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Anyone use MPX4115/6115 w/ ADS1100 AD?

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bobledoux

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I'm working on an altimeter that reads the MPX4115 or MPX6115 pressure into the ADS1100 AD converter.

Has anyone here tried to do this?

My questions:

1. Did you avoid using an opamp to resolve the different voltage ranges between the two units? The ADS1100 is low impedance input so a classic resistor voltage divider seems unworkable.

2. Motorola Applications Note AN1646 suggests a low pass RC filter consisting of a 750 ohm resistor bypassed with a 0.33uf cap for the MPX4115 output. The MPX6115 manual suggests bypassing the output with a 47pf cap and a 51 kohm resistor, both to ground. Why the two different output schemes?
 

Shax

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I'm working on an altimeter that reads the MPX4115 or MPX6115 pressure into the ADS1100 AD converter.

Has anyone here tried to do this?
I didn't use the ADS1100, but did use the inbuilt A2D of a Pic18F2431 with 64x oversampling, a MPX4115 baro sensor and is good to about 35,000 feet for my rocketry altimeter.
I've attached a piccy..
 

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bobledoux

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I recently tested my altimeter. I took two aircraft altimeters, one recently calibrated, along with my project unit over the Cascade Mountains from about sea level. The road has several altitude markers and two summits with altitudes close to 5,000 feet. My project altimeter was within 10 feet of the calibrated altimeter over the trip. The uncalibrated unit was within 25 feet.

My project altimeter used an MPX4115 sensor feeding the ADS1100 A to D on the inverting input. The non inverting input was fed through a 10 turn pot. It was used to adjust the base altitude to surface level.

No op amp was employed. The PIC 16F676 read the ADS 1100 and performed a table lookup using a piece wise linear interpolation as described in Microchip Application Note AN942.

A 2x20 LCD is used for readout. My next step is to save a stack of recent history to calculate a rate of climb. I've already included the traces on the board to include a temperature gauge using a KTY-81 sensor, which I have tested in a separate unit.

When complete the unit will be used as a hot air balloon instrument package.

Balloon Builders Journal Compendium CD
 
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