TMP400 datasheet(20/25 Pages) TI

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LAYOUT CONSIDERATIONS

GND

(1)

D+

(1)

D-

(1)

GND

(1)

GroundorV+layer

onbottomand/or

top,ifpossible.

TMP400

0.1 FCapacitor

PCBVia

V+

GND

TMP400

SBOS404 – DECEMBER 2007

Remote

temperature

sensing

the

TMP400

measures

very

small

voltages

using

very

low

currents; therefore, noise at the IC inputs must be

minimized. Most applications using the TMP400 will

have high digital content, with several clocks and

logic level transitions creating a noisy environment.

Layout should adhere to the following guidelines:

1. Place the TMP400 as close to the remote

junction sensor as possible.

2. Route the D+ and D– traces next to each other

and shield them from adjacent signals through

the use of ground guard traces, as shown in

Figure 19. If a multilayer PCB is used, bury these

traces between ground or VDD planes to shield

them

from

extrinsic

noise

sources.

mil

(0.127mm) PCB traces are recommended.

(1) 5mil traces with 5mil spacing.

3. Minimize

additional

thermocouple

junctions

caused by copper-to-solder connections. If these

Figure 19. Example Signal Traces

junctions are used, make the same number and

approximate

locations

copper-to-solder

connections in both the D+ and D– connections

to cancel any thermocouple effects.

4. Use a 0.1

µF local bypass capacitor directly

between the V+ and GND of the TMP400, as

shown in Figure 20. Minimize filter capacitance

between D+ and D– to 1000pF or less for

optimum

measurement

performance.

This

capacitance

includes

any

cable

capacitance

between the remote temperature sensor and

TMP400.

5. If

the

connection

between

the

remote

temperature sensor and the TMP400 is less than

8 inches (203.2mm), use a twisted-wire pair

connection. Beyond 8 inches, use a twisted,

shielded pair with the shield grounded as close to

the TMP400 as possible. Leave the remote

sensor connection end of the shield wire open to

Figure 20. Suggested Bypass Capacitor

avoid ground loops and 60Hz pickup.

Placement

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