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Original question:

TPS61390: Is an APD Boost Converter with 100µV Ripple Too Good To Be True?

TPS61390: Purpose of Current Mirror and Measuring Current in the Microamp Range

Charles Gervasi
Expert 2270 points
Part Number: TPS61390
Other Parts Discussed in Thread: ACF2101

The TPS61390 has built in current mirrors.  You can select between two current mirrors, sample the voltage at a resistor connected to the mirror at a moment in time, and read back this voltage multiplied by 4 at some time after sampling.  

The current mirror has about 20uA of bias current.  

For this to be useful in an APD or MPPC, the bias current would have to be in the µA range.  Is this a normal way to use MPPCs/APDs?  For low light levels, I believe you can get sensitivity if bias current is in the nA range. or lower, allowing us to count pulses and compare the dark pulse count with the pulse count while sampling a light source.  For larger signals, we integrate using a TI ACF2101 switched integrator.  In the testing I'm doing now, I'm using a 10,000pF integration cap, integrating for 2s, to measure current in the nA range.  

In what scenarios is bias current in the µA range?

A few years ago  addressed the issue of measuring the voltage at the MON pins, not using the internal 4x voltage amp.  

 touched on this issue in a response to my question, but we never got into why we would want to measure currents in the µA range.  

  • 0
    TI__Expert 4760 points

    Hi Charles,

    Ibias is 20uA and it is always uA range(min 15uA, max 25uA). It is bias current for the current mirror itself and not related to APD.

  • 0 in reply to Yichen Xu
    Expert 2270 points

    But my reading of the excerpt of the datasheet I pasted above is that the 20µA of bias current goes through the resistors connected to the MON pins, that voltage is amplified, and it shows up as part of the total current. We must subtract bias current to find the APD current.  If the bias current is 15-25µA, it will be impossible to measure nAs of APD current.

  • 0 in reply to Charles Gervasi
    TI__Expert 4760 points

    Hi Charles,

    Yes, it will be much less accurate if you want to measure nA APD current. This function is mainly used for high current. You can measure the voltage on MON pin when APD current is 0, and use this voltage as offset to calculate the current flowing through APD. Why do you need accurate nA level current sensing? 

  • 0 in reply to Yichen Xu
    Expert 2270 points
    Yichen Xu said:
    Why do you need accurate nA level current sensing? 

    We're using it to detect small amounts of light. I'm trying different bias voltages.  More bias voltage means more dark current but also more gain due to the avalanche effect.  

    Do you know of a useful detection mode where the reverse bias current (dark current) is in the microamps?  My thought is if I had such high currents, I would be better off using a conventional photodiode (non-avalanche) and a sensitive detector.  

    Have you worked on APD or MPPC projects with bias current in microamp range?  I am new to APDs/MPPCs and would be interested to hear about the advantage for high bias current in certain applications.

  • +1 in reply to Charles Gervasi
    TI__Expert 4760 points

    Hi Charles,

    For nano level dark current TPS61390 can not sample it accurately but for current in microamp range the device can work well. The bias current in datasheet is actually offset current for current mirror, which is not the same concept as you mentioned because it will not flow in APD. I'm sorry we don't have solution for so small current now, maybe you can try amplifier.

  • 0 in reply to Yichen Xu
    Expert 2270 points
    Yichen Xu said:
    The bias current in datasheet is actually offset current for current mirror, which is not the same concept as you mentioned because it will not flow in APD.

    I should not have used the same word with two meanings.  I meant the APD dark current.  I was saying the dark current flowing in the APD would have to be >= the bias current in the mirror circuit to reliably measure it.  In all our experiments the APD dark current is << 20µA.  

    Yichen Xu said:
    maybe you can try amplifier

    We're having good results with the ACF2101BU integrator.  I'm just wondering if anyone knows for what types of applications you'd want to measure APD current in the µA range.