Performance indicators of the comparator

1, hysteresis voltage

changes, because the input terminal is often superimposed with a small fluctuation voltage, the differential mode voltage generated by these fluctuations will cause the comparator output to change continuously, in order to avoid output oscillation, the new type of comparator usually has a hysteresis voltage of several mV. The existence of the hysteresis voltage makes the switching points of the comparator become two: one is used to detect the rising voltage, the other is used to detect the falling voltage, the difference of the voltage threshold (VTRIP) is equal to the hysteresis voltage (VHYST), the hysteresis comparator The offset voltage is the average of TRIP and VTRIP-. The input voltage switching point of the comparator without hysteresis is the input offset voltage, not the zero voltage of the ideal comparator. The offset voltage generally varies with temperature and power supply voltage. The power supply rejection ratio is usually used to express the influence of power supply voltage changes on the offset voltage.

2, bias current

The input impedance of an ideal comparator is infinite, so theoretically, it has no effect on the input signal, but the input impedance of the actual comparator cannot be infinite. There is a current at the input end that flows through the internal resistance of the signal source and flows into the comparator, resulting in Additional pressure difference. The bias current (Ibias) is defined as the median of the input currents of the two comparators and is used to measure the effect of input impedance. The maximum bias current of the MAX917 series of comparators is only 2nA.

3, super power swing

In order to further optimize the operating voltage range of the comparator, Maxim uses the parallel structure of the NPN tube and the PNP tube as the input stage of the comparator, so that the input voltage of the comparator can be expanded, so that the lower limit can be as low as the lowest level. The upper limit is 250mV higher than the power supply voltage, thus reaching the Beyond-theRail standard. The input terminal of this kind of comparator allows a larger common-mode voltage.

4, drain-source voltage

Since the comparator has only two different output states (zero level or power supply voltage), and the output stage of the comparator with full power swing characteristics is an emitter follower, this makes its input and output signals only extremely small The pressure difference. This voltage difference depends on the emitter junction voltage in the saturation state of the internal transistor of the comparator, which corresponds to the drain-source voltage of the MOSFFET.

5, output delay time

includes the transmission delay caused 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 4.5ns and the rise time is 2.3ns. Need to pay attention to the influence of different factors on the delay time when designing, including the influence of temperature, capacitive load, input overdrive and so on.

Classification of comparators

1, zero-crossing voltage comparator

Typical amplitude comparison circuit, its circuit diagram and transmission characteristic curve are shown in the figure.

2, voltage comparator

Change an input terminal of the zero-crossing comparator from ground to a fixed voltage value to obtain a voltage comparator.

3, window comparator

The circuit consists of two amplitude comparators and some diodes and resistors. When the potential level of the high-level signal is higher than a certain specified value VH, it is equivalent to the positive saturation output of the comparator circuit. When the potential level of the low-level signal is lower than a certain specified value VL, it is equivalent to the negative saturation output of the comparator circuit. The comparator has two thresholds, and the transmission characteristic curve is window-shaped, so it is called a window comparator.

4, hysteresis comparator

Lead a resistor divider branch from the output to the non-inverting input. When the input voltage vI gradually increases from zero and VI is less than VT, the comparator output is a positive saturation voltage, and VT is called the upper threshold (trigger) level. When the input voltage VI "VT', the comparator output is a negative saturation voltage, and VT' is called the lower threshold (trigger) level.