ltspice
Another quirky LTSpice question. I am measuring input impedance and using a .meas directive to get the impedance at specific frequencies:
.MEAS AC Zi_freq1 FIND V(in)/I(v1) AT=10k
When I right click on the y-axis and change the axis from decibel to linear, the impedance in kOhms shows up. However, when I run the .meas directive the impedance is in dB again. How do I use the .meas directive to get impedance in a linear scale rather than in dB?
Aha (!) the infamous gain peaking problem seen in the non-inverting op-amp circuit!
This is one of those things you learn to watch out for. Remember that the gain of this type of amplifier configuration is: -
\$1+\dfrac{R_5}{R_6}\$ (for the right hand diagram)
But that formula hides the fact that the reactance of leakage capacitors needs to be taken into account. The op-amp has a common mode input capacitance of 0.45 pF and this will progressively shunt R6 as the operating frequency rises.
In other words, gain will start to rise with frequency and the gain will have increased by 3 dB when Xc = R6. This happens when: -
Frequency = \$\dfrac{1}{2\pi R_6C_{IN}}\$
So, input capcitance is 0.45 pF and R6 is 3000 ohms hence F = 117 MHz. At this frequency the circuit gain is rising and keeps rising at 6 dB per octave until it becomes limited by the open-loop gain of the op-amp. The data sheet isn't very good at pictorially showing this so I have taken a liberty with what they do show and extended it: -
The original extract from the DS is on the left and my mangled version is on the right. Hopefully this should explain why you see a peak at about 250 MHz. The bright red line is what I predict your gain profile will be until it hits the darker red line (what I expect the open-loop gain to be if the graph is extended).
When you chose lower value resistors (your left diagram), that peak got pushed out to 400 MHz and this generally ties in with what you should expect. It's never an exact science of course but this generally is what you are seeing.
Generally the best fix is to lower your resistors so that the peak is pushed out way beyond the open-loop gain of the op-amp. I don't see any problem in making R6 = 300 ohms (ten times lower) and making R5 a 2k7 (ten times lower too).
Remember that when you build this there will be stray capacitance that can make the problem worse so keeping the resistor values low is going to be sensible. Note that the DS also states this: -
Since LTC6268-10 is a decompensated op amp with gain-of-10 stable, it requires that CIN/CF ≥ 10.
This means that adding a capacitor across the feedback resistor has to be done with care.
I believe that the reference level for LTspice ac plots is that 0 dB = 1 V. So, it looks like your ac signal is very, very small. Keep in mind that this is a simulation, not real life, so there are round-off errors that can accumulate and look like very small noise signals.
EDIT: Your simulation provides an input voltage of 1V for the regulator. An LM340T-12 is not going to function properly with that small of an input voltage, you need at least 14V to make this device work. I think your simulation results are probably bogus.
Best Answer
I've been looking for that feature as well for a long time and came across the site linked below. There are quite some more interesting explanations, features and documentation on it (e.g. Undocumented LTspice).
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