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LMP91002: Some design question

Kami Huang60
Genius 3830 points
Part Number: LMP91002
Other Parts Discussed in Thread: CC2640

Dear Sir/Ms.

My customer designed his product with reference to LMP91002EVM. May I  to ask some questions about the design.

The input signal specification is approximately

Signal: 500mV

Frequency: Frequency below 30 Hz

1. How accurate is the voltage of the VREF Pin of LMP91002 provided? What is the recommended voltage? What is the impact?

2. What is the practical application of Pin C1~C2 of LMP91002? How to adjust C1 and C2? Is there any reference material available?

3. Do customer need to add an RC filter to the Vout Pin of LMP91002? Any suggestions? The ADC uses our CC2640.

Best Regards,

Kami Huang

  • 0
    TI__Genius 9310 points

    Kami,

      The LMP91002 uses a zero reference for taking measurements. The reference voltage is 1/2 of VDD. This sets a zero at one half of VDD. If the customer uses 5V for VDD then the reference is 2.5V. If the customer uses 3.3V for VDD then the reference is 1.65V. The reference is derived from VDD and operates in a ratio metric mode to reduce the noise and eliminate variations in the VDD.

    The zero is used for measuring positive and negative with reference to the zero. (an example is PH). The PH probe will measure PH7 which is neutral or zero. When the ADC measures 1.65V with VDD at 3.3V, the PH level is 7. When the PH level goes up it is considered acidic and the Vout will move up away from the 1.65V reference. When the Vout goes below 1.65V then the PH level is going down. 

    The main amplifier is a transimpedance amplifier. The internal Rtia resistor sets the gain of the amplifier. It may be necessary to adjust the gain at a different level than one of the internal resistors. You have the ability to disconnect the internal resistor and use an external resistor to set the gain. The customer will connect the external resistor to C1 and C2 to set the new gain. Remember to disconnect the internal resistor in software when using an external gain resistor. Most of the time the internal gain setting will work just fine.

    As for as an RC filter, some customers are not sampling fast so an RC is good. Make sure the low pass frequency does not interfere with the speed of the sampling or it will interfere with the measurement and provide a slow response.