The difference is what is amplified and transferred to the output, not the actual voltages at the inputs. In a basic op-amp, the parameter measured is the difference in voltage between the two inputs.
Because the LM311 can operate from single rail supplies, and we are going to use it that way, this is how we’ll move forward. The basic comparator has five connections: Inverting input, non-inverting input, as well as an output, positive power supply, and negative power supply. While we very briefly recapped the operation of comparators, we’ll give a bit more detail here. Note: To learn more about op-amps, you can refer to our previous Classroom articles, including ‘Operational Amplifiers - Super Electronic Building Block’ from Issue 7, January 2018 or ‘Operational Amplifiers.Revisited’ from Issue 13, July 2018. While specifications should be the same for all manufacturers the location of information, and indeed the amount of information may vary. We used Texas Instruments’ LM111/LM211/LM311 document, revised March 2017. We always summarise the most relevant information for our maker audience, but there is always more in the datasheets. The outputs rated highly enough to be capable of driving some loads without additional amplification.Īs always, having a copy of the manufacturer’s datasheet for the IC is useful. The minimum operating voltage unfortunately rules out 3.3V operation. The LM311 is designed with versatility in mind as it can operate from a range of supply voltages, including +/- 15V DC, and +5V DC for logic circuits. Voltage comparators like the LM311 do this with high accuracy, fast response, and minimum external component count. The output will go high or low, depending on set-up, as the sensor voltage goes above or below the reference. The other is used to measure the target signal, such as the voltage on a sensor. Usually, this is done by using a voltage divider to produce a known voltage, and then feeding it to one of the inputs. Op-amps have many other modes of operation, but the comparator is the one relevant to this article. Op-amps can be arranged as a comparator, where one input is compared to the other and the output goes high or low depending on the situation in the inputs. In single rail situations, a synthetic ground is arranged. Op-amps are often driven from a positive and negative split rail power supply with a zero volt ground in the middle, allowing them to operate negatively or positively.
One input is inverting and the other is non-inverting, which means that a positive voltage on the inverting input will cause a negative voltage on the output, and a positive voltage on the non-inverting input causes a positive voltage on the output. Operational amplifiers are an IC with high-impedance inputs which means they are voltage-sensitive rather than current-sensitive. Of course, not every reader will be familiar with op-amps or previous articles, so here is a very brief recap. However, the LM311 Integrated Circuit (IC) is a dedicated device which does only this job, and does it well. We’ve previously come across the comparator before when discussing Operational Amplifiers (Op Amps), and we have even made them out of op-amps in projects. Comparing voltages or switching at a threshold can be done with the Analog to Digital Converter (ADC) built into Arduino and Raspberry Pi, but we will discuss later some very valid reasons why you may want to do this externally. This IC is a low-component-count way to compare two signal voltages or a signal and a reference, and switch the output either on or off.Ĭomparators do a very specific job that presents some advantages to the maker which may not be immediately obvious.
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