Freescale Pressure Sensor + INA128P Problems

Hello,

I am new to this forums and I would greatly appreciate any help I can get with my project.

I am using a MP3V5004G ( http://cache.freescale.com/files/sensors/doc/data_sheet/MP3V5004G.pdf?fpsp=1&WT_TYPE=Data%20Sheets&WT_VENDOR=FREESCALE&WT_FILE_FORMAT=pdf&WT_ASSET=Documentation ) pressure sensor by Freescale. It has three pins Vin, Vout, and GND. I apply 3.3V to the Vin, GND to GND, and Vout to A0 on a UNO.

Without further amplification circuits I get an expected 0.50V voltage reading and a significant sensitivity to pressure changes. However when I use the INA128P ( http://www.ti.com/lit/ds/sbos051b/sbos051b.pdf ) instrumentation amp to up the voltage by twice, I get a undesirable result. The voltage reading stays at 0.50V and loses all form of sensitivity.

Here is a picture of my circuit:

In text form: Vin (pressure sensor) to 3.3V GND (pressure sensor to GND UNO V+ to 3.3V UNO V- to GND UNO Vout (pressure sensor) to Vin+ Vin- to UNO GND, Ref to UNO GND, Vout (AMP) to A0 UNO

I would like for the amplifier to enhance the voltage reading while retaining the sensitivity. Apologies if my posting is unclear, I am striving to become better at communicating technical details.

Thank you very much for your time.

You're not using the differential amplifier in accordance with its datasheet:

  • The differential amplifier that you're using has a minimum power supply requirement of -2.25V and +2.25V. You're operating it single-ended at 3.3V - 1.2V less than the 4.5V it requires.
  • The common-mode input range extends down to 1.7V above the negative supply. Your v- input is 0V, and V+ doesn't reach 1.7V until the pressure is about 1.5 kPa. This op amp, or another 5V rail-to-rail unit, ought to work: https://www.adafruit.com/products/808

Thank you for the reply.

I had a strong feeling that my Amplifier Circuit was funky. I am going to review the specifications for amplifiers. I don't yet understand Common-Mode Input Range, would you happen to have a nice resource to learn about it.

And I will surely try out your suggested amp for my application.

Thank you again :)

Why use an extra amplifier ? The pressure sensor has an amplifier inside, to get a signal which is good for a microcontroller. If you double the signal, and double the noise at the same time, the result is not better. There are pressure sensors for 5V.

The pressure sensor is for 3.0V, and you use 3.3V. The maximum voltage for the sensor is 3.3V, but the best accuracy is at 3.0V.

Thanks for the feedback. Although the signal is awesome without any addition amplification, the application I am using it for requires the detection very minute changes. I agree, I am a little skeptical that the additional amplification can improve the signal.

I did some measurements before hand and noticed that there was a 1 or 2 count deviation from the norm. I assumed it is noise produced by the A/D converter because I held the pressure "relatively fixed". I proposed that amplification prior to A/D conversion can help me distinguish conversion noise versus pressure change noise.

Please let me know if any of this sounds crazy. I am eager to learn!

Sounds good.

For higher accuracy you need perhaps an digital pressure sensor with I2C and with a 14,15 or 16 bit ADC in the sensor. But the sensors I can find are for baromic pressure and have no connection for an air tube.