LTC1749 datasheet(18/20 Pages) LINER

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LTC1749

1749f

sheet. Bypass capacitors must be located as close to the

pins as possible. Of particular importance are the capaci-

tors between REFHA and REFLB and between REFHB and

REFLA. These capacitors should be as close to the device

as possible (1.5mm or less). Size 0402 ceramic capacitors

are recomended. The large 4.7

µFcapacitorbetweenREFHA

and REFLA can be somewhat further away. The traces

connecting the pins and bypass capacitors must be kept

short and should be made as wide as possible.

The LTC1749 differential inputs should run parallel and

close to each other. The input traces should be as short as

possible to minimize capacitance and to minimize noise

pickup.

An analog ground plane separate from the digital process-

ing system ground should be used. All ADC ground pins

labeled GND should connect to this plane. All ADC VDD

bypass capacitors, reference bypass capacitors and input

filter capacitors should connect to this analog plane. The

LTC1749 has three output driver ground pins, labeled

OGND (Pins 27, 38 and 47). These grounds should con-

nect to the digital processing system ground. The output

driver supply, OVDD should be connected to the digital

processing system supply. OVDD bypasscapacitorsshould

bypass to the digital system ground. The digital process-

ing system ground should be connected to the analog

plane at ADC OGND (Pin 38).

HEAT TRANSFER

Most of the heat generated by the LTC1749 is transferred

from the die through the package leads onto the printed

circuit board. In particular, ground pins 12, 13, 36 and 37

are fused to the die attach pad. These pins have the lowest

thermal resistance between the die and the outside envi-

ronment. It is critical that all ground pins are connected to

a ground plane of sufficient area. The layout of the evalu-

ation circuit shown on the following pages has a low ther-

mal resistance path to the internal ground plane by using

multiple vias near the ground pins. A ground plane of this

size results in a thermal resistance from the die to ambient

of 35

°C/W.Smallerareagroundplanesorpoorlyconnected

ground pins will result in higher thermal resistance.

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