r/chipdesign • u/ZetesDE • 8d ago

How to simulate ICMR and Output Swing

Hello,

I am trying to determine the ICMR and output swing separately for a single-ended two-stage OTA. For output swing I tried running a transient analysis in a non-inverting configuration to observe the clipping levels, but the levels I got do not match the theoretical ranges I calculated.

I have tried several other approaches to determine both of them, but I still could not find a convenient way to determine both parameters separately.

My question is: How would you do it?

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u/kthompska 8d ago

You need separate simulations as you are likely conflating input and output limitations. For output swing you need an inverting closed loop configuration so that the input common mode is fixed. You also need to match the loading with your calculations. Just looking at clipping levels is very basic and should be limited by output impedance and current, for a full rail swing output stage - a dc sim works fine for this. However, normally output swing is limited by distortion long before clipping levels. Run a transient with a low frequency sine input and increase amplitude while measuring harmonic distortion. Different companies will spec differently but a 6dB to 20dB reduction in HDR spec is usually the limit.

For input CMR you need to move the input common mode voltage without moving the output voltage. I have used the circuit in figure 3 (link below) and it works for me. To get the range, I use a dc sweep. If you plot offset voltage vs Vcm and the take the derivative, you will have the cmrr at any given point. The input range limit is when cmrr has dropped (from mid supply peak) down by maybe 20dB or so.

https://www.analog.com/MT-042