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AFE44xx with phototransistor

Guru 20090 points
Other Parts Discussed in Thread: AFE4490, AFE4400
Hello,
 
I'm considering using AFE44xx with phototransistor(NJL5303R).
 
Output current of Phototransistor(NJL5303R) is 1mA to 4.3mA.
And Full-scale input current of AFE4490 is 0.5uA to 50uA.
I think that we can't use this phototransistor with AFE44xx directly.
But , if we can use this phototransistor by AFE44xx with any external circuit, could you please kindly let me know the best method ?
 
Best Regards,
Ryuji Asaka 
  • Hello Ryuji,

    We have not evaluated the phototransistor with Green LED (NJL5303R) with AFE44xx.

    But we have done some preliminary evaluations on NJL5501R (phototransistor with red and IR LEDs) with AFE44x0.

    You can use the configuration below to connect the phototransistor with AFE44x0.

    You may need to select the R and C  values such that the phototransistor is operating in the active region.

     

     

    Regards

    Praveen.

  • Hello Praveen,

    I am with a group working on a similar project which is using the NJL5501R in conjunction with an AFE44xx board.

    Do you have any further information on this? Also what specific resistor/capacitor values should be used?

    Thanks,
    Daniel 

  • Hello Daniel,

    We have not done exhaustive testing with NJL phototransistor sensor.

    The resistor values set the DC value that will be seen at the output of the ADC for a given LED current.

    10k and 1uF was selected by experimentation based on LED currents < 20mA (red and infrared), such that the signal chain is not saturated.

    Regards

    Praveen.

  • Hey Praveen,

    Your sharing is very interesting. I am also currently trying to build a hardware of pulse oximeter using AFE4490 and NJL 5001R and I am very new here, so pardon me if my questions will be very basic...

    1. If I only bought chip of AFE4490 and NJL 5001R, If it still possible to use the GUI of AFE44x0SPOGUI? 

    2. beside of the configuration that you attached for Daniel previosuly, is there anything that i need to connect in the AFE4490?

    3. If I have AFE4490 evaluation mode, can i also test it my NJL5001R chip?


    Thank you in advance!

    Stella

  • Hello Stella,

    It may not be straight forward to use the GUI of AFE44x0SPO2EVM with your hardware directly.

    There are 2 things that need to be taken care of.

    1.  The processor on your board must implement the message communication protocol (refer to E2E post below) to communicate with the GUI.

    http://e2e.ti.com/support/applications/high_reliability/f/30/p/273490/955180.aspx#955180

    2. Your board must support USB connectivity and the USB must be enumerated as virtual COM port with

    "AFE44x0SPO2-FE EVM" as description.

     

    No, you do not need anything else other than the configuration to interface to AFE4490.

     

    Yes, the EVM can be used to test the sensor but you may need to build a special cable to connect the sensor to the DB9 connector of the EVM.

    Best Regards.

  • Hello Praveen,

    I tried to build the NJL5501r yesterday,
    Here, I attached how I designed the circuit into PCB board.
    But when I connect it with the power supply with 3V, there is short circuit. 
    So before i investigate the PCB board itself, 
    I need you to verify whether my circuit design has complete, or there is any mistake that I failed to interpret from your above picture.
    I am very appreciate your help. Thank you!

    The J-1 will be connected into DB9 connector and J-2 to the power supply
      

  • other solution: 100kohm between 6 and INM, 5 and INP to ground (AFE in voltage amplifier mode)

  • Hello Stella,

    Your sensor board schematics are incorrect. The sensor collector and emitter terminals must be biased before AC coupling to the input channels of the AFE.

    That is the collector terminal must be pulled up to 3.3V through R1 before C1 and the emitter terminal must be pulled down to GND through R2 before C2.

     

    Regards.

  • oh,Yes! I just noticed it now too!
    Thanks a lot! 
    I should be more careful next time.
    Thank you again!


    Stella


  • Hey, all

    one problem solved, but I got another problem...
    I tried my NJL5501r into AFE4490SPOEVM, 
    it gives good result, but sometimes it is unstable, 
    so when I placed my finger too close, the oxygen saturation is not read but the heart rate is giving correct result, 
    however if I give some distance between the sensor and my finger, the oxygen saturation is read, but the heart rate gives the wrong result.

    So I wonder, if anyone know how to improve its sensitivity?
    Thank you in advance!


    Stella

  • Praveen Aroul said:

    Hello Ryuji,

    We have not evaluated the phototransistor with Green LED (NJL5303R) with AFE44xx.

    But we have done some preliminary evaluations on NJL5501R (phototransistor with red and IR LEDs) with AFE44x0.

    You can use the configuration below to connect the phototransistor with AFE44x0.

    You may need to select the R and C  values such that the phototransistor is operating in the active region.

     

     

    Regards

    Praveen.

    Hey Praveen, 

    Can you explain me what is actually the purpose of the Resistor and the Capacitor that are connected to the collector and emitter?


    Stella

  • Hello Stella,

    Photo-transistor would need a biasing network before the INP, INN pins. Texas Instruments does not recommend a photo-transistor based solution.

    But if it is a must to use a photo transistor, the above configuration needs to be used to bias the phototransistor.

     

    Regards

    Praveen

  • Hello Praveen,

     

    May I use other MCU to simulate the processor and use USB to serial UART convertor (FT232) communicate with the GUI?  and if the description is not "AFE44x0SPO2-FE EVM" , is it possible?

     

  • Hello Siu,

    The GUI works only for the USB port description "AFE44x0SPO2-FE EVM".

    If you can change the USB serial port description to "AFE44x0SPO2-FE EVM", and follow the message communication protocol, then I do not find any reason why it will not work.

    Refer to the link below for the updated message communication protocol.

    http://e2e.ti.com/support/applications/high_reliability/f/30/p/273490/955180.aspx#955180

     

    Regards

    Praveen.

  • hellow,Praveen Aroul
    I'm sorry to bother you . I used dcm03 sensor to generate Signal , then the Signal transferr to the AFE4400. But I find the ppg of the signal is 1MV to 3 mv by AFE44xx0spo2evm gui. I think the signal is too small so that the MCU can‘t deal with it. What should I do to the problem? can I Increase the current before it is transferred to the AFE4400?
    Thank you very much!
    XLL
  • Hello,

    When you refer to the PPG of the signal, are you referring to the AC component of the pleth signal or the DC level of the signal?

    To increase the signal level, you could increase the TIA gain, second stage gain and/or LED current.

  • Thank you very much!