LTC1588 datasheet(13/16 Pages) LINER

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LTC1588/LTC1589/LTC1592

1588992fa

APPLICATIO S I FOR ATIO

with low output voltage initial tolerance, like the LT1236

(

±0.05%), minimizes the gain error caused by the refer-

ence; 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

reference with a low output voltage temperature coeffi-

cient 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

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.

Table 5. Partial List of LTC Precision References Recommended

for Use with the LTC1588/LTC1589/LTC1592 with Relevant

Specifications

INITIAL

TEMPERATURE

0.1Hz to 10Hz

REFERENCE

TOLERANCE

DRIFT

NOISE

LT1019A-5,

±0.05%

5ppm/

°C12µVP-P

LT1019A-10

LT1236A-5,

±0.05%

5ppm/

°C3µVP-P

LT1236A-10

LT1460A-5,

±0.075%

10ppm/

°C20µVP-P

LT1460A-10

LT1790A-2.5

±0.05%

10ppm/

°C12µVP-P

Grounding

As with any high resolution converter, clean grounding is

important. A low impedance analog ground plane and star

grounding techniques should be used. IOUT2 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 IOUT2,

a low resistance trace should be used to route this pin to

star ground. This minimizes the voltage drop from this pin

to ground caused by the code dependent current flowing

to ground. When the resistance of this circuit board trace

becomes greater than 1

Ω, a force/sense amplified con-

figuration should be used to drive this pin (see Figure 2).

This preserves the excellent accuracy (1LSB INL and DNL)

of the LTC1588/LTC1589/LTC1592.

An Isolated 16-Bit Subsystem Using the LTC1592

The circuit in Figure 4 is a complete example of an optically

isolated analog output subsystem that supports most of

the legacy ranges that are still common in industrial

environments. This circuit uses only two optoisolators,

the load pulse (CS/LD) being derived from a series of

transitions on the data line (SDI) after the clock (SCK) is

halted high. If a single chip microcontroller with an auto-

mated SPI interface is to be used, the SPI port can transfer

the 24 bits as three bytes. Subsequently, the data output

port pin can be reassigned to general purpose port opera-

tion and exercised to produce a number of transitions to

generate the load pulse. Alternatively, the entire sequence

can be programmed bit by bit with a general purpose port.

Figure 5 shows the timing.

The DC/DC converter, Figure 3 based on the LT

®3439

ultralow noise transformer driver provides a compact

means of powering this circuit, and allows the output to

deliver output current that is only limited by the LT1468

capabilities. The output capability of the DC/DC converter

itself is 80mA at

±12V and is available as demo board

DC511A. This circuit as shown requires approximately

130mA of the 5V supply (no load). The total surface area

required is less than 2 square inches.

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