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OPA2317-Q1: Input range question

Sherry Liang
Genius 10255 points
Part Number: OPA2317-Q1
Other Parts Discussed in Thread: OPA317, LM7705

Dear team,

My customer wants to change a negative value into a positive value through a amplifier as below. The input is -200V , and then the output is 4Vpp. In my understanding, the power supply is single polar, so the input must be larger than 0V, but in this solution, the input is negative, and there is still a normal output. Could you please tell me whether below solution is right?

Thanks & Best Regards,

Sherry

  • 0
    TI__Guru 61980 points

    Sherry,

    There are several ways to implement such function but we need to know what is the exact range of the input/output signall you need?  Is it 0 to -200V input that you want to convert to 0 to 4V output or something else?   Also, OPA317 may be powered from a single supply (0,5V), dual supply (+/-2.5V), negative supply (-5V,0) or assymetric supply (e.g. -1, +4V), thus you are not limited to 0 to 5V supply.   The circuit above will NOT work since your overall attenuation is: G=24k/(24k+3.08M)*(33) =~1/4, which would cause 200V input signal to result in 50V output, which is not possible on 5V supply.

  • 0 in reply to Marek Lis
    TI__Genius 10255 points

    Hi Marek,

    Yes, the input range is 0 to -200V, and I want t o covert to 0 to 4V. And the OPA317 is powered form a single supply(0,5V).

    And the simulation shows that the circuit can achieve this function, right?

    Thanks & Best Regards,

    Sherry

  • 0 in reply to Sherry Liang
    TI__Guru 61980 points

    Sherry,

    For Vin=0, output cannot swing all the way to negative rail - for that reason, output needs to be lifted at least 100mV above ground so it is within its linear range (see AOL conditions). This may be accomplished with R1/R2 resistors.  Since the output needs to be attenuated by a factor of 50, this is achieved with RF/Rin voltage divider - see below. 

    Sherry OPA317.TSC

  • 0 in reply to Marek Lis
    Guru 48185 points

    And Sherry, 

    That 0.1V on the V+ input gets a 1.02V gain due to inverting attenuator for a 0.1022 output for 0V input. 

    And of course you now have the supply tolerance in your output error calculation. To swing to ground without this input offset, you could also using the LM7705 to provide a -0.23V fixed negative supply. 

  • 0 in reply to Michael Steffes
    TI__Guru 61980 points

    Yes, if you are willing to add the negative charge pump, LM7705, you will eliminate the need for R1/R2 level-shift and achieve desired output voltage range of 0 to 4V for 0 to -200V input voltage range - see below.