Low-Cost, +5V, Serial-Input,

Voltage-Output, 16-Bit DAC

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7

Applications Information

Reference and Analog Ground Inputs

The MAX5541 operates with external voltage references

from 2V to 3V, and maintains 16-bit performance with

proper reference selection and application. Ideally, the

reference’s temperature coefficient should be less than

0.4ppm/°C to maintain 16-bit accuracy to within 1LSB

over the commercial (0°C to +70°C) temperature range.

Since this converter is designed as an inverted R-2R

voltage-mode DAC, the input resistance seen by the

voltage reference is code dependent. The worst-case

input-resistance variation is from 11.5k

Ω (at code 8555

hex) to 200k

Ω (at code 0000 hex). The maximum

change in load current for a 2.5V reference is 2.5V/

11.5k

Ω = 217µA; therefore, the required load regulation

is 7ppm/mA for a maximum error of 0.1LSB. This implies

a reference output impedance of <18m

Ω. In addition,

the impedance of the signal path from the voltage refer-

ence to the reference input must be kept low because it

contributes directly to the load-regulation error.

The requirement for a low-impedance voltage reference

is met with capacitor bypassing at the reference inputs

and ground. A 0.1µF ceramic capacitor with short leads

between REF and AGND provides high-frequency

bypassing. A surface-mount ceramic chip capacitor is

preferred because it has the lowest inductance. An

additional 10µF between REF and AGND provides low-

frequency bypassing. A low-ESR tantalum, film, or

organic semiconductor capacitor works well. Leaded

capacitors are acceptable because impedance is not

as critical at lower frequencies. The circuit can benefit

from even larger bypassing capacitors, depending on

the stability of the external reference with capacitive

loading. If separate force and sense lines are not used,

connect the appropriate force and sense pins together

close to the package.

AGND must also be low impedance, as load-regulation

errors will be introduced by excessive AGND resis-

tance. As in all high-resolution, high-accuracy applica-

tions, separate analog and digital ground planes yield

the best results. Connect DGND to AGND at the AGND

pin to form the “star” ground for the DAC system. For

the best possible performance, always refer remote

DAC loads to this system ground.

Unbuffered Operation

Unbuffered operation reduces power consumption as

well as offset error contributed by the external output

buffer. The R-2R DAC output is available directly at

without degradation at zero-scale. The DAC’s output

impedance is also low enough to drive medium loads

Ω) without degradation of INL or DNL; only

the gain error is increased by externally loading the

DAC output.

External Output Buffer Amplifier

In unipolar mode, the output amplifier is used in a volt-

age-follower connection. The DAC’s output resistance

is constant and is independent of input code; however,

the output amplifier’s input impedance should still be as

high as possible to minimize gain errors. The DAC’s

output capacitance is also independent of input code,

thus simplifying stability requirements on the external

amplifier.

In single-supply applications, precision amplifiers with

input common-mode ranges including AGND are avail-

able; however, their output swings do not normally

include the negative rail (AGND) without significant per-

formance degradation. A single-supply op amp, such

as the MAX495, is suitable if the application does not

use codes near zero.

Since the LSBs for a 16-bit DAC are extremely small

external amplifier’s input specification. The input offset

voltage can degrade the zero-scale error and might

require an output offset trim to maintain full accuracy if

the offset voltage is greater than 1/2LSB. Similarly, the

input bias current multiplied by the DAC output resis-

tance (typically 6.25k

Ω) contributes to the zero-scale

error. Temperature effects also must be taken into con-

sideration. Over the commercial temperature range, the

offset voltage temperature coefficient (referenced to

+25°C) must be less than 0.42µV/°C to add less than

1/2LSB of zero-scale error. The external amplifier’s

input resistance forms a resistive divider with the DAC

output resistance, which results in a gain error. To con-

tribute less than 1/2LSB of gain error, the input resis-

tance typically must be greater than:

The settling time is affected by the buffer input capaci-

tance, the DAC’s output capacitance, and PC board

capacitance. The typical DAC output voltage settling

time is 1µs for a full-scale step. Settling time can be sig-

nificantly less for smaller step changes. Assuming a

single time-constant exponential settling response, a

full-scale step takes 12 time constants to settle to within

1/2LSB of the final output voltage. The time constant is

equal to the DAC output resistance multiplied by the

total output capacitance. The DAC output capacitance

is typically 10pF. Any additional output capacitance will

increase the settling time.

6.25k

1

2

1

2

205M

14

ΩΩ

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