Voltage comparator - a unit that performs the comparison of the available voltage level to the reference signal. The answer, usually, it is a binary value - yes or no, zero or one.
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The comparator compares the two voltages, hence the name. If necessary, a conditioned signal is formed as a binary code or sign issued difference otherwise:
- The steep voltage drop (front or decline).
- Pulse with desired characteristics.
- Changing the polarity of the output voltage.
- Binary code in the system logic of the chipset.
Comparator geographically part of the analog-to-digital converter, the method is applied separately. The element accuracy depends directly, as well as on the bit. The characteristics of the comparator include:
- Load capacity.
- Input resistance and so on.
Most comparators implemented on the basis of operational amplifiers, data directories are common. This is achieved by the introduction of feedback that coined in the 30s of XX century.
Sensitivity refers to the minimum voltage comparator, fit for perception. Differential pairs of transistors used in the operational amplifiers, increased temperature stability, therefore serve to create comparators. Parameter closely related to the resolution or precision. The sensitivity strongly depends on the circuit design, it is an obvious fact.
In addition to thermal stability and architecture for a parameter affecting the reliability and noise immunity. In practice, the optimal sensitivity say, equal to half the discharge analog-to-digital converter. This means that because the comparator is not reduced measurement accuracy. At the present stage of technological development is often very different values.
Speed digital technology is great, but considering the fact that the transmitter need to get done sample clock frequency of the processor must be hundreds, if not thousands of times higher than the discrete counts. And the main limiting factor is the speed characteristics of the comparator. At its second input at the time of measuring the reference voltage gradually increases until reaching a match. And generates the digital code result.
The sampling rate is determined by the process speed was investigated. If this sound range, the values start from 45 kHz and can be four times more for studio recording. At each time interval the comparator must have time to compare the voltage, the lowest frequency for accuracy in 0.5% lies already in the region of 10 MHz. In practice, there are much larger quantities, but remember, the main bus of the motherboard becomes the fastest section of the system unit (PC).
comparator performance expressed time between adjacent measurements. It consists of increasing the interval compares the voltage to a desired level and the speed of the electronic components. The latter figures include the period from the decision comparator for issuing the signal pulse to its actual appearance on the findings. The second parameter is considered the edge steepness, as the logic chip set to the threshold. It is considered an important recovery time for which the comparator returns to its original state.
These parameters determine the amount of the clock frequency of the comparator. Under loading capacity refers to the ability to issue a signal powerful enough to trigger dependent schemes. Distinguish so-called overload capability, showing how large the difference in times at adjacent reference voltage. To shorten the measurement interval, starting from the second comparator can carry two parallel measuring process:
- Increasing the voltage in comparison to the previous readout.
- Voltage reduction compared to the previous readout.
So it will be able to quickly find the result without going through the entire range of from the beginning. Although it will take as much as two parallel comparator included. But the savings in time costs of this struggle. The success of such an event directly affects the overload capability.
The input resistance forms a resistive divider signal source, and the smaller it is, the higher the accuracy, the majority of voltage drops here. With an increase in the parameter and decreasing consumption current. Most comparator input impedance adapts specifically taken need for separate circuits.
varieties of comparators
Most of the comparators is based on schemes of operational amplifiers, covered by a chain of positive feedback. Due to the high gain is possible to achieve vertical transfer stage functions.
Characteristics of the operational amplifier in a kind of plot is linear. The graph is symmetrical with respect to zero. At a certain value Uogr saturate and the output voltage does not grow further. This is observed in the positive range of input values and negative. The described property is used for the construction of the comparators.
The operational amplifier is covered by a positive connection, with a coefficient of its transmission coefficient is inversely proportional to the transfer of the operational amplifier, the formula goes to infinity. From this parameter depends on the steepness of the graph, it becomes vertical. What is required in practice for comparing voltages.
Standard any value is allowed. For example, a possible circuit implementation of the voltage transition through zero. But as a part of the analog-digital converter measures the value within the range considered to be constant, the reference voltage increases until it equals. And at this moment produced coincidence pulse.
a threshold comparator
The threshold voltage comparator - mentioned in the literature. The transfer characteristic of his straightforward - when the difference at the inputs of the operational amplifier becomes zero, there is a response at the output. The reverse movement along the transfer characteristic goes by the same path.
It is organized as described above: an operational amplifier feedback loop is covered for a steep, vertical transfer characteristic. But remains some small error. The reference voltage supplied to the accepted non-inverting input.
Hysteretic comparator was named for the fact that the transmission ratio of the feedback loop varies in absolute value and sign. The result is a family of transfer characteristics, allows to create the comparator comprises a single value of voltage and is switched off otherwise.
The apparatus is useful in the case of high-frequency noise on the line. And when the value is changed repeatedly at a predetermined measurement interval, a conventional voltage comparator easily miss. At the same time hysteresis accurately estimated with an accuracy of interference and would keep the output until the process under study is close to the standard.
Any real hysteretic comparator is considered because of the error, in certain types of specially have an extended loop described in connection with nuances. Pronounced rectangular characteristic is characterized by a Schmitt trigger. Its hysteretic transfer function can be used to construct a comparator. Due to the presence of a positive feedback characteristic of the Schmitt trigger has appreciable slope.
Even for analog circuits sensitivity threshold is 5-10 mV, which is enough in most cases. Since the Schmitt trigger trip time is reduced to 0.1 ms, it becomes possible to process a frequency signal evaluation of hundreds of kHz (much higher ultrasound). Presented in Figure trigger has a large temperature drift and small measurement range.
Because of the simplicity popular balanced regenerative circuit with diodes. Feedback is accomplished through a transformer. By using the average operating point it is possible to simultaneously generate both positive and negative feedback. The compared voltages are applied to the cathodes of diodes (n-region in which the area drawn perpendicular line). Working point of the transistor is chosen at the beginning of the current-voltage characteristics, the base current is calculated so that no saturation has occurred.
Condenser performs galvanic isolation of the base and the input circuit. If the diode D1 is locked, and D2 - open, work negative feedback. As a result, the generation does not occur. Otherwise the blocking oscillator produces the first pulse. His positive edge indicates that the standard on par with the estimated value. Sensitivity balanced regenerative circuit can reach 1 mV.
Comparators tunnel diodes nice small size, excellent performance, low noise, low power switching thresholds. Mechanical strength and resistance are well-known semiconductors. Tunnel diodes are rare devices, not fearful of radiation, which makes them popular for special applications. In addition, the resistance of these comparators is extremely small, which reduces the sensitivity.
Characteristic of a tunnel diode includes a portion with negative differential resistance, which allows to realize the desired transfer function. The obvious disadvantage circuit becomes low accuracy. The current-voltage characteristic of the tunnel diode is too shallow. But the simplicity of the comparator can not be compared with any other type devices. It can not be called a hysteresis, for receiving this type of characteristics required at least two tunnel diode.
The simplest comparator
With two tunnel diodes can easily construct a simple comparator, including the scheme twin. It is assumed that the items are identical. The transfer characteristic of the system is strongly dependent on the supply voltage circuit. Features easily vary, resulting in greater flexibility. The sensitivity of the measured current and the experimentally obtained values lie in the range 8 mA at a frequency of 200 MHz clock and 3 microamps - at 50 MHz.
The division on the principle of action
Apart from the purely functional characteristics discussed above, the comparators are divided according to the principle of action on:
It is about generated outputs of the devices. In the voltage comparator emit two processes: comparison variables and forming the output signal. Static error is caused only by two reasons:
- Temperature drift and aging.