I'm trying to simulate a leaky integrator using an AD8603 op‑amp in LTspice.

My goal is to model the kind of slow‑changing signals that come from MOX gas sensors, but the gas sensor is not relevant here — the problem appears even with a simple triangular test signal.

I followed this document for calculation of Cf, Rf and Rin. I fixed Rin as 1Meg and calculated Cf using the Step two in the documents, Design Steps (Page no 2).

My opamp is AD8603:

Problem:

My integrator saturates (or runs into the supply rails) instead of producing the expected slowly rising and falling output.

My circuit and response is given below. The input waveform is attached as text.


May I know what mistake I am making?

 

hi hoy,
Your Triangle input has a positive voltage offset, it does not swing above and below zero volts, so the positive offset is integrated.
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Hi Eric,

May I know how do we integrate these kind of waveforms.

 

hi hoy
Is the waveform input you posted the actual waveform you want to integrate??

This is with a modified input
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My waveform will be something similar. The output from a gas sensor working on positive supply. When gas concentration increases voltage also increases and when gas concentration decreases output voltage of the sensor also decreases.It will never go below zero. So I choose this waveform.

 

hi hoy,
Look at this option with the 0.5V source offset input
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hi hoy,
Why do you need to integrate a slowly changing +DC waveform?
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The average DC value of the input signal (0.5V in your initial simulation) will be amplified by the op amp DC gain of R2/R1, so you need to adjust that ratio based on that DC value, to prevent op amp saturation at the rails.

 

hi @hoyyoth
Reference the sample triangular waveform you posted, is that the only waveform, 1 Volt amplitude level at that set frequency you are trying to integrate?
The suggested DC Voffset of 0.5V only applies to the posted waveform amplitude.

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You really need to describe what it is you NEED for your application. Not the problems that you are having with a circuit that you think you want, but what is the actual problem you are trying to solve.

You have a sensor that outputs a voltage that is related to the concentration of a gas. This concentration varies slowly over time. Got it.

Now, what is it your application needs to know about the gas concentration?

The average concentration over the last N seconds?

The min/max concentrations?

The total gas exposure over the last N seconds?

Once we have an idea of what it is that you are trying to actually accomplish, we are in a much better position to help you craft a circuit that will do it.

 

hi @hoyyoth
Reference the sample triangular waveform you posted, is that the only waveform, 1 Volt amplitude level at that set frequency you are trying to integrate?
The suggested DC Voffset of 0.5V only applies to the posted waveform amplitude.

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I provided the 0.5V, but still getting the same.

 

hi hoy,
What do you expect the Vout waveform to look like?
Post a simple drawing.
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Perhaps you may require a precision rectifier, not an integrator???

 

hi hoy,
Going back to basics on your LTS sim.
Why Is My Leaky Integrator Saturating With an 8.33 mHz Triangular Input in LTspice?

Why are you using a 60second period, in LTSpice, to generate a triangular waveform

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hi @hoyyoth
Basic examples of a precision rectifier and an integrator, using an 8.3mHz rise/fall pulse period

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I am expecting the same waveform you have shown.

 

hi hoy.
I would consider a precision rectifier circuit, it avoids the need for an offset voltage and the Vout is in the same polarity of the input signal, add a non inverting buffer on Vout.
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Hi Eric,

Thank you for your support.
I did a single supply design in TINA-TI, the output waveform is coming as expected.

 

hi hoy,
Unless I am missing something, that circuit is still going to integrate the V2 input offset voltage, so the Vout will not represent the amplitude of Vin.
Note how the Vout plot shows a gradual increase in the +DC level of the triangular input.

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