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Need similar op amp which can drive larger capacitive load

Brian Kaczynski
Prodigy 210 points
Part Number: LMC6084
Other Parts Discussed in Thread: TLC2264, TLC2272

Hello,

I am currently using LMC6084/6082 as the last stage of a low pass filter. It's in an inverting gain configuration with a parallel RC (R=36k, C=180pF) as the gain element and the input into the summing node is a current.

This circuit has to drive two analog switches in series (both elements in the MAX4750). According to the data sheet these switches each have a capacitance of 40pF in the on state. The output of these two switches gets buffered by a standard unity-gain buffer using a TLC2264.

This circuit should not oscillate, but it does. And yes, the traces are all very short. Frequency of oscillation is about 1.3MHz and amplitude about 200mV (and higher for higher output voltage).

So my question is simple: Can anyone recommend an op amp with similar parameters to the LMC6084 but which can drive a larger capacitive load? I need rail-to-rail output, operation from 9v single supply, input offset small (hopefully <1mV), power consumption similar to LMC6084 but it's OK if it's a but higher.

I know how to use the parametric search but unfortunately it doesn't consider capacitive load drive capability so I'm hoping a TI op amp expert can advise.

Thanks!

  • 0
    TI__Guru 53826 points
    Hello Brian,

    Can you post your schematic?

    Ideally, you should put a 100-200 ohm resistor in series with the output, before the switches (as shown in the datasheet Apps section). CMOS switches are a direct capacitive load, as the capacitance is not buffered by the Ron of the switch, and the majority of the switch capacitance is still there whether the switch is on or off.

    Actually, the parametric search does allow searching for high cap load drive - but it is hidden... Expand the "Additional Features" tile, scroll down and check "High CLoad Drive". These will be devices that can drive "unlimited" loads, or can drive >500pF directly.

    Regards,
  • 0 in reply to Paul Grohe
    Prodigy 210 points

    Here is the relevant part of my schematic.

    I can't add 100 ohms in series because the load on the MAX4750 isn't exactly just the buffer input.  There's a 50K resistive load there as well which I didn't draw but because of this resistive load I do not want to increase the output impedance that the MAX4750 has to drive.

    I'll check out parametric search with capacitive load driving turned on...

    Thanks!

  • 0 in reply to Brian Kaczynski
    Prodigy 210 points
    I found the TLC2272/4 in parametric search by turning on the "high Cload" option but the datasheet doesn't mention much about driving capacitive loads (except for a few transient response graphs with Cload=100pF, which is not very high). Can anyone comment on this op amp's capacitive load handling ability?
  • 0 in reply to Brian Kaczynski
    TI__Guru**** 151696 points

    Brian,

    Be sure to include all the parameters.

    Capacitance value.

    Forward voltage gain

    Forward bandwidth needed. 

    Output Amplitude.

    Stability and current drive are the key factors.

    Any op amp can drive any capacitor to a DC voltage if the proper frequency compensation is used.