In contrast, the op-amp goes high or low but not necessarily all the way to the power supply voltage. To do this, the comparator output goes quickly to V+ or V-, in other words, right to one of the rails. The logic output of a comparator may be TTL or CMOS-compatible, and this indicates which of the two inputs has a higher voltage level. There are, however, some important differences between them, and these determine when an op-amp can and when it cannot be used in a comparator application.īoth devices take note of the inputs in the same way, by ascertaining the relative values, but they form outputs differently. And both have low offset, high gain, and favorable common-mode rejection. So in view of the op-amp’s low cost and flexible packaging, why not use it wherever the more expensive comparator is suggested?īoth devices have inverting and non-inverting inputs and a single output that can go high or low. Both have differential inputs with a single output. If you look at comparator and operational amplifier (op-amp) schematics, you will see that externally, they are equivalent. If hysteresis is added, the comparator will not function within the revised hysteresis band. If the internal hysteresis is not present or is too low for the intended application, an external hysteresis network can be provided, comprised of positive feedback from the output to the non-inverting input. Contemporary comparators have a built-in hysteresis in the low-millivolt range. Rather than one transition, there are two, one for rising voltages and one for falling voltages. This is because of small voltage fluctuations caused by noise at the inputs, within the device, and imposed by loading variations. Just like a residential furnace or hot water heater, a comparator circuit must incorporate hysteresis to prevent uncontrolled cycling at transitions. In null detectors, the comparator identifies the point in time when an input is zero volts. Zero-crossing detectors use a comparator to detect when an ac pulse changes polarity, so the comparator outputs a square wave. Relaxation oscillators use both positive feedback with a Schmidt trigger configuration and negative feedback contributed by an RC circuit so the comparator becomes an astable multivibrator and thus a viable oscillator. This operation in conjunction with other devices performs multi-bit quantization as in a digital sampling oscilloscope.
In ADCs, a comparator functions as a digitizer when it ascertains whether a voltage at the input is above or below a predetermined threshold. The advantage in this arrangement is that power requirement is lower and accuracy is higher.Ĭomparators are widely used in numerous applications. (In many applications speed is of the essence.) Clocked (also called latched or dynamic) comparators are used when outputs are required only at discrete time intervals, as in analog/digital converters (ADCs). When the input status changes, the output updates more or less instantly. A continuous comparator reads the inputs and reports the results in real time. ICs are also available with a range of reference voltages.Ĭomparators may be continuous or clocked. In many devices, the reference voltage is integrated onto the chip. Typically, a comparator looks at a variable voltage in comparison to a stable reference. The usual implementation is with two inputs for the voltages that are to be compared and a single output that by mean of a logic high or logic low state indicates the higher of the two inputs. A comparator, as its name suggests, compares two voltages or currents and determines which is higher.
0 kommentar(er)
