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OPA2227 output saturation

Mike, Jung
Prodigy 230 points
Other Parts Discussed in Thread: OPA2227, OPA1652

Dear TI engineer

My customer use the OPA2227 to amplifier audio signal.

Non-inverting input is connected to 2.5V reference voltage, Vcc is 5V and load is not connected.

inverting input is connected to audio signal through input resistor and feedback resistor.

Input resistor 13k and feedback resistor 100k are connected to circuit,

when audio input signal is under 200mV, OPA2227 is saturaing the output.

Cf value change from 22pF to 150pF. and removed same result.

but when resistor change to 13k -> 2.7k, 100k -> 20k respectively, OPA2227 is not saturating.

We confirm the OPA1652 at same condition.

OPA1652 is not saturating.

I don't know what is reason for the OPA2227 saturation ?

Thanks & best regards

  • 0
    TI__Guru 60610 points

    Mike,

    There are two essential problems with your application:

    1.  The minimum specified supply voltage for OPA2227 is +/-5V (10V total) but you use just +5V supply.

    You may NOT rely on operating supply voltage of +/-2.5V since this condition refers to op amp's non-linear operation where PDS specs are not specified.

    2.  The specified output voltage swing is 2V from either rail, which on Vs=+5V supply would allow output signal of +/-500mV centered around +2.5V; but your gain of 100k/13k and 200mV input signal results in larger than +/-500mV output voltage thus violating the specified output voltage range - see PDS table below.

  • 0 in reply to Marek Lis
    Prodigy 230 points

    Thanks for your rely.

    I have additional question.

    when gain ratio is same to change 100k/13k to 20k/2.7k, OPA2227 is not saturation at same condition.

    I wonder what causes this saturation to occur due to feedback resistor increase

    Best Regards.

  • 0 in reply to Mike, Jung
    TI__Guru 60610 points

    Mike,

    First, the gain ratios you use are not the same for both resistor pairs: G1=100k/13k=~7.7 while G2=20k/2.7k=~7.4, thus using Vin=0.2V results in the output being driven to 1.54V and 1.48V, respectively (60mV difference).

    Second, there may be significant diferences in the OPA2227 performance of the output stage while using supply voltage much below minimum specified of +/-10V - for more info please read my blog by clicking on the link below:

    http://e2e.ti.com/blogs_/b/precisionhub/archive/2014/11/18/perils-of-using-op-amps-outside-the-specified-conditions.aspx