capacitanceinductance
An LC tank circuit produces oscillations, when the capacitor is charged and connected to the inductor, but the oscillations decay gradually. How to make it continue, i.e. oscillate continuously?
I know we need to add energy continuously, but I don't know how.
Could anyone describe a circuit that provides a non-decaying oscillation?
In your circuit you are using the tank as the load on the op-amp buffer circuit. The op-amp is trying to produce 4.5 V, but its output is short-circuited by the inductor.
To generate oscillations in a linear circuit, you need to meet the Barkhausen criterion:
The usual way to use a tank circuit to set the frequency of an oscillator is to put it in the feedback loop where it can cause the Barkhausen criterion to be met at (or very near) its resonance frequency.
In your circuit, the tank is not involved in feedback, so it can have no influence on oscillation.
A relatively similar circuit that can oscillate is shown here:
(source)
Notes:
R1 prevents the op-amp output being short-circuited at DC.
The tank forms a divider with R1 so that positive feedback is very low except when the tank is at resonance.
The designer might have intended to make a band pass or low pass filter, but neglected to put any impedance/resistance between the capacitor and the op amp. But this is purely speculation and EE.SE has no place for that.
One thing to remember is amplifiers and ADC's have input impedance, in this case R1 is 30kΩ (from the datasheet)
I can't see a reason to keep the capacitor on the board in its current configuration it's loading the op-amp with capacitance, either convert the circuit into a low pass filter (and cut back on your noise) A good reference is Op Amps for Everyone from TI or any op amp design guide from an analog manufacturer.
Learn how to do filter calculations, check the LT1001 datasheet for info on capacitance (fig 1001 G20) for info on overshoot. Design stuff with hand calculations, then maybe do a simulation in LT spice and then implement the changes and test them to see if they improve your situation.
Best Answer
simulate this circuit – Schematic created using CircuitLab
Here is an LC Oscillator that is easy to understand. All the values are examples only.
However, it is necessary to select the four equal resistors fulfilling the condition R2/R1=R4/R3.
In theory, you could drop the resistor R3 - however, in this case you should know the damping properties (that means the parallel loss resistance Rloss) of the tank circuit.
Because this is not possible it is wise to heavily damp the LC circuit with R3 because,in this case, the losses of the componenets L and C are "overshadowed" and play no role.
Thus, you have an opamp with two feedback pathes (positive and negative) which cancel each other for one single frequency only: The resonant frequency of the tank which gives the oscillation frequency.
Remark: For a safe start of oscillations it is necessary to slightly increase the non-inverting gain (1+R2/R1) above its nomional value. That means: R2/R1>R4/R3.