Electrical Engineering Dictionary
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## Active Low-Pass Filter

This circuit cover a second order low pass filter, using the LM324 as a possible operational amplifier. Any substitute amplifier could be used in it's place as long as the bandwidth of the device meets or exceeds the requirements of the circuit design.

The LM324 uses a single positive power supply up to 32 volts, with the other supply rail being grounded. The LM324 is also a quad package, but only a single Op Amp is being used here.

LM324 Low-Pass Filter

### Low-Pass Filter Design

This circuit design uses the standard non-inverting configuration, so the gain is: 1 + Rf/Ri, or [1 + R3/R4]. In this case the gain is set to two [Av = 2]. The gain was set to two only to show an example of a circuit with gain. However just by shorting out resistor R3, the circuit can be made into a unity gain amplifier or active filter. The resistor ratio could also be changed to provide a gain of other than two.

The filter shown is a second order low pass which means the signal is attenuated at 40dB/decade above the cutoff frequency. However by removing R1 and C1 from the circuit it becomes a first order low pass and will attenuate the signal at 20dB/decade above the cutoff frequency.

There is more than one method of coming of with the component values; for example, one uses a method of fixing the two capacitors at the same value, while another method uses a more simplistic calculation but ends up with capacitors of different values. However many of the more complex equations also configure the filter type: Bessel, Butterworth, or Chebyshev. The values presented here result in minimal peaking near the cutoff frequency, basically a Butterworth filter.

The equation represented here simply calculates the cutoff frequency with out regard to the filter type. In other words a Bessel filter would use different component values. The cutoff frequency [1kHz] is given by: 1 / (2 * 3.14) * ([1/R1*R2*C1*C2])1/2.

### LM324 Package Options

The LM324 is available in five different package options. Two 14-pin through-hole DIP versions are offered, in either a ceramic or plastic package. The photograph above shows a 14-pin plastic DIP package [indicated by the 'N' after the part number, LM324] produced by National Semiconductor [indicated by the symbol on the package, above the part number].

Surface mount versions of the LM324 include a 14-pin SOIC, a ceramic Flat-Pack, or a TSSOP package.

The pin out appears to be the same regardless of the package type. So at least for the through-hole variants, the plastic and ceramic versions may be traded out with one another. However; the surface mount versions have either different outer body dimensions or different pin lengths and may not be substituted for one another. The LM324 is a quad package device, meaning there are four operational amplifiers within the package. All pins are used on the 14-pin package, so the LM324 will not be found in another pin count package or a package with less than 14 pins.

The base part number will always indicate a LM324 regardless of the manufacturer. However each company uses their own packaging code, so the suffix will vary with manufacturer, even if the same package style is referred to. Of course an LM124 and LM224 are identical parts that operate to wider temperature ranges.