Low-Power, Quad, 12-Bit

Voltage-Output DACs with Serial Interface

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Detailed Description

The MAX5500/MAX5501 integrate four 12-bit, voltage-

output digital-to-analog converters (DACs) that are

addressed through a simple 3-wire serial interface. The

devices include a 16-bit data-in/data-out shift register.

Each internal DAC provides a doubled-buffered input

composed of an input register and a DAC register (see

the

Functional Diagram). The negative input of each

amplifier is externally accessible.

The DACs are inverted rail-to-rail ladder networks that

convert 12-bit digital inputs into equivalent analog out-

put voltages in proportion to the applied reference volt-

age inputs. DACs A and B share the REFAB input,

while DACs C and D share the REFCD input. The two

reference inputs allow different full-scale output voltage

ranges for each pair of DACs. Figure 1 shows a simpli-

fied circuit diagram of one of the four DACs.

Reference Inputs

The two reference inputs accept positive DC and AC

signals. The voltage at each reference input sets the

full-scale output voltage for the two corresponding

a digitally programmable voltage source as:

where NB is the numeric value of the binary input code

Gain is the externally set voltage gain.

The impedance at each reference input is code-depen-

dent, ranging from a low value of 10kΩ when both

DACs connected to the reference accept an input code

of 555 hex, to a high value exceeding giga-ohms with

an input code of 000 hex. The load regulation of the ref-

erence source affects the performance of the devices

as the input impedance at the reference inputs is code

dependent. The REFAB and REFCD reference inputs

provide a 10kΩ guaranteed minimum input impedance.

When the same voltage source drives the two reference

inputs, the effective minimum impedance is 5kΩ. A volt-

age reference with an excellent load regulation of

0.0002mV/mA, such as the MAX6033, is capable of dri-

ving both reference inputs simultaneously at 2.5V.

Driving REFAB and REFCD separately improves refer-

ence accuracy.

The REFAB and REFCD inputs enter a high-impedance

state, with a typical input leakage current of 0.02µA,

when the MAX5500/MAX5501 are in shutdown. The ref-

erence input capacitance is also code dependent and

typically ranges from 20pF with an input code of all 0s

to 100pF with an input code of all 1s.

Output Amplifiers

All DAC outputs are internally buffered by precision

amplifiers with a typical slew rate of 0.6V/µs. Access to

the inverting input of each output amplifier provides the

greater flexibility in output gain setting/signal condition-

ing (see the

Applications Information section).

With a full-scale transition at the output, the typical set-

tling time to within ±0.5 LSB is 12µs when the output is

loaded with 5kΩ in parallel with 100pF. A load of less

than 2kΩ at the output degrades performance. See the

Typical Operating Characteristics for the output dynamic

responses and settling performances of the amplifiers.

Power-Down Mode

The MAX5500/MAX5501 feature a software-program-

mable shutdown that reduces supply current to a typi-

cal value of 10µA. Drive PDL high to enable the

shutdown mode. Write 1100XXXXXXXXXXXX as the

input-control word to put the device in power-down

mode (Table 1).

In power-down mode, the output amplifiers and the ref-

erence inputs enter a high-impedance state.

The serial interface remains active. Data in the input

registers is retained in power-down, allowing the

devices to recall the output states prior to entering shut-

down. Start up from power-down either by recalling the

previous configuration or by updating the DACs with

new data. Allow 15µs for the outputs to stabilize when

powering up the devices or bringing the devices out of

shutdown.

OUT_

FB_

SHOWN FOR ALL 1s ON DAC

D0

D9

D10

D11

2R

2R

2R

2R

2R

RR

R

REF_

AGND

Figure 1. Simplified DAC Circuit Diagram