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I've got question about Wheatstone bridge. I've designed device which contain NCP1210 pressure sensor and works with 3.0V power supply. I've did calculations and now I've got some questions.

My device based on this eval board:

http://www.cdiweb.com/datasheets/ge_novasensor/CDI_Universal_Evaluation_Board.pdf

I read NCP1210 (I've got sensor with 0.025mbar resolution) values for 1.5mA current source. Typical output voltage value is 50mV (for 0.025mbar). Typical NCP1210 input impedance is ~4k.

So... with Ohm law:

U=RI

U=4000 Ohm * 0.0015 A=6V

I need ~6V to properly drive sensor wit 1.5mA. Furthermore voltage on resistor in current sink system has value of non-inverting input. So with this eval board I need at least 6V + 1.2V =7.2V to properly work with 1.5mA current source.

I've decided to use eval board ideas in my project. I use:

-> 1.6V Reference voltage

-> 3.0V power supply

With these values I calculate that I must use 8k resistor in current sink. With this resistor I should achieve current with value: 0.2mA.

And now my question: With this current (0.2mA) proportionally I can achieve ~6.6mV on NCP1210 output.I want to connect NCP1210 output to MCP6N11-100 instrumentation amplifier and I'm afraid that these values will be too small to reach good measurement.

Additionally I thought about build voltage doubler, but It will small battery-powered RF device and voltage doubler may take up too much space.

thank you in advance for your advices!

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  • \$\begingroup\$ This is the exact type of situation where instrumentation amplifiers shine. Why are you "afraid that these values will be too small to reach good measurement"? You can probably get away with a gain of just a few hundred depending on how much resolution you actually need. \$\endgroup\$
    – Matt Young
    May 13, 2014 at 13:11

1 Answer 1

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You really don't have much choice here. There are two options:

  1. amplify
  2. Sample using some technique such that the 6.6mV signal can be seen with enough precision.

Using a 3V reference, With 10 effective bits, you'll get about 3mV resolution, so your entire output range will be covered in 2 least significant bits. That doesn't sound like enough! With a 24-bit A/D that give you about 17 effective bits, you'll get about 23uV, and your range will be covered in about 260 least significant bits. That might be enough. Does being able to divide your output range into 260 chunks solve your problem?

Now you have to consider your noise levels to make sure your SNR is OK. You can use a smaller reference voltage, of course, but I haven't had much experience with reference voltages at tens of millivolts, which is where you'd likely need to be.

What is the sampling/measurement arrangement that drives your concern that 6.6mV isn't good enough?? I suggest you start at the other end. What resolution, in terms of a pressure change, do you need to reliably identify? Once you know that, you can design a measurement system that can meet that challenge.

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