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OPA855: TIA Design with offset between V_IL TYP and MAX

Ryan Syndergaard
Prodigy 10 points
Part Number: OPA855

Hello TI Team!

We have a two-stage transimpedance amplifier circuit for photodiode current conversion. The OPA855 is operating with VS+ at 3.3V and VS- at GND. Following the recommended TIA design, we have an offset voltage on the non-inverting input of 1.2V, which is between the TYP and MAX values of V_IL.

My questions are:

  • Why is there a TYP and MAX value for common-mode input voltage low? Is it always recommended to set the offset voltage between V_IL MAX and V_IH MIN?
  • What are the expected output effects if V_IL is marginally low (e.g., output instability)?
  • With V_IL TYP of 1.1V and V_IL MAX of 1.3V as given in the datasheet, what distribution of parts should we expect to maintain stability with the current-design offset voltage of 1.2V? 
  • +1
    TI__Genius 11020 points

    Hi Ryan,

    I see where your confusion is coming from so let me clarify this particular spec before we discuss the questions you are asking. Common-mode input range or input voltage swing is defined as the range of input common mode voltages that can be applied to the inputs where the device will operate in its linear operating region. We often define the limits as an input range high and low value in our datasheets like the OPA855 datasheet. For a 3.3V supply, the upper range limit (V_IH) was specified at 2.9V TYP and the lower range limit (V_IL) was specified at 1.1 TYP. This means that the device's input common-mode voltage range can be between 2.9V and 1.1V TYP and it will operate linearly. 

    We often give a MAX and MIN with these specs to guarantee the device will have a given input voltage swing range in a worst-case scenario. Meaning the MAX for V_IL is saying the worst the device's lower input voltage value will be is 1.3V. This is a worst-case scenario because now instead of working linearly all the way down to 1.1V TYP it will now work linearly down to 1.3V, limiting the input range you can work with, which is not ideal. The same logic can be applied to the MIN value we have for the V_IH spec, as now 2.7V will be the highest common-mode voltage value the device will operate linearly in when it was all the way up to 2.9V TYP. This does not mean the device's common-mode voltage must be within the TYP and MAX for V_IL, it is saying the common-mode voltage must be above this lower limit. Putting it in this region of 1.2V will likely result in some devices not operating linearly as there are some devices that will not work lower than 1.2V due to process variation. Therefore, we recommend the device operating between the input voltage range as operating outside this range will result in a non-linear amplifier and erroneous results.

    Section 10.4 shows how to bias the device for different supply configurations, and as with most amplifiers it will be Vs+/2 when biasing a device with a single supply configuration. In your case we would recommend biasing the non-inverting pin to 3.3/2 or 1.65V or something close to mid-supply depending on your application. This will ensure you are well within the input voltage range. The same considerations must be taken for the output swing of the device. There is a great set of TI videos that highlight these concepts and many more for op amps that I recommend. The link can be found here. The videos that highlight the topics we discussed can be found under "Input and output limitations."

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 10 points

    Thanks for the reply and clarification! I think you answered my first two questions, so I'll summarize:

    • TYP and MAX or MIN values for V_IL and V_IH are given for input voltage swing range in a worst-case scenario. Setting an offset voltage in the range between TYP and the extreme may produce nonlinearities and erroneous results in some parts due to process variation, in some scenarios.
    • A device operating outside the lower limit may not operate linearly.

    The third question is more a process variation question then, which is not specified so I'm hoping you can answer it here. Given that there is a process variation that can produce TYP and MAX V_IL:

    Ryan Syndergaard said:
    With V_IL TYP of 1.1V and V_IL MAX of 1.3V as given in the datasheet, what distribution of parts should we expect to maintain stability with the current-design offset voltage of 1.2V? 
  • +1 in reply to Ryan Syndergaard
    TI__Genius 11020 points

    Hi Ryan,

    Unfortunately, there is no clear answer on the distribution for this spec and due to lot-to-lot variation some wafers might not follow a similar distribution as a different wafer. This is why we always advise to give yourself some headroom and to design the circuit that it will work even with the specified maximum for a given spec. We guarantee that the device will be 1.3V at worst but there is no clear distribution that can gauge how many devices will perform non-linearly with a 1.2V common-mode input as there is a chance a device's common-mode input range is from 1.3V and above.

    Best Regards,

    Ignacio