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LOG114: Dynamic range and resolution ?

PYTsai
Genius 9315 points
Part Number: LOG114
Other Parts Discussed in Thread: TINA-TI

Hi 

Can Log114 dynamic range support (+10dBm ~ -80dBm)? 

The resolution of Log114, can it achieve 0.01dB  or 0.05dB resolution ? 

  • 0
    Guru 140200 points

    Hi,

    pleae tell more about your application. What is your signal bandwidth?

    Kai

  • 0
    TI__Guru* 93105 points

    Hello PY,

    The LOG114 has a logarithmic gain determined by the ratio of an input current (I1) level and a reference current (I2) level. The gain equation is provided in the datasheet. The input current is specified be a minimum of 100pA, and a maximum of 10 mA. The LOG114 input is most often an input current, but in some cases it may be a voltage applied through a high value series resistance that is converted to a current.

    You mention a dynamic range expressed in dBm, where dBm is a dB scale referenced to one milliwatt of power. Therefore, we would need to convert input power to input current. What impedance is the dBm unit referenced to in your system? It is commonly referenced to 600 Ohms in audio systems, and 50 Ohms in radio frequency systems. The input impedance of a transimpedance amplifier is nearly zero Ohms at DC, but becomes complex with increased frequency. Thus, it doesn't represent a fixed impedance as I expect would be needed for a dBm scale.

    If you can provide us more information about your application and a proposed schematic for your application that would be helpful. And as Kai asked, we do need some idea of the signal bandwidth that would be applied to the LOG114. Do note that it isn't usable for high RF frequencies.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    TI__Genius 9315 points

    Hi Thomas,

    The application is Optical Switch System.

    The signal bandwidth is 1KHz and impedance is 600 ohm 

    Customer is asking, can LOG114 meet their requirement 

    1. Maximum dynamic detection range: (+10dBm ~ -80dBm)
    2. Minimum resolution: 0.1dB
    3. Bandwidth: 1KHz

    And any suggestion using LOG114 to connect to such signal ?

  • +1 in reply to PYTsai
    TI__Guru* 93105 points

    Hello PY,

    The LOG114 accepts a current input and that is why I had requested more information. It is not clear to me how a signal expressed in terms of power is going to be applied to the input.

    Putting that aside for the moment, here's my analysis:

    1. Maximum dynamic detection range: (+10dBm ~ -80dBm)

    Converting the -80 dBm to +10 dBm range to actual power levels -

    P(mW) = 1 mW [10 ^(PdBm/10)]

    For -80 dBm

    P(mW) = 1 mW [10 ^(-80/10)] = 10 e-9 mW = 10 pW   (10 e-12 W)

    For +10 dBm

    P(mW) = 1 mW [10 ^(10/10)] = 10 mW

    If these power levels are with a 600 Ohm load the current range is -

    P = I E = (I ^2) E

    I = (P / R) ^0.5

    I (-80 dBm) = [(10 e-12 W) / 600 Ohms)] ^0.5 = 129.1 e-9 A = 129.1 nA

    I (+10 dBm) = [(10 e-3 W) / 600 Ohms)] ^0.5 = 4.08 e-3 A = 4.08 mA

    There is an immediate issue because the LOG114 minimum specified input current is 100 pA. The -80 dBm minimum current is decades nearly 3 decades below the minimum. The 4 mA is within the maximum 10 mA input range, but log non-conformity between 1 and 10 mA is higher due to heating of the internal logging transistors.

    2. Minimum resolution 0.1 dB

    That 0.1 dB would need to be applicable over the entire -80 dBm to +10 dBm range. We can already see that the LOG114 cannot be applied to the ultra low current range. The LOG114 datasheet does not specify resolution, but instead log conformity error which is typically 0.08 dB from over the input current range of 100pA to 3.5mA (7.5 decades). 

    3. 1 kHz

    The LOG114 datasheet specifies FREQUENCY RESPONSE, Core Log (note 6). Where note 6 states, "Small signal bandwidth (3dB) and transient response are a function of the level of input current. Smaller input current amplitude results in lower bandwidth."

    BW, (3dB) I1 or I2 = 1 nA, 5 kHz typical. 

    Therefore, if the current is reduced below 1 nA the bandwidth will be reduced. The small signal ac graphs do not provide a curve for the minimum 100 pA input current level. And that is the minimum usable input current.

    It appears the LOG114 is not a good fit for this particular application.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    TI__Genius 9315 points

    Hi Thomas, 

    In your calculation 

       I (-80 dBm) = [(10 e-12 W) / 600 Ohms)] ^0.5 = 129.1 e-9 A = 129.1 nA 

    And our spec is 100pA, look like   I (-80 dBm) is within our spec is it correct ?

  • 0 in reply to PYTsai
    TI__Guru* 93105 points

    Hi PY,

    Yes, it looks like I got nA (10e-9) and pA (10e-12) turned around in my head. You are correct, 129.1 nA should be within range.

    Sorry for any confusion.

    Regards, Thomas

    Precision Applications Engineering

  • 0 in reply to Thomas Kuehl
    TI__Genius 9315 points

    Hi Thomas,

    Thanks, Is the conclusion still the same LOG114 can not meet the requirement, or LOG114 OK for this application ?

  • 0 in reply to PYTsai
    TI__Guru* 93105 points

    Hi PY,

    I am still not completely sure how the customer intends to apply the LOG114 in the application. Guessing, my thought is that the power source would be applied to a 600 Ohm load, and the resulting current would be the input current to the LOG114. The LOG114 input impedance at DC and very low frequencies is very low, nearly zero - just as it is for a transimpedance amplifier. Then, the input current would be converted to a voltage output appearing at the VLOGOUT whose level would follow the LOG114 transfer function. Do keep in mind that the LOG114 log conformity does degrade when the input current exceeds approximately 1 mA. There is a technique to improve that, but it is at the expense of the low current log conformity.

    I put together a TINA-TI simulation circuit that is seen below. This is just a concept circuit at this point, but it is something that the customer could review in relation to their application. It may, or may not, be what they have in mind.

    Once they look the idea over, then they can decide if it does what they need.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

    LOG114_Pwr_600_Ohm_01.TSC