Serie Precision labs: amplificadores de instrumentación

Hello, and welcome to the lecture for the TI Precision Lab discussing instrumentation amplifiers. Specifically, what is an instrumentation amplifier and what does it do?

An instrumentation amplifier, or IA, can be idealized as an electrical circuit that amplifies the signals that differ between the inputs and rejects the signals that are the same on both inputs. We will first discuss the first characteristic, the ability of an IA to amplify the signals that differ between the two inputs.

Before delving into this function, though, let us first define the various pins on an IA. The inputs are n minus and n plus. N minus means it has inverting gain to the output. N plus has non-inverting gain to the output. Either pin can be positive or negative voltages and be greater or less than each other.

VS Plus and VS minus are the power supply pins that can be dual supply like plus minus five volts, or single supply like ground to five volts, or asymmetrical supplies, like minus five volts to plus 10 volts. The V out pin is the output of the amplifier. It's important to note that this output pattern is with respect to a reference pin, labeled rough. The rough pin is used to level shift the output of the amplifier. A typical reference voltage for level shifting is mid-supply, as this provides the widest dynamic range. For the rest of this presentation, Ref will be set to ground so the output will not be level shifted.

Now let's discuss this operation of amplifying the signals that differ between both inputs. In basic terms, an instrumentation amplifier amplifies the voltage difference between its two inputs. Its output is a single-ended signal proportional to the voltage difference between its two inputs given with respect to a set reference voltage. In this example, we have a generic IA. The inputs are set to generic voltages V1 and V2. The difference between these two inputs is simply V2 minus V1, and we refer to this as the differential input voltage, VD.

The gain of the circuit or differential gain AD may be set externally, or sometimes internally fixed to the IA. The instrumentation amplifier output will be equal to the product of the differential gain and the differential input voltage plus the reference voltage. Remember that we are going to set this reference voltage to zero volts for the remainder of the video.

In addition to amplifying the differential signal, IAs also reject the signals which are common to both inputs. We will now discuss what this means.

Let us examine now a case where the inputs are equal to each other and both are tied to a common mode input voltage, or VCM. By the virtue of differential input voltage amplification, we expect to see 0 zero at the output. In other words, if the two input terminals are tied together to a common mode input voltage, then there is no input voltage difference, so VD is zero volts and the expected output is zero volts.

But in actuality, there will be some small voltage at the output. This is called the common mode output voltage, or the VOCM. VOCM is usually much less than the applied input voltage, so the ratio of the common mode output voltage to the common mode input voltage is much less than one. This ratio is called the common mode gain, or attenuation of the instrumentation amplifier or ACM.

For an ideal instrumentation amplifier, ACM is equal to zero. In a later practical example, we will see how you can use a product data sheet to find the practical value for this gain. When the IA inputs are tied together to a common mode voltage, the experienced output voltage, VOCM, could be due to common mode noise sources. Examples of sources of noise signals common to both input terminals can be 50 to 60 Hertz, power line noise, or other signals capacitively coupled in from adjacent conductors. These are common mode input signals, an error that we don't want to see at the output of the amplifier.

To recap our original circuit, we have a differential input voltage equal to V2 minus V1, and the differential gain AD. Again, for an ideal IA, we expect the output to be equal to the differential gain AD multiplied by the differential input voltage, BD. However, because of the common mode input signal dependence discussed previously, a non-ideal IA will exhibit some degree of output voltage error. This error, the common mode output voltage, is equal to the common mode gain multiplied by the common mode input signal.

The goal of any instrumentation amplifier is to make this common mode output voltage error term as small as possible. Therefore, a large common mode rejection ratio, or CMRR, is desired. This CMRR parameter measures how well the amplifier is able to suppress the common mode input signal and is defined as the ratio of the differential gain to the common mode gain. The equivalent value of CMRR (dB) can be calculated as 20 times log base 10 of the differential gain to the common mode gain ratio. In a future video, we will cover CMRR in more depth with a practice example.

In real world applications, there is often a large unwanted signal common to both inputs of the amplifier. In some instances, the unwanted amplitude may exceed the amplitude of the signal itself. Historically, a number of methods have been developed to increase CMRR and thus reduce the associated common mode error. The wires to the two input terminals can be twisted and shielded to suppress the common mode error, for example.

Ultimately, engineers devised the instrumentation amplifier to achieve high rejection of noise that is common to both inputs while also achieving excellent amplification of the differential input signal.

That concludes our first video discussing what an IA is and what an IA does. Please try the quiz to check your understanding of this video's content.