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LOG200: Technical questions

Shinya Sawamoto
Expert 5285 points
Part Number: LOG200

Hello team,

I have a couple of questions as follows.

1) How to set the RBIAS: It is described that IBIAS is used as a measure to reduce the effect of a leak current. By passing 0.1 times the current of IPD to RBIAS, PD applied voltage is suppressed and the effect of the leak current would be reduced. I would like to measure the PD current by fluctuating from 822uA to 46.2nA. Please tell me how to set the resistance value of RBIAS.

2) For use with single supply (+5V-0V): In case of detecting the I1 input current between 822uA and 46.2nA, guessed it would be necessary to increase the internal offset by 0.5V or more Since OUTA below 1uA becomes negative from the characteristic graph. An offset of 1.65V is achieved by connecting the REFA and VCM to the REF165. Is this correct?

3) About temperature characteristics: A transistor is used for OP-AMP feedback from the internal circuit of the device. That transistor(hfe) would have a temperature dependency. If you have any data with OUTA variations when entered the same IPD, that is helpful to be shared it. (There is a concern that the operating temperature will affect to fluctuate exponentially.)

Thanks in advance.

  • +1
    TI__Guru 55226 points

    HI Shinya,

    1)  The RBIAS voltage sets the reverse bias voltage for your photodiode. The optimal RBIAS voltage is dependent on your sensor and application requirement.

    As you have mention, a percentage of the I1 current flows through the RBIAS resistor. The IBIAS pin produces a current output ratio of  ~1.143 A/A @ I1=10uA and 1.175 A/A  @ I1= 10mA typical. When an RBIAS resistance is placed in parallel with the photodiode, 1.0 times the input current is drawn through the photodiode, and the remaining ~0.175 times the input current flows through RBIAS

    For example, if you chose RBIAS = 7.5kΩ, and the PD current is fluctuating from 900uA to 40nA, and assume that approximately ~1.175*I1 current flows through IBIAS, the current through RBIAS is 0.175*I1, and the bias voltage across the RBIAS resistor will be around ~1.181V for I1= 900uA, and  ~52.5µV for I1=40nA.   

    When the LOG200 is powered with unipolar +5V supply, with VCM set to REF25 = +2.5V; and if the max current expected from the sensor is around 900µA, using RBIAS = 7.5kΩ will be in range. 

    Please note, the IBIAS pin voltage must always remain 1V below the VS+ supply, to meet the IBIAS current source compliance voltage. Hence, limit the voltage across RBIAS resistor to less than 1.5V, so the IBIAS pin voltage remains below 4V when VS+ = 5V. 

    See below:

    2) The LOG200 logarithmic amplifier input has an input common-mode voltage requirement in the range of (VS-) + 2.3V and (VS+) - 2.0V.  

    Therefore, when using the LOG200 with unipolar 5V supply, (VS-) = GND and (VS+) = +5V, the LOG200 logarithmic amplifier input must remain in the range of +2.3V to +3V to meet the input VCM spec.

    The LOG200 offers the REF25 2.5V reference, hence the VCM pin needs to connect to the REF25 +2.5V reference output through an RC low-pass filter, with R=100Ω and C=1µF. The LOG200EVM supports these connections through jumper J8.  See image below.

     

    3) The LOG200 data sheet provides performance specifications for the logarithmic conformity, and transfer function (gain error) as a function of the input current over the 0°C to +85°C temperature range, as well as -40°C to +125°C temperature range on the Electrical Characteristics Table on page 5.  Also, Figure 3 provides the logarithmic amplifier OUTA voltage at temperatures -40°C, +25°C, +85°C, as a function of input current. Hence the errors contributed by the LOG200 device as it is operated over temperature, as a function of I1 current are relatively small.

    Thank you and Regards,

    Luis