MP3V5004GP range

[rkris]

Hi Freescale,

We are evaluating the MP3V5004GP sensor in an application where water level can range between 0 and 300 mm. We got some samples today. We are finding that this sensor works ok up to around 155 mm of water. Beyond that the sensor is not tracking the water level properly. We have interfaced the sensor o/p to an MSP430 with 10 bit ADC through an op amp buffer. Vcc is around 3.3 V. ADC reference voltage is 2.5 V. We are using the bypass capacitors shown in the datasheet. This sensor's range is 0 to 3.92 kPa (400 mm of water).

Thanks in advance

WiSense.in

[Joshevelle]

Hello r krishnan,

You're correct, the MP3V5004GP has a pressure range from 0 to 400mm H2O, so you shouldn't have any problems using it in your application.

What is the output voltage that the pressure sensor provides at 150mm H2O?

Also, I'm not familiar with the MPS430 but if the +Vref of the ADC is 2.5v, then the maximum voltage that you could read with it would be 2.5v.

I would recommend you to increase the Vref to at least Vcc (3.3v).

Hope it helps.

Josh

[rkris]

Hi Joshevelle,

Thanks for the quick response.

When water level is 160 mm, the sensor o/p is 1.681 volts (ok).

At around 185 mm, the sensor o/p is 1.744 volts. Should be more.

At around 200 mm, the sensor o/p is 1.791 volts. Should be more (around 1.942 V).

Vcc is fixed at 3.281 V.

I am using this transfer function:  Vout = Vs * (0.2 * P  + 0.2)

Reference voltage of 2.5 V should be enough to measure 300 mm of water.

[rkris]

HI Josh,

   I just tried another sample but see the same issue.

Ram

[rkris]

Hi Josh,

  Any update on this ? I increased the supply voltage to 5 volts. The sensor o/p is now going up to 3 V but the error is increasing with the water level. At 300 mm, the difference is around 50 mm.

[Joshevelle · Accepted solution]

Hello r krishnan,

The MP3V5004GP is not designed for 5v supply voltage, its maximum operating voltage is 3.3v.

The transfer function that you're using is not taking consideration of the offset, I would recommend you to take a look at the following document, it contains all the required information to properly "Auto-Zero" our pressure sensors:

http://cache.freescale.com/files/sensors/doc/app_note/AN1636.pdf

Also, I would strongly suggest you to increase +Vref, otherwise you won't be able to read voltages greater than 2.5v.

hope it helps.

Josh

[rkris]

Hi Josh,

     I measured the sensor o/p at two levels (100 mm and 200 mm) and derived a different transfer function (compared to the one in the datasheet). When I use this new transfer function, the sensor o/p is properly tracking the actual water level. I guess we will have to calibrate every sensor during final assembly.

  One more thing, I measured the current consumption and it is around 15 mA at around 2.7 V Vcc. The datasheet mentions max 10 mA.

Thx

Ram