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AFE4490

Other Parts Discussed in Thread: AFE4490, AFE4490SPO2EVM

Hello,

I'am using afe4490 for a reflective PPG wrist application and iam trying to find the best configuration for the TIA amplifier.

I choosed SFH 2430 photodiode which has a capacitance of 1000pF. Do you have any method to calculate Rf and Cf of TIA to achieve a stable frequence response?

Best Regards, 

Vítor Pinto

  • Hello Vitor,

    The photodiode's capacitance is in the max range of the external differential input capacitance across INM and INP pins.

    I am checking with the design team to find out if the photodiode can be used with the AFE.

    The CF capacitors (along with Rf) set the low pass corner of the TIA. Without these capacitors, the dominant pole that sets the corner frequency would be the combination of Rf and Cin (which is the equivalent capacitance of the PD). What Cf does (along with Rf) is to introduce a zero in the Bode plot to offset the effect of this pole and thereby extend the frequency response. The new corner now gets set by the combination of Rf and Cf. The value of Cf should be chosen based on the choice of Rf and the LED pulse width. As per the datasheet, the time constant from Rf x Cf should be about a tenth of the LED pulse width.

     

  • Hi Praveen,

    Thank you for the answer, i got it! So then let me know if that photodiode is suitable to use with AFE.

    Best Regards,

    Vítor Pinto

  • Hello Vitor,

    Yes the photodiode can be used with the AFE.

    The large PD cap causes the TIA noise to get amplified. This noise amplification is governed by (Cin/Cf).

    The other repercussion could be the slow settling time in case of a transient.

    The LED pulse width setting might need to account for it.

     

    Regards

    Praveen.

  • Hi Vitor Pinto,

    I am also planning to design a reflective pulse oximeter using the afe4490. Is it possible for you to give me the design of the probe with the same photodiode. I am currently using the afe4490spo2evm board to learn the operation. 

    I would be grateful if you could help me out because I am new to this field.

    I would also like to know if we can interface optical sensors like DCM03 to the evaluation module of afe4490 for testing . If so could you help me out on the same with respect to the connections. Should the DB9 connector be used to interface the sensor to the board? 

    Thanks.

    Santosh

  • Santosh,

    The AFE44x0 users guide found on our product fold has a table (Table 6) that shows the connections required if you want to use your own sensor. It only supports H-bridge configuration. To support push-pull (common anode) configuration you can blue wire using the extra unused pins to connect LED_DRV_SUP to external common anode supply.