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OPA454: OPA454 Insdtrumentation Amplifier implementation(Figure 67 of datasheet) in Tina Ti not working. Please help!

Jean Rintoul1
Prodigy 70 points
Part Number: OPA454
Other Parts Discussed in Thread: TINA-TI, , THS3110

Hello, 

I am implementing the application 10.2 from the datasheet in Tina-TI, and I am getting a saturation effect. Is there anything wrong with my design here? I've lifted it exactly from the datasheet(Figure 67), though am not getting anything like the suggested output. I am sending in a 4.75V PP sine wave at 500kHz.

Below is a picture of the transient response, and below that the circuit which is a copy of Figure 67 of the datasheet for OPA454. 

  • 0
    TI__Guru 62290 points

    Jean,

    Your circuit does not work because you shorted VDD (100V) to +V and VEE (-100V) to -V but they are completely different voltages - see below.

  • 0
    TI__Guru* 93105 points

    Hi Jean,

    In addition to Marek's comments about the power supplies:

    Once you have the circuit supplies and biasing set up correctly it should function as expected. Do note that Tina's convergence time for this circuit is long so you have to be patient. It takes about a minute, or more to run.

    A problem with the application is the frequency you are attempting to amplify with the OPA454 amplifier circuit. The OPA454 has a slew rate (SR) 13 V/µs, for G = ±1, VO = 80-V step, RL = 3.27 kΩ. If we calculate the minimum SR for your 500 kHz requirement:

    SR = 2 pi x f x Vp

    SR = 6.28 (500 kHz) (95 V) = 298 V/µs

    about 23x what the OPA454 is capable of slewing. You would need a much higher SR high-voltage op amp to do what you need.

    When I simulate the OPA454 under the original conditions with a 4.75 Vpk, 20 kHz sine input I receive similar results as in the datasheet. But when I apply the 4.75 Vpk, 500 kHz input the results are not useable.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Marek Lis
    Prodigy 70 points

    Thank you. This is my first experiment with op amps and Tina TI. I have made these changes, yet I do not get the an output waveform. It still appears to be saturating. Do you happen to know something else I could check to make this work? Thank you so much. 

  • 0 in reply to Thomas Kuehl
    Prodigy 70 points

    Thanks. I am still not getting the simulation to work though I've made the changes. The slew rate is a good point and perhaps a higher slew rate op amp like the THS3110 would be a better fit? www.ti.com/.../ths3110.pdf

    I'm trying to make an instrumentation amplifier for a 500kHz signal, which can deliver up to 50mA. 

  • 0 in reply to Jean Rintoul1
    TI__Guru* 93105 points

    Hi Jean,

    I have attached my original OPA454 Tina file that I used for the datasheet circuit. It runs fine, but as I mentioned earlier it takes more than a minute for the circuit to converge.

    The THS3110 is certainly a much higher slew rate op amp than the OPA454, but a stack of them won't be able to handle as high of supply voltages as the OPA454. Additionally, the THS3110 is a current feedback op amp (CFA) so it is a bit different beast than a voltage feedback op amp (VFA) such as the OPA454. The non-inverting input of a CFA is the high impedance input, while the inverting input is a low impedance input. Most users tend to apply the CFA as a non-inverting amplifier. 

    The THS3110 is from TI's High-Speed Amplifiers product line and is supported by that team. You can run your ideas past that team and they should be able to give you a good idea if the THS3110 will work in the high output current, instrumentation amplifier application. They have their own High-Speed Amplifiers e2e forum where you can post your questions.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

    OPA454_output_boost_01.TSC

  • 0 in reply to Thomas Kuehl
    Prodigy 70 points

    Thanks! This is really helpful as I am also new to Tina TI. Your implementation looks more organized than mine, so perhaps I had a wire that was connected where it shouldn't be. I will look at it in further detail and experiment with it. 

    Also - thanks for pointing me to the high speed op amps - it seems like that is where I need to go.