input voltage

represented by a digitally programmable voltage

source, as expressed in the following equation:

where NB is the numeric value of the DAC’s binary

and Gain is the externally set voltage gain.

The impedance at the reference input is code dependent,

ranging from a low value of 14k

Ω when the DAC has an

input code of 1554 hex, to a high value exceeding several

gigaohms (leakage currents) with an input code of

0000 hex. Because the input impedance at the reference

pin is code dependent, load regulation of the reference

source is important.

In shutdown mode, the MAX5302’s REF input enters a

high-impedance state with a typical input leakage current

of 0.001µA.

The reference input capacitance is also code dependent

and typically ranges from 15pF (with an input code of

all 0s) to 50pF (at full scale).

The MAX873 +2.5V reference is recommended for the

MAX5302.

Output Amplifier

The MAX5302’s DAC output is internally buffered by a

precision amplifier with a typical slew rate of 0.6V/µs.

Access to the output amplifier’s inverting input provides

the user greater flexibility in output gain setting/signal

conditioning (see the Applications Information section).

With a full-scale transition at the MAX5302 output, the

typical settling time to ±1/2LSB is 14µs when loaded

with 5k

Ω in parallel with 100pF (loads less than 2kΩ

degrade performance).

The amplifier’s output dynamic responses and settling

performances are shown in the Typical Operating Char-

acteristics.

Shutdown Mode

The MAX5302 features a software-programmable shut-

down that reduces supply current to a typical value of

4µA. Writing 111X XXXX XXXX XXXX as the input control

word puts the device in shutdown mode (Table 1).

In shutdown mode, the amplifier’s output and the refer-

ence input enter a high-impedance state. The serial

interface remains active. Data in the input registers is

retained in shutdown, allowing the MAX5302 to recall

the output state prior to entering shutdown. Exit shutdown

mode by either recalling the previous configuration or

OUT

FB

SHOWN FOR ALL 1s ON DAC

MSB

2R

2R

2R

2R

2R

RR

R

REF

AGND

Figure 1. Simplified DAC Circuit Diagram

_____________________Pin Description

NAME

FUNCTION

1

OUT

DAC Output Voltage

2

CS

Chip-Select Input. Active low.

PIN

3

DIN

Serial-Data Input

4

SCLK

Serial-Clock Input

8

Positive Power Supply

7

GND

Ground

6

REF

Reference Voltage Input

5

FB

DAC Output Amplifier Feedback