Slight Instability at start on active phase shift circuit

[pgiles]

When I simulate a active phase shift circuit using a LT6210 operational amplifier with feedback resistors around the negative side of 1K, and a capacitor down to ground from the positive terminal of 35pF with an input resistor of 100R, to that positive input.

I get a slight instability at the start of the simulation, I can't work out what that is

 

[LvW]

pgiles, the LT6210 is a current-feedback amplifier which requires a certain feedback resistor (lower limit).
You should check if you are complying with the specification.
More than that, your circuit description is not clear - please provide a circuit diagram.
Furthermore, a "slight instability at the start" is an indication for a poor phase margin.

 

[Arouse1973]

What input signal are you using?
Adam

 

[pgiles]

Here is the circuit for the active phase shift circuit

What input signal are you using?
Adam

I am using a LTSPICE Voltage source set to sine an offset of 2V and a amplitude of 2V, the Voltage source does not have any impedance set.

 

[Arouse1973]

So your circuit is completely different to what you original mentioned.

 

[Arouse1973]

Can you explain the voltages you show on the photo. Which one is unstable? I can't see signs of instability but then you have chopped of the sides of the photo, we can't see the scales. Can you explain what this circuit is suppose to do? What is it being used for?
Thanks
Adam

 

[pgiles]

So your circuit is completely different to what you original mentioned.

Hello It's as the screen dump of LTSPICE

 

[LvW]

I think, it is a classical allpass cicuit: Constant amplitude and frequency-dependent phase shift.
But I don`t understand the problem. Where do you see some kind of instability?
Sinus in and sinus out - where is the problem?

 

[pgiles]

Can you explain the voltages you show on the photo. Which one is unstable? I can't see signs of instability but then you have chopped of the sides of the photo, we can't see the scales. Can you explain what this circuit is suppose to do? What is it being used for?
Thanks
Adam

The green trace is the output from the op-amp, the blue trace is the output from across the voltage source, the red and grey traces are more or less the same they are the inputs to the op-amp

The circuit is intended to alter the phase of a 80MhZ clock pulse, as compared to the input, it is to be used for aligning a optical mirror signal from a laser so a trigger can be created at the instance the mirror on the laser in in a certain position.

 

[pgiles]

I think, it is a classical allpass cicuit: Constant amplitude and frequency-dependent phase shift.
But I don`t understand the problem. Where do you see some kind of instability?
Sinus in and sinus out - where is the problem?

Hello
The green trace is the output from the op-amp, the blue trace is the output from across the voltage source, the red and grey traces are more or less the same they are the inputs to the op-amp

The instabilty is the first half of the output clock pulse

 

[Arouse1973]

Settling time I think you'll find, not instability.
Adam

 

[pgiles]

Settling time I think you'll find, not instability.
Adam

Thanks, is that due to the power supply, or is it signal related

 

[LvW]

Hello
The green trace is the output from the op-amp, the blue trace is the output from across the voltage source, the red and grey traces are more or less the same they are the inputs to the op-amp

The output shows a ggod portion of triangular distortions caused by insufficient slew rate. Is this a problem for you?

The instabilty is the first half of the output clock pulse

This is no indication of instrability. It is, rather, a normal transient behaviour as a result of the inpiut signal switch-on event.
No reactive circuit is able to reach its steady-state condition immediately after switch-on of an input signal..

 

[pgiles]

The output shows a ggod portion of triangular distortions caused by insufficient slew rate. Is this a problem for you?

This is no indication of instrability. It is, rather, a normal transient behaviour as a result of the inpiut signal switch-on event.
No reactive circuit is able to reach its steady-state condition immediately after switch-on of an input signal..

Hello
I shifted the start of the stimuls past the power on time of the Simulation, and I still got the same result, so do you think it's due to the transient behavior of the initial applied signal

Im not too worried about the Slew rate of this op-amp, as I will be applying a pulse to stimulate the op-amp, and I am going to square it up again with a LT1715

 

[LvW]

Hello
I shifted the start of the stimuls past the power on time of the Simulation, and I still got the same result, so do you think it's due to the transient behavior of the initial applied signal

Yes - that´s what I have tried to explain in my former post.

 

[pgiles]

Thanks for your support

 

[Arouse1973]

Ok I know what it is, as LVW said it is slew rate limiting. Drop your input voltage to 0.5 V.
Thanks
Adam

 

[Arouse1973]

So for your input of 2 V you will need an opamp with a slew rate of 2 * PI * F * V = 2 * 3.1416 * 100*10^6 * 2 = 1256637061 V/s or 1256 V/us. Yours is 700 V/us typical.

So the max input you will be allowed is V=SR(Slew Rate) / 2 * P I* F = 700*10^6 / 2 * 3.1416 * 100*10^6 = 1.114 V

Thanks
Adam