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OPA2189: Problems with the OPA2189 operation amplifier.

Rhea Gao
Expert 2580 points
Part Number: OPA2189
Other Parts Discussed in Thread: OPA189

Hi,

When I use OPA2189,

The values of ELVSS_DAC3 and VGNG_ELVSS remain unchanged,

The ratio of R45, R204, R52 and R53 remains the same, and the maximum output range measured at opamp output pin 1 changes when the resistance value changes (the maximum output value can reach -20V when the resistance value becomes smaller =1.5Kohm, and the maximum output value can reach -18V when the resistance =100Kohm). Please help analyze the reason for this problem:

1. Help check whether +5V and -23V power supply can be used for the op amp?

2, R45, R204, R52, R53 these four resistance values are different will affect the performance of the op amp?

Many thanks~

  • +1
    TI__Guru* 77541 points

    Hi Rhea,

    Rhea Gao said:
    1. Help check whether +5V and -23V power supply can be used for the op amp?

    In terms of range of supply rail, the configuration is ok, where 5V - (-23V) = 28V < 36Vdc. The issues are that OPA189 is not rail to rail input (RRIO) op amp, therefore, the Vcm of the input signal is limited to 2.5V. see the captured specification below. If the common mode voltage of the difference amplifier has to be operated up to 2.5V, there is no margins left over the wide temperature range. 

    You need to increase the positive supply voltage rail from 5Vdc to 8Vdc or higher.  Or you may swap -23Vdc and +5Vdc rails around to power the amplifier, since the Vdac signal is ranged from 0 to 2.5V. 

    Note: OPA189 is specified as RRO op amp, but you still need to provide the output swing margin because the output voltage swings are load dependent. At heavier load, say 2kΩ, the output voltage swing may be lowered as much as 500mV from supply rail in certain parts.  

    OPA189 diff amp 10072023.TSC

    Rhea Gao said:
    2, R45, R204, R52, R53 these four resistance values are different will affect the performance of the op amp?

    For precision difference amplifier application, the feedback resistors shall be matched over temperature and use lower tolerances ones. As you have found out, the CMRR errors may be large and are unable to meet your design requirements, if the feedback resistors are not matched in low tolerances and temperature coefficient for precision application. 

    https://e2e.ti.com/blogs_/archives/b/thesignal/posts/difference-amplifiers-the-need-for-well-matched-resistors

    BTW, could you explain your attached image. You may have to negative Vcm configuration in the setup and I need to know in order to simulate the condition. Also, please specify the operating frequency. There is 15kHz LPF at the output, and the input frequency has to be <1kHz in order to avoid signal attenuation at the output. If you are using switching power rails to drive the difference amplifier, please use low ESR (~10mΩ ESR, MLCC 1-10uf bypass capacitor) to filter out the switching noises from the rails. 

    If you have other questions, please let us know. 

    Best,

    Raymond 

  • 0 in reply to Raymond Zhang1
    TI__Guru* 77541 points

    Hi Rhea,

    I am going to close this inquiry. 

    Please let me know if you have other questions. 

    Best,

    Raymond

  • 0 in reply to Raymond Zhang1
    Expert 2580 points

    Hi Raymond,

    OK. Thanks for your support.