Using the formula, , where f is the cut-off frequency, and R and C are the resistance and capacitance of resistor and capacitor components used in the circuit. The R and C values were selected based off of available components, and desiring a resistor in the kilo-ohm order of magnitude. Measuring the components, we obtained two R values at 3. This results in an expected cut-off frequency of The transfer function of an arbitrary Butterworth filter is:.
The amplitude A is the theoretical amplitude of the first stage. We designed the circuit such that the first stage gain is 1. This results in the amplitude A being 1, resulting in the following transfer function, after substitution of known values:. For the gain stage, we wanted to obtain a gain of This was accomplished by using a non-inverting amplifier configuration.
Two resistors are used in the gain stage, accomplishing a gain which can be seen in the following equation:. The gain of was accomplished by using resister values of and ideally. We measured and. This results in a gain of ideally , but with an expected gain of We had similar results with the other filter.
The final filter and amplifier can be seen in the following figure:. We included potentiometers used in the null-offsets of all of the op-amps. This was done in order to accommodate an offset with the output voltage. The overall transfer function including both stages is:.
The resulting bode plot for this transfer function is the following:. The next step is to construct the amplifier Non-Inverting for the output of the filter. A resistor Ra is connected to ground and connected to the negative input signal - Vin Port 2, of the operational amplifier for the amplifying stage.
Another resistor Rf is connected in parallel with resistor Ra and connected to the output Port 6 of the operational amplifier. The Non-Inverting amplifier is completed below. The same construction of the first filter-amplifier circuit is used as a basis for building the second filter-amplifier circuit. The components were then soldered on to a printed circuit board to complete the assembled filter and amplifier circuit for the pressure transducers seen below:.
The board was then included as a part of the power supply used for the project. As seen below, there are external connections to the primary connection points, for the inputs, and outputs. This allows for a convenient connection point when using this with the project as a whole.
Calibration was done to make sure there were no initial errors with the filter. Without calibration, there may be an offset of the output. To calibrate, the input to the filter was grounded and an oscilloscope displayed the resulting output signal of the filter stage.
The potentiometer connected to the null offset pins of the filter stage was adjusted to obtain an average offset of approximately zero. Next, the oscilloscope was connected to the output of the amplifier stage. Then, the potentiometer connected to the null offset pins of the amplifier stage was adjusted to obtain an average offset of approximately zero.
This process was repeated for both filters and both amplifiers. AC Testing. In order to test the filters and amplifiers, a controlled input from a signal generator was used. With a low frequency, it was expected to get minimal filtering, and with an amplified output. The results were successful; the signal output was still intact, and the overall gain measured in this case was measured to be This can be seen in the following figure:.
To test the filtering aspect of the circuit, a larger frequency was used. The output range is in the hundreds of millivolts order of magnitude, therefore the filter is successfully working. Note, however, the DC offset is noticeable at higher frequencies. How to add a low pass filter to this non-inverting amplifier circuit?
Ask Question. Asked 3 years, 1 month ago. Modified 3 years, 1 month ago. Viewed times. K H 3, 1 1 gold badge 9 9 silver badges 24 24 bronze badges. Add a comment. Sorted by: Reset to default. Highest score default Date modified newest first Date created oldest first. So you are saying I need to select a cut freq. And the filter order depends on how low or high I select the cut freq. Is that correct? It's 8th order and have a very sharp cut-off point.
Right now I have no LPF at the input and I've managed to get audio from Youtube and process it and it didn't sound that bad to be honest. I guess the only way to figure this out is by testing different Fcut points if not going "brick-wall".
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This low-pass non-inverting circuit amplifies the signal level by 20V/V (26dB) and filters the signal by setting the pole at 10kHz. The simplest form of a low pass active filter is to connect an inverting or non-inverting amplifier, the same as those discussed in the Op-amp tutorial. OP-AMP Filter Examples: The two examples below show how adding a capacitor can change a non-inverting amplifiers frequency response.