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ACF2101: Why does the ACF2101output +0.5 voltage?（为什么ACF2101能够输出+0.5电压？）

Yue Li
Prodigy 10 points
Part Number: ACF2101


The datasheet mentions that the integrator output voltage range is from +0.5V to –10V. As charge accumulates, the voltage across the integration capacitor rises, and the output voltage of the ACF2101 gradually decreases from 0V to a minimum of -10V.  So, under what circumstances would it output a positive voltage?
(ACF2101数据手册中提到积分器输出电压范围为+0.5V至-10V。随着电荷的积累，积分电容两端的电压升高，ACF2101的输出电压从0V逐渐降低到最低-10V。那么，在什么情况下它会输出正电压呢？）

  • 0
    TI__Mastermind 41600 points

    I believe the output range specification is to explain the output swing range the device is capable of. So the device is able to swing from -10V to +0.5V min and max respectively but typically can swing to -13.5V to +1.0V.

    As outlined in the transfer function, the device will output a negative voltage from 0 to -10V, shown in the typical applications in the datasheet.

    There is not a specific circumstance that the device is going to output a positive voltage. In most application use cases, the device will be outputting a negative voltage.

    Please let me know if there are further questions.

  • 0 回复 Vivian Gao
    Prodigy 10 points

    I am using the ACF2101 chip to perform photodiode output current-to-voltage conversion. The circuit design is shown in the figure, with the S1336-8BQ photodiode's anode connected to the SWIN pin of the ACF2101.
    When there is no light, the output of the ACF2101 is +380mV. How should I interpret this phenomenon? Additionally, could you help verify if the placement of the photodiode in the circuit design is correct?

  • 0 回复 Yue Li
    TI__Mastermind 41600 points

    The photodiode configuration looks correct with the anode connected to the SWIN pin and the cathode connected to ground. 

    Could you clarify how low of a current you are trying to detect in the low-light conditions?

    Is the output of +380mV a measured or simulated value? If this is measured, have all the possible leakage currents been mitigated? Such as clean PCB board and guard patterns. This device has a very low input bias current but leakage currents can cause additional errors. 

  • 0 回复 Vivian Gao
    Prodigy 10 points

    Thank you! I will double-check the hardware circuit design. Additionally, I would like to know what the limit is for the ACF2101's integration period. Since I am using a pulsed light source, the integration time I have set is 2 µs.

  • 0 回复 Yue Li
    TI__Mastermind 41600 points

    Please see the Reset time vs C_integration to see what capacitor values correlate to integration period. The default internal C_integration is 100pF but can be added with an external capacitor as explained in the datasheet. 

    For our information, is this device being used in a customer application currently? We do not receive much feedback about this device so would like to understand if this is an active use case and how the customer is using it. 

  • 0 回复 Vivian Gao
    Prodigy 10 points

    We are using the ACF2101 to detect the current from a photodiode (Hamamatsu S1336-8BQ). In our application, the light source flickers at 50 Hz. During each illumination period, we control the ACF2101 to integrate the output current of the photodiode with an integration time of 2us.
    Our testing shows that under no-light conditions, the maximum output voltage from the ACF2101 can reach +380 mV. As light intensity increases, the output gradually shifts to negative values. We recognize a flaw in our PCB design: the lack of an input guard ringis likely introducing noise. Could TI provide a hardware evaluation board for the ACF2101 to assist with our debugging and validation?

  • 0 回复 Yue Li
    TI__Mastermind 41600 points

    This chip does not have an evaluation board.

  • 0 回复 Vivian Gao
    Prodigy 10 points

    Okay, thank you for your repeated replies!