What does a simple triangle symbol mean?

As we will see, this can cause problems. In the analog world, triangles can represent operational amplifiers, comparators, or instrumentation amplifiers. You can use one of them to achieve the function of the other, but the system performance will not be optimal. This article will discuss the differences and the points that need attention so that we can avoid troubles when designing. We will see that in some cases, you simply don’t want to try to design with the wrong type of device.

Author: Harry Holt, Application Engineer Mike Skroch, Application Engineer

Do symbols help or hinder us from thinking about design?

Symbols are important, but what if one symbol can represent multiple things?

As we will see, this can cause problems. In the analog world, triangles can represent operational amplifiers, comparators, or instrumentation amplifiers. You can use one of them to achieve the function of the other, but the system performance will not be optimal. This article will discuss the differences and the points that need attention so that we can avoid troubles when designing. We will see that in some cases, you simply don’t want to try to design with the wrong type of device.

Looking at Figure 1, which triangle represents an op amp? Which triangle represents the comparator? Which triangle represents the instrumentation amplifier? Answer:

They are all!

What does a simple triangle symbol mean?
Figure 1. Operational amplifier, instrumentation amplifier, and comparator.

So, what is the difference between them and why should we care? It can be seen from Table 1 that certain characteristics are quite different, but what do they mean for circuits and systems?

Table 1. Comparison of operational amplifiers, comparators, and instrumentation amplifiers

Operational Amplifier

Comparators

Instrumentation amplifier

Feedback

burden

No/positive

internal

Open loop gain

5k to 10 million

3k to 50k

Fixed at 0.2 to 10k

Closed loop gain

usually

Fixed at 0.2 to 10k

Input capacitance

none

might have

good

Output

Analog/Linear

number

Analog/Linear

Important specifications

VOS, GBW/PM

Transmission delay

CMRR

Programming

R or C

none

R, SPI, jumper

Let’s take a look at how everyone got into trouble…

Feedback

Operational amplifiers have huge gains. The school teacher taught us that at the beginning of the analysis, the difference between the two inputs is equal to zero. But in real life, this is impossible. If the open loop gain is one million, then to get 5 V on the output, there must be 5 μV on the input. To make the circuit usable, we need to apply feedback. When the output is going to be too high, the control signal will be fed back to the input, canceling the original excitation—for example, negative feedback. When used as a comparator, if there is no feedback, the output will directly rush to one rail or the other. If it is positive feedback, the output will be driven farther in the same direction. Therefore, the op amp needs negative feedback.In fact, when some operational amplifiers are used as comparators without feedback, the supply current may be 5 to 10 times higher than the maximum value on the data sheet1.

However, for the comparator, 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. If there is noise in the system, such as ground bounce, the output may be reversed, which is of course undesirable in the control system. But then it began to look back, producing an oscillating behavior, sometimes called tremor (see MT-0832Figure 5). Reza Moghimi’s article “Eliminate Comparator Instability by Hysteresis” fully introduces the benefits of adding positive feedback (also called hysteresis)3.

What does a simple triangle symbol mean?
Figure 2. Classic three-op-amp instrumentation amplifier

For the instrumentation amplifier, the feedback is already internal, and adding feedback will only produce inaccurate gain. Figure 2 shows a typical method of using an operational amplifier to build an instrumentation amplifier.

Note: Each op amp has feedback. We start with the standard negative feedback diagram (see Figure 3), the instrumentation amplifier is G, and the expected gain is 10, which means that the feedback coefficient is 0.1. Next, choose a fixed gain of 100 for the instrumentation amplifier. Using Equation 1, the actual closed-loop gain will be 9.09, which is almost a 10% error. Therefore, it does not make sense to use an instrumentation amplifier as an operational amplifier and add feedback to it.

What does a simple triangle symbol mean?
Figure 3. Schematic diagram of classic feedback

What does a simple triangle symbol mean?

Operational amplifiers require negative feedback; comparators require positive feedback; instrumentation amplifiers do not require any feedback.

Open loop and closed loop gain

For op amps, see Equation 1, open loop gain (AVOLThe higher the ), the more accurate the closed-loop gain will be. The open loop gain of most op amps is between 100,000 and 10 million, but some earlier high-speed op amps may be as low as 3,000. As mentioned earlier, the higher the open-loop gain, the smaller the closed-loop gain error.

For the comparator, if the output logic swing is 3 V and you need a 1 mV threshold, the minimum gain must be 3000. A higher gain will make the uncertainty window smaller, but if the gain is too high, microvolt noise will trigger the comparator.

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

Input capacitance

Capacitors are often added to circuits to limit bandwidth. Examining Figure 4, at first glance R1 and C1 seem to form a low-pass filter. This will not work and may cause oscillations. The feedback coefficient of the inverting amplifier is R2/R1, but in Figure 4, the feedback coefficient is R2/(R1 // Xc). As the frequency increases, the feedback coefficient also increases, so the noise gain increases at a rate of +20 dB/10 octave, while the open loop gain of the op amp decreases at a rate of C20 dB/10 octave. They cross at 40 dB, which is bound to produce oscillations according to control system theory. The correct way to limit the bandwidth of the circuit is to place a capacitor across R2.

What does a simple triangle symbol mean?
Figure 4. Try to reduce the op amp bandwidth

The comparator usually does not have a negative feedback network, so the low-pass filter formed by the simple R and C in front of the comparator in Figure 5 works well. RHYSShould be much larger than R7, the two split the output swing to provide a small amount of positive feedback (hysteresis). If the comparator has built-in hysteresis, such as LTC6752 or ADCMP391, do not use R7 and RHYS.

What does a simple triangle symbol mean?
Figure 5. Comparator with LPF and hysteresis

For instrumentation amplifiers, it is completely acceptable to place capacitors at the input, as shown in C4 in Figure 6. ADI Instrumentation Guide4The graph in Chapter 5 shows a good thing to do every time you use an instrumentation amplifier. If you lay out the printed circuit board with appropriate traces and pads to allow the addition of two resistors and three capacitors, you can start with 0Ω resistors and no capacitors to measure system performance. By adjusting the values ​​of the five components, the common mode roll-off and the normal mode roll-off can be set separately (see the guide for details).

What does a simple triangle symbol mean?
Figure 6. The RFI filter is placed before the instrumentation amplifier

Output

The output of an operational amplifier or instrumentation amplifier will swing from close to one rail to another. Depending on whether the output stage uses a common emitter or a common source configuration, the output may reach the range of 25 mV to 200 mV of any supply rail. This is considered rail-to-rail output. If the operational amplifier is powered by +15 V and C15 V, it is inconvenient to interface with digital circuits. A poor solution is to place a diode clamp on the output to protect the digital input from damage. But instead, the operational amplifier is damaged due to excessive current. There are more complicated methods for interfacing operational amplifiers with digital logic, but why bother? Just use the comparator.

The comparator can have CMOS totem pole output, or have NPN or NMOS open collector or open drain output. Although open-collector or open-drain output requires a pull-up resistor, resulting in unequal rise and fall times, it has the following advantages: the comparator uses one voltage (such as 5 V) to supply power, and is connected to another voltage (such as 3.3 V). Logical interface.

Important specifications

The operational amplifier needs a gain bandwidth higher than the highest signal frequency to keep the closed loop error low. Looking at Equation 1, we know that the gain bandwidth should be 10 to 100 times the highest signal frequency. As mentioned earlier, it can be seen from Equation 1, that AVOLIt is a function of frequency and will affect the accuracy of the closed loop. The phase margin is also very important, it will change with capacitive load, so the specification sheet should clearly state the test conditions. To ensure DC accuracy, the offset voltage should be low. For adjusted bipolar operational amplifiers, 25 μV to 100 μV is better; for FET input operational amplifiers, 200 μV to 500 μV is better. Auto-zero/chopper/zero-drift op amps are almost always below 20 μV (maximum), which is in terms of the entire temperature range. Please refer to the data sheets of some typical op amps, such as ADA4077, ADA4084, ADA4622 or ADA4522.

Propagation delay is the key specification of the comparator. Op amps will slow down when they are overdriven, unlike the comparators, they will go faster when they are overdriven. The specification sheet sometimes provides transmission delays under a small amount of overdrive (for example, 5 mV), and different transmission delays under larger overdrives of 50 mV or even 100 mV.

The most important indicator of an instrumentation amplifier is the common-mode rejection ratio (CMRR), because the application needs to extract a very small differential-mode signal above a large common-mode voltage. Like many specifications, this specification varies with frequency and sometimes lists DC CMRR or CMRR at very low frequencies. A diagram of the relationship between CMRR and frequency is usually provided. For example, when detecting the current in an H-bridge motor driver, this graph will be very important, as shown in Figure 7.

This may be the most difficult application for instrumentation amplifiers, because the common-mode voltage changes from near one rail to close to the other, and the current reverses rapidly. Both gain bandwidth and slew rate are important.

Programming

Programming here does not mean writing code, it means configuring the device to meet system requirements (although some instrumentation amplifiers do have traditional software programming capabilities through SPI ports and registers).

The operational amplifier needs to be configured for negative feedback. This can be a purely resistive element, but usually resistors and capacitors are used in parallel to limit the bandwidth. This helps to improve the signal-to-noise ratio, because the noise will be integrated over the entire range, even if we only use part of it. It is also possible to use only capacitors to obtain an integrator or differentiator.

The comparator should always have a bit of positive feedback to ensure that once the input forces the output to move, the output will intensify the movement (see Figure 4 and Figure 5). See MT-083 for pictures and calculations. Some comparators have internal hysteresis, but more hysteresis can usually be added if needed. Some comparators with internal hysteresis have a pin to add a resistor to change the amount of hysteresis.

Operational amplifiers can be used as comparators, but this is not ideal. There are some things to note. You must be a good analog designer to do this well. MT-083 introduces some precautions, and there are many related articles discussing its pros and cons. If you are not afraid of danger, you can consult reference materials.

What does a simple triangle symbol mean?
Figure 7. Bidirectional current sensing with high common-mode swing

The comparator is almost always programmed with a resistor. You can add a high-value resistor to provide a bit of positive feedback, or you can use a capacitor to provide AC feedback to avoid increasing DC hysteresis. Some comparators have built-in hysteresis, but this can also be improved by adding a small amount of positive feedback.

Final note

When trying to use an op amp as a comparator, subtle things happen. Many low-noise bipolar operational amplifiers have anti-parallel diodes between their inputs. The input common-mode range of most comparators accounts for 80% or more of the total range. However, some low-noise bipolar op amps have one or two series diodes between the inputs. This is to prevent the formation of a Zener effect between the input stage and one of the emitter base junctions, causing the noise performance to degrade over time.

In a 3.3 V system, if a 5 V operational amplifier is used as a comparator and the threshold level of the power good indicator is 3 V, there will be a problem that one input is 3 V and the other is 0 V, because of these The diode limits the maximum differential voltage allowed at the input of the op amp.

Summarize

For many applications, the choice of op amp depends on whether the user focuses on DC accuracy, AC accuracy, input offset voltage, gain bandwidth, or supply voltage. By 2020, there will be more than 700 devices to choose from. The key parameters of the comparator are usually propagation delay and power supply voltage. It is relatively easy to choose, there are 122 devices to choose from. The main criterion for instrumentation amplifiers is the relationship between CMRR and frequency, but near DC, offset voltage and gain accuracy are also important. Since the instrumentation amplifier is a dedicated device, there are “only” 63 models to choose from.

Only by choosing the right device can products and designs that are trouble-free and mass-produced in the next few years can be realized.

references
1 Harry Holt. “Operational amplifier “maximum supply current” specifications”. Analog Devices, November 2011.
2 MT-083 tutorial: “Comparator”. Analog Devices, 2009.
3 Reza Moghimi. “Eliminate the instability of the comparator through hysteresis.” “Analog Dialogue”, Volume 34, Issue 7, November 2000.
4 “Instrumentation Amplifier Application Engineer’s Guide”, 3rd edition. Analog Devices, 2006.

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