Reasonable selection and application of comparators

For a long time, the application of comparators has been impacted by operational amplifiers. Until now, with the continuous improvement of comparator performance indicators, this situation has been improved. This article mainly introduces the performance of new comparators and their typical applications.

The two inputs of the comparator are analog signals, and the output is a binary signal. When the difference in input voltage increases or decreases, its output remains constant. Therefore, it can also be thought of as a 1-bit analog-to-digital converter (ADC). In principle, an operational amplifier can be used as a comparator without negative feedback, but because the open-loop gain of the operational amplifier is very high, it can only handle signals with very small input differential voltages. Moreover, in general, the delay time of operational amplifiers is long and cannot meet actual needs. The comparator is tuned to provide very small time delays, but its frequency response characteristics are somewhat limited. To avoid output oscillations, many comparators also have internal hysteresis circuitry. Therefore, the comparator cannot be used as an operational amplifier. This is also one of the main principles why operational amplifiers have a relatively wide range of applications.

1 Comparator supply voltage

Traditional comparators require ±15V dual power supply or single power supply up to 36V. These products are still in demand in industrial control, and many manufacturers are still providing such products. However, judging from the market development trend, most current applications require the comparator to operate within the single power supply voltage range allowed by the battery voltage. Moreover, the comparator must have low current and small package. Some applications also require the comparator to have relevant break function. For example: the MAX919 comparator can operate in the voltage range of 1.8V to 5.5V, and the maximum sink current in the full temperature range is only 1.2μA. It is packaged in SOT23. The similar MAX965 comparator can operate with a voltage as low as 1.6V, making it very suitable For battery-powered portable products.

2 Main performance indicators of comparator

The voltage between the two input terminals of the comparator will change the output state when it crosses zero. Since the input terminals are often superimposed with small fluctuation voltages, the differential mode voltage generated by these fluctuations will cause the comparator output to change continuously, as To avoid output oscillations, newer comparators typically have a hysteresis voltage of several mV. The existence of hysteresis voltage changes the switching points of the comparator into two: one for detecting the rising voltage, and one for detecting the rising voltage. The difference between the voltage threshold (VTRIP-) is equal to the hysteresis voltage (VHYST). The offset voltage of the hysteresis comparator is TRIP+ and the average value of VTRIP-. The input voltage switching point of a comparator without hysteresis is the input offset voltage, not the zero voltage of an ideal comparator. The offset voltage generally changes with changes in temperature and power supply voltage. The power supply rejection ratio is usually used to represent the impact of power supply voltage changes on the swapping voltage.

The input impedance of an ideal comparator is infinite. Therefore, in theory, it has no effect on the input signal. However, the input impedance of an actual comparator cannot be infinite. There is a current at the input end that passes through the internal resistance of the signal source and flows into the comparator, resulting in additional pressure differential. The bias current (Ibias) is defined as the median of the two comparator input currents and is a measure of the effect of input impedance. The maximum bias current of the MAX917 family of comparators is only 2nA.

In order to further optimize the operating voltage range of the comparator, Maxim uses a structure in which NPN tubes and PNP tubes are connected in parallel as the input stage of the comparator, thereby expanding the input voltage of the comparator. In this way, its lower limit can be as low as the lowest level. The upper limit is 250mV higher than the supply voltage, thus reaching the so-called Beyond-the-Rail standard. This comparator allows large common-mode voltages at its input.

Since the comparator has only two different output states (zero level or supply voltage), and the output stage of the comparator with full supply rail characteristics is an emitter follower, its input and output signals have only minimal pressure difference. This voltage difference depends on the emitter junction voltage of the comparator's internal transistor in the saturated state, corresponding to the drain-source voltage of the MOSFFET.

The output delay time is a key parameter for selecting a comparator. The delay time includes the transmission delay generated by the signal passing through the components and the rise time and fall time of the signal. For high-speed comparators, such as MAX961, the typical value of the delay time can reach 4.5 ns, with a rise time of 2.3ns. When designing, you need to pay attention to the impact of different factors on the delay time, including the impact of temperature, capacitive load, input overdrive, etc.

Some applications require a tradeoff between comparator speed and power consumption, and Maxim offers a variety of chip types to choose from, ranging from the MAX919, which consumes 800nA and has a delay time of 30μs, to the MAX919, which consumes 6μA and has a delay time of 540ns. From the MAX9075, which consumes 600μA and has a delay time of 20ns, to the package which consumes 11mA and has a delay time of 4.5ns, the delay time is as low as 5ns and the power supply current is only 900μA, thus providing more choices for product design.

3 Typical Comparator

Comparators are usually used to compare two input voltages. One input voltage is a fixed value and the other input is a changing quantity. To meet the needs of this application, Maxim integrates the reference source and the comparator into the same chip. This not only It saves space and consumes less power than an external reference. For example, the maximum current consumption of the MAX918 in the full temperature range is only 1.6μA (including its internal quasi-source). Taking into account changes in ambient temperature and the type of reference source, the accuracy of the integrated reference source is generally in the range of 1% to 4%. For applications with higher accuracy requirements, you can consider choosing the MAX9040 series of products. The initial accuracy of its built-in reference source can reach 0.4$ and the maximum temperature drift is 30ppm/℃.

The dual comparators MAX923 and MAX933 and the open-drain output MAX973 and MAX983 are very suitable for window function comparator applications. The internal reference can be connected to the non-inverting input terminal or the inverting input terminal, and three external resistors are used to set the overvoltage and undervoltage. Press the threshold. In addition, these chips also contain hysteretic input pins,Two external voltage divider resistors are connected to this pin to set the hysteresis voltage threshold. For ease of use, some comparators also provide two outputs, non-inverting and inverting.

4 Typical applications

Figure 2 is a level converter that can complete the conversion from 3V logic to 5V logic. The open-drain output comparator MAX986 provides an extremely simple implementation solution. Similarly, if the comparator supply voltage allows (such as MAX972), it can also be Realize level conversion from ±5V bipolar logic to +3V unipolar logic. In specific applications, care should be taken that the input signal does not exceed the swing of the power supply voltage, and the current flowing into the output terminal is limited by a large value pull-up resistor.

The specific circuit for processing bipolar signals using a single power supply comparator is shown in Figure 3. This circuit can convert the bipolar input sine wave into a unipolar square wave output. The external bias voltage is:

Vos=[(VccR1R2+V2R1R3)/R1R2+R1R3+R2R3]

In the formula: V2 is peak-peak value. Two resistors of the same value (R4) set the comparator switching detection threshold at half the supply voltage.

Figure 4 shows a current detection circuit using four comparators, which can be used to indicate the four states of the input current. The resistor "Shunt" is used to convert the input current into a voltage signal, and R1 and R2 are used to set the operational amplifier. gain and provides the required reference voltage for the comparator. R4~R7 can be used to set the detection thresholds corresponding to different digital output states.