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LMC7101: Output of square wave PWM not as expected

Part Number: LMC7101

Hi,

Customer has the following schematic. Positive input is a 0-3.3v analog input.  The negative input is a 0-3.3v pwm input.  Both inputs need to be gained up to 0-5v at the output.

Simulation works great on an ideal op-amp but breaks when the actual LMC7101 is used.  The analog input to gained output works great.  However, the PWM only goes to 1V peak to peak.   PWM is 10khz frequency 0-3.3v square wave.

The designer is primarily a digital designer so this is likely a cookbook problem.  Does a 3.3v input on the DAC input for IN+ effectively make a 1.6V IN+ node with the 20K resistor to ground?

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I pulled up the following circuit that looks very similar - albeit with negative voltages rather than ground.  It would imply they should ground IN+ rather than take it to 3.3v?   Thoughts?

  • Hi Blake,

    Didn't get a chance to simulate this circuit in-depth today.  But, think I can offer some useful advice right off the bat.

    First, a couple of other things.  Regarding your first question, yes if I've understood you correctly.  A 3.3V input from the DAC will effectively make the IN+ pin of the amp 1.65V.  Also, the image of the second schematic is not displaying properly.  So, I can't comment on it.

    Now for some thoughts on the circuit:

    1. Before considering your results, I'd wonder if the customer has simulated for stability.  I say this because the op amp is a low-power part, which typically have a tougher time stabilizing.  Additionally, the C57 load capacitance of 1uF is quite heavy.  R43 may act as a sufficient isolation resistor and make the system stable, but I'd have to check in sim.  Again, might want to ask the customer if they've simmed for stabilty.

    2. Are you sure the R44 and R46 resistors, and R45/R47 for that matter, are set to the correct gain?  This circuit is known as a "Difference Amplifier" or a "subtractor circuit" and, using the transfer function from the top of page 2 of this cookbook, I'm not sure you're going to get the right gain from your resistors.  Don't know what the input signal look like, so I can't tell you with complete confidence.

    3. The first two points are just some things I noticed. However, more directly relating to your problem may be the fact that the PWM and analog signals are applied with a timing that makes the op amp try to swing negative.  If you again look at the transfer function of the circuit in the cookbook, you'll notice that the first term is negative.  So, the amp will try to invert the PWM signal.  It's possible that the amplifier is trying to do something along the lines of swing from +1V to -1V, which it can't due to its supply voltages.  And, this may be the cause of the strange behavior.  I'd have to see what the input signals are like and their timing.

    Please let me know if this is of use or if you have any further questions.

    Regards,

    Daniel

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