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How Fast is Your Amplifier?

Tue, 12/06/2011 - 4:53am
Kevin Tretter, Microchip Technology Inc., www.microchip.com

Kevin TretterThe world of industrial data acquisition is extremely diverse, encompassing measurements of temperature, pressure, humidity, strain and a variety of other physical phenomenon. Operational amplifiers are an integral part of the signal-conditioning circuitry typically found in industrial control and automation applications. These amplifiers provide fundamental building blocks that can be used for the buffering, amplification and/or filtering of signals. In addition, these measurements may be required under harsh conditions, such as extreme temperatures, centrifugal forces and electrically noisy environments. System designers must be diligent in designing the appropriate signal conditioning to deal with these environmental issues, and their operational amplifier’s speed should not be overlooked.

When it comes to operational amplifiers, the subject of speed has many facets. Perhaps bandwidth is the first specification that comes to mind. As in, ‘Does this particular amplifier have the bandwidth required for my application?’ While bandwidth is definitely a critical specification, when it comes to speed, an amplifier’s slew rate must also be taken into account.

The Slew Rate

An amplifier’s slew rate is the maximum rate of change for its output voltage, and is typically specified as V/µs. In order to reproduce a signal without distortion, the amplifier must be able to change the output just as fast (or faster) than the input signal (see Figure 1). The full power bandwidth of an amplifier is the maximum frequency at which the amplifier does not distort the signal, based on its limited slew rate. The equation can be shown as:

amplifier equation 1 new

Where: FPBW is the Full Power Bandwidth
Vp is the peak output swing of the amplifier

op amp speed figure 1 An Example Application

Let’s look at an example in which a designer needs to develop a circuit for monitoring the vibration of an industrial motor, as part of an early fault indicator. For this application, the designer determines that the signals of interest have a maximum frequency of 100 kHz. An operational amplifier is used to capture and amplify these vibration waveforms for further analysis. The amplifier is powered from a 3-V source and the output can swing rail-to-rail, so the peak voltage at the output of the amplifier is 1.5 V.

In terms of bandwidth, a general rule of thumb is to select an amplifier with a bandwidth that is an order of magnitude higher than the signal of interest (assuming unity gain, in this case). So, in this example, an amplifier with a bandwidth of 1 MHz should be just fine. Looking at Microchip’s portfolio of amplifiers, for example, the MCP6V01 seems like a good fit. It has a bandwidth of 1.3 MHz and its auto-zero architecture is well suited for the harsh, noisy environment in which this circuit will be located. But let’s take a look at its slew rate. The typical slew rate of the MCP6V01 is specified as 0.5 V/µs (500,000 V/s). Plugging these numbers into the earlier equation, the full power bandwidth is:

amplifier equation 2 new

This suggests that, at the maximum input frequency of 100 kHz, the MCP6V01 will be slew limited; hence causing distortion in the output signal. So, for this application, selecting a higher-slewing op amp (such as the MCP6V26, see Figure 2) would be the better choice, since it will not be slew limited — even with an input signal at 100 kHz.

amplifier speed fig 2If one were to look at a number of op amps, it may seem that the bandwidth and slew rate are directly related, because as the one goes higher, so does the other. In general, the slew rate of an amplifier is determined by its tail current divided by its compensation capacitor (part of the internal design of the amplifier). Higher-bandwidth op amps generally have higher tail currents, and thus have higher slew rates. However, it is possible to keep the same tail current while lessening the compensation capacitor. Although this will result in a higher slew rate, the trade off is reduced bandwidth, higher noise and higher voltage offset. There are other techniques and architectures that enable higher slew rates, but for general-purpose op amps, the bandwidth and slew rates tend to track each other.

Conclusion

There are many considerations that must be taken into account when designing signal-conditioning circuitry for industrial applications. Extreme temperatures, electrical noise and long signal runs are just a few of the factors that may come into play. In light of these design challenges, it is easy to overlook the speed requirements of the circuit. So, the next time you need to select an operational amplifier and are concerned about speed, be sure to consider the slew rate of the amplifier; not just its bandwidth. 

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