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OPA388: Max. Capacitive Load

Part Number: OPA388
Other Parts Discussed in Thread: OPA392, TINA-TI, INA240

Hi,

We are using the design below. We observe that the output of the OPA388IDBV oscillates. We think that is due to the capacitive load on its output (400nF)

Could you please provide us the Max. capacitive Load allowed for stability ? Do you have cross references of amplifier (same package and pin configuration) that could accept a load of 400nF ?

regards,

  • Hi Pascal,

    According to OPA388 datasheet like most op amps, it can directly drive in a buffer configuration (G=1) only few hundred pF. - see below.

    Having said that, I do not see on your schematic any capacitive loads (see below) - is this the circuit you refer to?  BTW, I have hard time reading it because of poor resolution.

    OPA388 is a chopper amplifier that may require matching of the input impedances in order to minimize conversion of the IB spikes across the unmatched input impedance - this makes the output look noisy and the input offset voltage may increase above the maximum specified limit.  In order to eliminate these problems, instead you should consider using a linear op amp with a similar bandwidth like OPA392.

    If you insist on using OPA388 because of its superior input offset voltage drift, you should match the input impedances by adding feedback cap/resistor as shown below.

    If in fact you do attempt to drive 400nF load, OPA388 will be unstable - see below.

    Running AC stability analysis show that you need to add at least 10 ohm to 50ohm Riso resistor to assure stable operation - see below.

    Running a small-signal transient stability confirms very stable operation with series output Riso resistor of 50ohm - see below.

    With 10ohm Riso resistor the circuit still meets the maximum recommended small-signal overshoot of 25% - see below.

    Below I have attached Tina-TI schematics for your own simulation.

    OPA388 AC stability.TSC

    OPA388 Transient stability.TSC

  • Hi,

    Thanks for your answer. We will try to use the OPA392 instead of the OPA388...

    Regards.

  • Yes, using OPA392 would make things simpler.

  • Hi Pascal,

    do you need the 100nF cap at the REF pin of each INA240 at all? I ask because the current flowing through the REF pin is only very small and the bandwidth of INA240 is only 400kHz. The bandwidth of OPA388 (and OPA392), on the other hand, is beyond 10MHz and because of this these OPAmps should be fast enough to serve the REF pins of INA240 without causing an extra signal distortion compared to what the INA240 is doing. I would first confirm by measurement whether the additional 100nF cap is of benefit at all. Otherwise adding a huge filtering capacitance of 400nF may do more harm than good.

    I see another issue: You have mounted huge filtering capacitances from the inputs of INA240 to signal ground. Common mode filtering in this way is criticial with current sense amplifiers because any missmatch between the two common mode filtering caps can considerably degrade the common mode rejection of current sense amplifier.

    I would reduce the common mode filtering caps to the absolute minimum and guarantee lowest mismatch between the two caps in order to keep the balance at the inputs of INA240 high and by this the common mode rejection. 1%-toleranced NP0 of C0G capacitors are recommended at this place. Only these are showing lowest temperature drifts and long term drifts.

    Kai