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OPA2677: driving large capacitive loads and with feedback

Maher Dyayn
Prodigy 20 points
Part Number: OPA2677
Other Parts Discussed in Thread: TINA-TI

Tool/software:

Hi, 

I'm trying to use the OPA2677 current feedback op-amp to drive a large capacitive load (around 10 uF). The op-amp is configured as non-inverting and with a gain of 2x using feedback resistors. I'm concerned that with driving such a large capacitive load, the feedback resistors would have almost no current flowing into them untill this large output capacitance is charged and so the feedback gets delayed? 

Can I drive a 10uF load with this op-amp with feedback resistors (Rf = 100k, Rg = 100k)?

is there an LTSpice model for this part?

Thanks

  • 0
    TI__Genius 16645 points

    Hi Maher,

    Driving this large of a capacitive load could result in an unstable circuit due to the interaction between the output impedance of the device and the capacitive load. I was able to find a model for this device. However, I highly recommend trying this type of test on a PCB in lab. Simulation models do not capture the devices performance perfectly and could show erroneous results. We do not typically test our amplifiers with this large of a C-load so I am not certain what performance can be expected. I would recommend implementing an isolation resistor in any testing you attempt as this is the best way to compensate for this type of load and could help stabilize the circuit. Resistance values in the single digits would be a good starting point.

    Best Regards,

    Ignacio

    OPA2677_TINA-TI_Reference_Design-A.tsc

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 20 points

    Hi Ignacio, 

    Thanks for providing the model, I just imported this to LTSpice from TINA-TI and I followed a circuit configuration from the datasheet, however the results are strange. This setup should give a gain  = 4. My input to INA+ is 1Vpk-pk signal, however in the output I only see about 20mV pkpk. I have attached screenshot below. I'm not able to attach the LTSpice .asc file on this forum for some reason

    I couldn't find the DisA pin mentioned in the datasheet. Setting this to Gnd gives me 20-30mV ripple at the output (node B). setting it to Vcc gives almost no ripple

  • 0 in reply to Maher Dyayn
    TI__Genius 16645 points

    Hi Maher,

    I was able to run the simulation in the same gain configuration and got the expected output. DisA should be to VCC as grounding disabled the device. I am not certain if importing this model will cause some issues in LTSpice as our models were tailored to run on our own Spice simulator. Due to licensing issues, we cannot use LTSpice. Although you have already tried, could you set the DisA pin to VCC and reroute your traces to rule out it is the circuit itself. You could maybe try a quick dc bias simulation to see if its maybe due to the transient simulation.

    Best Regards,

    Ignacio