Integrator Circuit Issue

#12

Joined Nov 30, 2010
18,159
I keep seeing this as a voltage driving a proportional current generator. Problem for me is that the accumulating capacitor is always inside the feedback loop (no end grounded). I suppose one could take that information to a differential amplifier in the following stage.
 

crutschow

Joined Mar 14, 2008
23,486
Here's a circuit that generates a current proportional to the positive input voltage into a grounded cap to provide the integrate function.
The circuit uses a differential connection op amp with its input referenced to the plus supply along with a P-MOSFET to generate a current to a grounded load.
The LTspice simulation below shows that it only integrates on the positive half of the input sine-wave.
You will need to add a switch to ground the cap and reset it for the next measurement.

Integrator.PNG
 

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crutschow

Joined Mar 14, 2008
23,486
As an alternate circuit (offset not sensitive to resistor values), below is the simulation of dl324's circuit using MOSFETs.
I had to change to a rail-rail input and output op amp since the the common-mode range of the LM358 only goes to within about 1.5V of the positive rail.


Integrator.PNG
 

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Thread Starter

Eoin_oc

Joined Oct 24, 2015
23
Here's a circuit that generates a current proportional to the positive input voltage into a grounded cap to provide the integrate function.
The circuit uses a differential connection op amp with its input referenced to the plus supply along with a P-MOSFET to generate a current to a grounded load.
The LTspice simulation below shows that it only integrates on the positive half of the input sine-wave.
You will need to add a switch to ground the cap and reset it for the next measurement.

View attachment 93428

Which resistor is used here to set the current that flows into the capacitor?
Is it Rsense? I would like for a capacitor of around 1uF to charge up to 3.5V in 100ms with a constant 3.5V input level.
 
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crutschow

Joined Mar 14, 2008
23,486
Which resistor is used here to set the current that flows into the capacitor?
Is it Rsense? I would like for a capacitor of around 1μF to charge up to 3.5V in 100ms with a constant 3.5V input level.
Yes, Rsense is one resistor that sets the flow.
For 3.5V out in 100ms for a 3.5V input requires an integrator time-constant of 100ms.

That time-constant with 1μF will give an offset problem with the circuit in post #25 so you would need to use the one in post #26, modified as shown here.

Integrator.PNG
 

JamesBond007

Joined Oct 25, 2015
24
OK. What is summation (integration) ?
The addition of all values of a variable (voltage) over a period of time.
Integration is: over some time, a voltage is sampled at infinitely small periods of time, and measurements are taken over such a short period of time that they are equivalently zero, yet multiplying these infinity small and discrete sums over an infinite time frame (according to Newton), gives us a finite value.
 
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