Hy, I have some questions about opamp circuitry layout.
1.) Is it better to run input signal path over supply rail, if it means that running it around would result in much longer path... like 3 times longer?
2.) Is it bad to run signal input or output path over feedback loop or realy close to it?
3.) Can two feedback loops of different opamps be close together?
4.) Running input path of one opamp close to feedback loop or output path of other opamp?
5.) Running ground above anything should not be too big of a problem I suppose?
Thank you for your help 🙂
1.) Is it better to run input signal path over supply rail, if it means that running it around would result in much longer path... like 3 times longer?
2.) Is it bad to run signal input or output path over feedback loop or realy close to it?
3.) Can two feedback loops of different opamps be close together?
4.) Running input path of one opamp close to feedback loop or output path of other opamp?
5.) Running ground above anything should not be too big of a problem I suppose?
Thank you for your help 🙂
A good general strategy is to consider the impedance (or how strongly driven, or how strongly held) of each signal. Signals with high impedance (like inputs) are susceptible to coupling because the strength of the parasitic capacitance or inductance (made worse by long length) inducing an errant signal may be somewhat larger relatively when compared to the weak strength of the driver or components holding the desired signal. Signals with low impedance (like outputs and feedback) are not likely to get signal coupled into them, because the strength of the parasitic capacitance and inductance is rather weak compared to how strongly the signal is being driven with the correct signal. Think of outputs or feedback or strongly driven signals as being "aggressors" and weakly held inputs as being "victims". Ground and VDD are extremely low impedance, and are considered "neutral", so routing a weak signal vertically over GND or VDD will really not help and it's not worth a long trace length (which adds coupling cap and inductance, making a victim more prone to victimization). Most aggressors couple to victims from side to side when the traces run parallel along each other, either in the same plane or adjacent planes. So, much like Switzerland, placing a trace of VDD or GND =between= aggressor and victim on the same plane will keep them separated.
Keeping traces short allows less opportunity (paracitic capacitance and inductance) for aggressors to bully the weak signals around them, and short traces also minimize the suseptibility of weak signals to be bullied. It cures many evils. Be most protective of inputs, as by definition they are small and sensitive, and outputs/feedback are much larger and stronger. Where you cannot keep those two groups separated, shield with Switzerland. In the end, no layout will be perfect- the best layout is one that works and does not oscillate or have high distortion, it's probably dense, so tradeoffs will be required.
Keeping traces short allows less opportunity (paracitic capacitance and inductance) for aggressors to bully the weak signals around them, and short traces also minimize the suseptibility of weak signals to be bullied. It cures many evils. Be most protective of inputs, as by definition they are small and sensitive, and outputs/feedback are much larger and stronger. Where you cannot keep those two groups separated, shield with Switzerland. In the end, no layout will be perfect- the best layout is one that works and does not oscillate or have high distortion, it's probably dense, so tradeoffs will be required.
Yes- the note about bypassing the op-amps is critical. I recommend placing at least one 0.1uF = 100nF (marked 104) ceramic capacitor from -VDD to +VDD if using complimentary supplies or from +VDD to GND if a single ended supply as close to the supply pins of each op-amp as possible. Ceramic has much better very high frequency performance than a poly cap, and will go a long ways to preventing oscillation or ringing in the op-amp.
This is exactly what I did read before coming here. I was just looking if there is more to it. And I did get additional info 🙂
Edit: didn't see the pdf under at first. Will dive into it.
Edit: didn't see the pdf under at first. Will dive into it.