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RF430FRL152H: Sensor data problem.

Minhyung Kang
Intellectual 490 points

Part Number: RF430FRL152H

hello, my name is minhyung.

I am using RF430FRL152H and MPS-EXP430G2ET + DLP7970ABP to create a custom board.

The RF430FRL152h custom board which operates with NFC is to receive the LED light by the light sensor and send the data to the master kit MPS-EXP430G2ET + DLP7970ABP and send it to the PC to display it. We were going to display the data in Matlab.

I succeeded in plotting graphs in real-time by receiving raw data with Matlab. However, I have an additional problem.

When the RED LED and the Infrared LED are blinking at the intersection, data is strange Each time RED and Infrared flashed, I thought I would get different raw data. However, unexpectedly, I got some strange raw data like the graph below. This is similar to the charge and discharge graph of the capacitor. Why does the graph look like this?

However, the novelty is that if we do not apply any light except the fluorescent lamp while the LEDs are turned on crossroads, We can get constant raw data as shown in the graph below.

I want to solve the problem of the graph appearing strange. I'm currently trying to create a Pulse Oximetry. However, if this strange graph is derived, the oxygen saturation value cannot be obtained.

Finally, I have a question. For the raw data I get, the value is saturated at 0.5V, so no higher value can be obtained. Why is the value saturated at 0.5V? Is there a way to solve this?

To help solve the problem, I've attached a schematic of my custom board.

Thank you.

  • 0
    TI__Guru*** 135355 points

    Hello Minhyung,

    I'll preface this all by saying I'm not an expert in this field, but I'll propose a theory as to what is going on.

    The whole premise being that it could be that the Red LED is introducing noise/error to the measurement of the IR LED. I think the difference in your measurements are because the Red LED does not interfere with the Fluorescent light notably but it does with Infrared light.

    Let's have a look at the Electromagnetic Spectrum:

    Based on this, you can see that Red light is around 4x10^14 Hz, while yellow light from a Fluorescent will be closer to 5-5.5x10^14 Hz. So if these signals are overlapped, and the Fluorescent light is the stronger light source, it would be at a higher frequency and show up without much noise influence from the Red Light which is at a lower frequency.

    However, Infrared is at a lower frequency.10^13 or 10^12 Hz. So as a result of this, if the two lights are similar in strength, then it could be that the higher frequency of the red light is adding or removing signal from the infrared source (constructive or destructive interference).

    That's my best guess anyways.

    I am not really familiar with the physics behind Pulse Ox, but is it intended to have both lights flash on at the same time in a typical use case? May be a question to investigate.

    As for why the value is saturated, that would probably be a sensor issue though it actually looks to be saturated to less than 0.45 volts which would be in your benefit if so because you could use the internal PGA to boost the resolution that way (it would put a gain of 2 on the measurement, letting you use the full range available for analog signals).