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I am trying to use LMH6642MAX op amplifier in the inverting closed loop configuration (gain 3.5) for a periodic square input signal 20 kHz (2 Volts High, 0 Volts Low). The problem comes when I checked the ouput and I find that the output signal rings. When the input is low the output swings around 0 V and when is high it oscillates around 7 Volts.

My first thought was that the problem was coming from the power supply but I have added two decoupling capacitors in the positive rail of the op-amp, but it didn't fix the problem. The datasheet doesn't say the magnitude of the decoupling capacitor but I have used 0.01uF and 2.2uF because they have worked very well for similar op-amps.

My second thought was that the op-amp was unstable working at that frequency. That's is, the gain is 1 and the phase between the input and output is 180 degrees. However, after inspecting the datasheet, it seems that the open-loop gain reaches 0 dB when the frequency of the signal is of the order 100 Mhz.

I am running out of ideas about what could be the root cause of this ringing.

enter image description here

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  • \$\begingroup\$ what is attached to the output that you are not showing ? capacitance? \$\endgroup\$
    – tobalt
    Nov 1, 2021 at 17:37
  • \$\begingroup\$ CH 11.1 explains about parasitic capacitances possibly causing ringing. Is this on a breakboard or PCB? \$\endgroup\$
    – rdtsc
    Nov 1, 2021 at 17:37
  • \$\begingroup\$ It is in a PCB and there is no output connected yet. Maybe is this the cause of the ring? I have simulated the circuit (output not connected) with LTSpice and I don't see the ring. \$\endgroup\$
    – Jonw3
    Nov 1, 2021 at 19:13
  • \$\begingroup\$ I am using the single-channel op amp. \$\endgroup\$
    – Jonw3
    Nov 2, 2021 at 7:41
  • \$\begingroup\$ Yes sorry. I was misled by the name.. I am out of ideas. Could you post a scope shot of the ringing ? It could be that 2.5k feedback resistance is already somewhat large for a really fast opamp. In that case, you should add a small capacitor across that feedback resistor. Something on the order of 10 pF would suffice. \$\endgroup\$
    – tobalt
    Nov 2, 2021 at 10:13

1 Answer 1

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when your OPAMP is powered with a single power supply, you have to make a Vcc/2 voltage to provide a mid-voltage on inputs of OPAMP.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ -1 neither does this explain the ringing, nor is the op trying to amplify a gnd referenced signal that would require a dc level shift, nor is your schematic the usual way to do it ( I am not sure it would even work) \$\endgroup\$
    – tobalt
    Nov 1, 2021 at 17:39
  • \$\begingroup\$ @tobalt missing a proper DC reference for OPAMP always cause unpredictable problems including instability . By the way, what exactly would you do for a single supply OPAMP to bias correctly? It's been working for me hundred times. So I know what I said. \$\endgroup\$ Nov 1, 2021 at 18:19
  • \$\begingroup\$ Op says that it is also unstable for the high input where everything is properly biased. in your circuit R3/4 have way too high impedance to set a stable midpoint reference. as a result, gain will not be 2 but will vary based on input voltage.. that high midpoint impedance also allows positive feedback which invites noise at best, instability at worst. \$\endgroup\$
    – tobalt
    Nov 2, 2021 at 4:40
  • \$\begingroup\$ also, that opamp have common voltage input range below the negative supply and rail-to-rail output, so no problem with the function desired by the OP \$\endgroup\$
    – tobalt
    Nov 2, 2021 at 4:57
  • \$\begingroup\$ @tobalt My circuit is for AC signals and not for DC precision signals. with C2 the effect of impedance of R3 and R4 will be removed. of course generally, impedance of R3 R4 should be lower, I just meant a general idea with these values in the circuit. The 100k would never make any positive feedback and stability at all. \$\endgroup\$ Nov 2, 2021 at 12:55

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