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Humidity Sensor PICtail™ Demo Board User’s Guide
DS51594A-page 20
©
2005 Microchip Technology Inc.
The test points, TP3 – TP5, have 10 k
Ω
placed in series to isolate the circuit from
external measurement equipment. This will help prevent over-voltage events, and will
help the op amp, U2, operate in a stable condition.
a) This is a two-layer board. It has a solid ground plane on the bottom layer to
minimize EMC issues, and routes the traces in the top layer. The complete
schematic is in Figure A-1 and Figure A-2, the board layer plots in Figure A-3,
Figure A-4 and Figure A-5. The Bill of Materials (BOM) is in
Appendix
B. “Bill Of Materials (BOM)” . The Gerber files for this board are available on
the Microchip web site (www.microchip.com) and are contained in the zip file
00084R1.zip
.
A.4 THE “REDUCED CURRENT” MODIFICATION
Figure A-1 shows the Humidity Sensor PICtail™ Demo Board as it comes from
Microchip. This board can be modified by the user to implement the “Reduced Current”
circuit discussed in application note AN1016 and
Chapter 4. “Modified Circuit” .
A.5 ADDITIONAL COMMENTS
The demonstration board includes test points for convenience on the bench. The “+5V”
and “GND” test points (TP1 and TP2) connect to the board’s supply voltages.
The “V
INT
” test point (TP3) connects to U1’s pin 14 to allow the user to monitor the
square wave into the integrator. The “V
CM
” test point (TP4) allows the user to measure
the op amp’s common mode input voltage. The “V
SEN
” test point (TP5) allows connec-
tion to the op amps output to allow the user to monitor the triangle wave at the integra-
tor’s output.
The PICkit™ 1 Flash Starter Kit provides a +5V single supply voltage. It can drive up to
5
μ F on the supply; a larger capacitive load causes current loading and timing issues.
This demonstration board also uses the +5V single supply voltage. It has two 1 μ F bulk
bypass capacitors and two 0.1 μ F local bypass capacitors.
High frequency design practices are used to minimize digital interference:
• Solid ground plane
• Surface mount devices for the analog circuitry
• Separate digital and analog lines and sections
Note: Do NOT connect a power supply to the “+5V” and “GND” test points, unless
the board is NOT connected to the PICkit™ 1 Flash Starter Kit (i.e., it
stands alone).