Have you noticed that in analog circuit diagrams, whether it is an operational amplifier, comparator, or instrumentation amplifier, engineers will use the same pattern to express it (see Figure 1 below).

Figure 1: Circuit diagram symbol for an operational amplifier, instrumentation amplifier, or comparator at the same time

If we see a "triangle" device in a chip specification, does it mean that it can be applied anywhere when selecting materials? In theory, yes. You can force one of them to implement other functions, but the system performance will not be optimal. Therefore, the original manufacturer generally lists the recommended applications of its devices in the specification.

In this article, let’s look at the differences between them and what to look out for when selecting an application so that we can design around them as much as possible, while also gaining insight into how to use parameter screening to find the right op amp.

The differences in the key parameters or specifications of the three devices are summarized in Table 1 below.

Table 1: Comparison of operational amplifiers, comparators, and instrumentation amplifiers (Source: ADI)

Let's first look at what can be achieved in "op amp" applications. Since op amps have huge gain, theoretically, we need to apply feedback to make the circuit usable. When the output is about to get too high, the control signal will be fed back to the input to offset the original excitation, which is the need for "negative feedback." If the op amp is deliberately designed as a comparator, when it works at high speed, in order to prevent the output from rushing directly to one rail or the other, "negative feedback" is needed.

However, for comparator applications, positive feedback is what we need. Without feedback, if one input of the comparator slowly exceeds the level of the other input, the output will begin to change slowly. But if there is noise in the system, such as ground bounce, the output may be affected, so adding positive feedback to the comparator can make the comparator react more slowly, causing hysteresis, making it insensitive to small changes, which helps with anti-interference ability.

In contrast, the instrumentation amplifier itself already has feedback internally, so it does not make sense to use an instrumentation amplifier as an op amp and add feedback to it. Take the AD621 series of instrumentation amplifiers from Analog Devices as an example. Figure 2 shows that the device already contains three op amps.

Figure 2: AD621 series “three op amp” instrumentation amplifier (Image source: ADI)

The operational amplifier's feedback equation is:

Using (1), assuming the instrumentation amplifier is G, the desired gain is 10, and if negative feedback is connected, this means the feedback factor is 0.1. Next, the instrumentation amplifier is selected to have a fixed gain of 100, and the actual closed-loop gain will be 9.09, which is almost 10% error.

• Component Selection

To help engineers quickly find and access technical information for operational amplifiers, comparators or instrumentation amplifiers, Digi-Key's official website lists the types of corresponding components in its product catalog.

Figure 3. Op amps, comparators, and instrumentation amplifiers as categorized in the Digi-Key product catalog.

As mentioned above, for an operational amplifier, referring to equation (1), the higher the open-loop gain (AVOL), the more accurate the closed-loop gain and the smaller the error.

Comparators are used in open-loop systems to drive logic circuits from their outputs. If the output has a 3V logic swing and a 1mV threshold (threshold) is required, the minimum gain may be around 3000. In addition to considering the bandwidth requirements of the system, noise also needs to be addressed. Higher gains can reduce the uncertainty window, but if the gain is too high, microvolt-level noise will trigger the comparator.

For instrumentation amplifiers, the concept of open-loop gain does not apply.

• Component Selection

Digi-Key's official website has clearly listed the bandwidth parameters in the operational amplifier screening table, which shows that its coverage is very wide, so that when engineers choose these three products, they can have a general understanding of them. For example, -3db bandwidth, if you ignore the instrumentation amplifier and comparator options, since operational amplifiers include many types, you can see that -3db bandwidth covers a wide frequency range, up to tens of GHz.

Figure 4: The "-3db Bandwidth" option on the Digi-Key website

As mentioned above, for the op amp, let's look at the way to add input capacitors in Figure 5 below. At first glance, R1 and C1 seem to form a low-pass filter, but this is not feasible and may cause oscillation. The feedback factor is R2/R1 without adding capacitors, but if the capacitor is added between the two, the feedback factor will become R2/(R1 // Xc). As the frequency increases, the feedback factor will also increase, so the noise gain increases at a rate of + 20dB/10 octaves, while the op amp open-loop gain decreases at a rate of –20 dB/10 octaves. They will cross at 40dB and meet, which will cause oscillation.

Figure 5: One approach to “trying” to reduce op amp bandwidth (Image source: ADI)

A more feasible method to limit the circuit bandwidth is to place a capacitor (C2) across R2, as shown in Figure 6.

In contrast, comparators usually do not have a negative feedback network, so a simple R and C low-pass filter in front of the comparator in Figure 7 is feasible.

Figure 7: A simple R and C low-pass filter in front of the comparator (Image source: ADI)

For instrumentation amplifiers, it is perfectly acceptable to place capacitors at the input, if the circuit allows for additional components and proper layout and routing on the board. For example, the AD8220 from ADI in the figure below is a widely used differential radio frequency interference (RFI) filter application circuit diagram, which also adds capacitors to stabilize the system and improve performance.

Figure 8: AD8220 "RFI filter" application circuit diagram (Image source: ADI)

In addition to RFI suppression, this filter provides additional input overload protection, as resistors R5 and R6 help isolate the instrumentation amplifier's input circuitry from external signal sources. But the only thing to consider is that the filter forms a bridge circuit, whose output appears at the instrumentation amplifier's input pins. Therefore, any mismatch between the time constants of C3/R5 and C5/R6 will unbalance the bridge and reduce high-frequency common-mode rejection. Therefore, resistors R5 and R6 and capacitors C3 and C5 need to be equal.

In applications, the output of an op amp or instrumentation amplifier will swing from close to one rail to the other. Depending on whether the output stage uses a common emitter or common source configuration, the output may reach a range of 25mV to 200mV within any power rail. However, if the op amp is powered by +15V and -15V, this type of rail-to-rail is not convenient for interfacing with digital circuits. A poorer and more complicated method is to place a diode clamp at the output to protect the digital input from damage, but the op amp will also be damaged by excessive current, so using a comparator is the simplest solution.

Comparators can have different output types, such as CMOS, TLL, NMOS, or open-drain outputs. Although an open-collector or open-drain output requires a pull-up resistor, resulting in unequal rise and fall times, it is desirable that it can be powered at one voltage (such as 5V) but still operate with logic interfaces at other voltages (such as 3.3V).

• Component Selection

The output of an operational amplifier, comparator, or instrumentation amplifier (including output type and current output) is very important for circuit design. The product screening items on the Digi-Key website are also listed in detail.

Figure 9: Output options for operational amplifiers and instrumentation amplifiers on Digi-Key’s website

Figure 10: Comparator "Output" option displayed on Digi-Key's official website

For the op amp, we need a gain bandwidth that is higher than the highest signal frequency to keep the closed-loop error low. Looking at equation (1) above, we can see that the gain bandwidth should be 10 to 100 times the highest signal frequency. From equation (1), we can see that A _VOL is a function of frequency, as mentioned before, which affects the closed-loop accuracy.

• Phase Margin

Phase margin varies with capacitive loads, so the data sheet should clearly state the test conditions. To ensure DC accuracy, the offset voltage should be low. For trimmed bipolar op amps, 25μV to 100μV is good. For FET input op amps, 200μV to 500μV is good.

• Transmission delay

For comparators, propagation delay is one of the important parameters. Compared with op amps, op amps become slower when overdriven, but comparators become faster when overdriven. The data sheet sometimes provides propagation delay under a small amount of overdrive (such as 5 mV), or provides different propagation delay data under a larger 50mV or even 100mV overdrive.

• Common Mode Rejection Ratio (CMRR)

An important parameter of an instrumentation amplifier is the common-mode rejection ratio (CMRR). CMRR is the ratio of differential gain to common-mode gain and is expressed in V/V or dB according to the following formula.

Common-mode rejection ratio varies with frequency, as shown in Figure 11 below for the AD8422 common-mode rejection ratio vs. frequency. In addition, sometimes the data sheet will also list the DC CMRR or the CMRR at very low frequencies.

If you are sensing current in an H-bridge motor driver, or using an instrumentation amplifier such as the AD8207 for bidirectional common-mode, high-swing current sensing, use the drive application diagram shown in Figure 11.

Figure 12: Bidirectional common-mode high-swing current sensing using the AD8207 (Image source: ADI)

This is probably the most difficult application for an instrumentation amplifier, because the common-mode voltage changes from near one rail to near the other, and the current reverses rapidly. Gain-bandwidth and slew rate are both important.

"Programming" here does not mean writing code, but rather configuring the device to meet the system requirements (although some instrumentation amplifiers do have traditional software programming capabilities with SPI ports and registers). For op amps, we configure the device for negative feedback. This can use purely resistive elements, but typically resistors are used in parallel with capacitors to limit the bandwidth. This will help improve the signal-to-noise ratio because even if we only use a portion of the noise, it will be integrated over the entire range, and different capacitors can be used to obtain an integrator or differentiator.

Comparators should be designed with positive feedback to ensure that once the input forces the output to move, the output will move. Some comparators do have internal hysteresis, but you can usually add more hysteresis if you want. Some comparators with internal hysteresis have a pin for adding a resistor to slightly change the amount of hysteresis.

Engineers can use op amps as comparators, but this is not ideal and, as mentioned earlier, there are many things to be aware of. The benefit of using a comparator directly is that it requires almost no programming except resistors. You can add a high-value resistor to provide a little positive feedback.

Summarize

In the past, when engineers selected operational amplifiers, they would focus on individual characteristics, such as whether specifications such as DC and AC accuracy, input offset voltage, and gain bandwidth are suitable for the system. However, there are many operational amplifier products today. In this case, many manufacturers have classified operational amplifier products according to different applications, such as the comparators and instrumentation amplifiers introduced in the article. This helps to narrow the target range when selecting materials. In addition, the Digi-Key official website provides clear classification and parameter screening tools, which allows engineers to find suitable operational amplifiers more accurately and save time.

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