12

LTC1821

Precision Voltage Reference Considerations

Because of the extremely high accuracy of the 16-bit

LTC1821, careful thought should be given to the selection

of a precision voltage reference. As shown in the section

describing the basic operation of the LTC1821, the output

voltage of the DAC circuit is directly affected by the voltage

reference; thus, any voltage reference error will appear as

a DAC output voltage error.

There are three primary error sources to consider when

selecting a precision voltage reference for 16-bit applica-

tions: output voltage initial tolerance, output voltage tem-

perature coefficient (TC), and output voltage noise.

Initial reference output voltage tolerance, if uncorrected,

generates a full-scale error term. Choosing a reference

with low output voltage initial tolerance, like the LT1236

(

±0.05%), minimizes the gain error due to the reference;

however, a calibration sequence that corrects for system

zero- and full-scale error is always recommended.

A reference’s output voltage temperature coefficient af-

fects not only the full-scale error, but can also affect the

circuit’s INL and DNL performance. If a reference is

chosen with a loose output voltage temperature coeffi-

cient, then the DAC output voltage along its transfer

characteristic will be very dependent on ambient condi-

tions. Minimizing the error due to reference temperature

coefficient can be achieved by choosing a precision refer-

ence with a low output voltage temperature coefficient

and/or tightly controlling the ambient temperature of the

circuit to minimize temperature gradients.

As precision DAC applications move to 16-bit and higher

performance, reference output voltage noise may contrib-

ute a dominant share of the system’s noise floor. This in

turn can degrade system dynamic range and signal-to-

noise ratio. Care should be exercised in selecting a voltage

Table 2. Partial List of LTC Precision References Recommended

for Use with the LTC1821, with Relevant Specifications

INITIAL

TEMPERATURE

0.1Hz to 10Hz

REFERENCE

TOLERANCE

DRIFT

NOISE

LT1019A-5,

±0.05%

5ppm/

LT1019A-10

LT1236A-5,

±0.05%

5ppm/

LT1236A-10

LT1460A-5,

±0.075%

10ppm/

LT1460A-10

LT1790A-2.5

±0.05%

10ppm/

reference with as low an output noise voltage as practical

for the system resolution desired. Precision voltage refer-

ences, like the LT1236, produce low output noise in the

0.1Hz to 10Hz region, well below the 16-bit LSB level in 5V

or 10V full-scale systems. However, as the circuit band-

widths increase, filtering the output of the reference may

be required to minimize output noise.

Grounding

As with any high resolution converter, clean grounding is

important. A low impedance analog ground plane and star

grounding should be used. AGNDF and AGNDS must be

tied to the star ground with as low a resistance as possible.

When it is not possible to locate star ground close to

AGNDF and AGNDS, separate traces should be used to

route these pins to the star ground. This minimizes the

voltage drop from these pins to ground due to the code

dependent current flowing into the ground plane. If the

resistance of these separate circuit board traces exceeds

1

Ω, the circuit of Figure 3 eliminates this code dependent

voltage drop error for high resistance traces.

To calculate PC track resistance in squares, divide the

length of the PC track by the width and multiply this result

by the sheet resistance of copper foil. For 1 oz copper

(

≈1.4 mils thick), the sheet resistance is 0.045Ω per

square.