AN607 datasheet(9/41 Pages) SILABS

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AN607

Rev. 1.7

3.4. PCB Layout

The Si70xx should be thermally isolated from the equipment connected to it to prevent heat from the equipment

from affecting RH. The Si70xx should be thermally immersed in the ambient environment it is intended to sense.

One strategy for accomplishing this is to put the Si70xx on a small PCB and run a ribbon cable to the host

processor. The small PCB should be placed away from heat sources and should be placed in the ambient

environment as much as possible. That said, even with the hydrophobic filter, keeping dust, liquids and cleaning

agents away from the sensor is required.

3.5. Design and Bring-Up Checklist

Be sure the sensor is placed away from heat sources and exposed to the environment being measured.

Prevent the active area of the sensor from being exposed to liquids, dust, and other contaminants as well

as sunlight or other UV sources. The optional filter cover available with Si70xx parts serves this purpose

and is compatible with soldering.

Generally, avoid the use of ground planes around the Si70xx, which could conduct heat from external

sources. Route the ground connection.

Do not connect unused pins. Make sure the CSb pin of the Si7015 is low prior to starting I2C

communications.

The Si7005 should not be on the same bus as other I2C devices when it is active. It acknowledges data

bytes that match its address. This issue has been resolved with other members of the Si70xx family

Be sure to meet all of the timing and level requirements of the device. The Si7005 can tolerate SDA or SCL

higher than VDD and has 8.5 mA drivers. The Si7006, Si7013, Si7020, and Si7021 have 2.5 mA drivers

and do not tolerate I2C pins higher than VDD. The Si7034 is a 1.8 V part but can tolerate 3.3 V on its inputs.

It's drive strength is 1.5 mA. See also application note, “AN883: Low-cost I2C Level Translator” for a low

cost I2C level translator circuit.

Route the I2C signals away from analog nodes and noisy digital nodes.

Use 0.1 µF bypass capacitors on VDD placed close to the sensor.

Pay careful attention to I2C protocols, such as start and stop conditions, the repeated start of a read

transaction, and proper treatment of the Acknowledge bit.

Allow adequate time for initialization (per data sheet).

If the optional thermistor sensing of Si7013 is used, make sure the thermistor is thermally isolated. If there

are long leads to the thermistor, use a twisted pair. Avoid noise pick up; use either a shield or capacitive

filter.

3.6. Si70xx Self Test

The following steps define a reliable test of the Si7006, Si7013, Si7020, Si7021, and Si7034 family that uses the

integrated heater:

1. Read and write all I2C registers checking for expected values and capability of modifying where

appropriate

2. Perform an RH and temperature measurement.

3. Turn on the heater and wait 60 seconds.

4. Check for delta temperature with heater on. This can be adjusted changing the heater setting. A setting of

0x3 will give over 3 °C.

5. Check for delta RH is > RHinitial 4x (delta temperature in °C).

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