Understanding precision comparator applications (Part 2 of 2)

Editor's note : Part 1 looked at basic operation and configurations; you can read it here

The preceding discussion highlighted several significant error sources that affect the basic comparator with external hysteresis. For some comparator applications, the accuracy of the state-transition voltage is not critical, but there are a wide variety of applications that would benefit from an accurate, easily controlled state-transition voltage. One such class of applications is in “dosing” applications, where “dose” is the integral of the rate. For example, if a pipe has one gallon per minute of a liquid flowing though it, the dose, or total volume of liquid in a specific time interval, is the sum, or integral, of the flow rate over the period of interest.

A specific application for this example is medical x-ray dosimetry, which is used to control x-ray film exposure. The accurate exposure control of x-ray film during diagnostic x-rays procedures helps to minimize a patient's exposure to x-rays. A circuit for this application is shown in Figure 6 :

(Click to enlarge image)

and would be composed of two functions: an ion chamber to detect the x-rays and generate a current, IIC, proportional to the x-ray's intensity, and a transimpedance amplifier composed of amplifier A1 and resistor RF which converts the ion chamber current to a voltage:

Dose = IIC x RF

Amplifier A1 is a LMP7721 designed for very low input bias current, typically 3 fA, and works well with high source-impedance signals such as an ion chamber. Amplifier A2 is an integrator used to measure the dose, which is the integral of the Dose Rate:

The comparator, a LMP7300, is used to signal that the required dose has been reached when the output of the integrator, applied to pin 1, is equal to the threshold voltage which is applied to pin 2. In this type of application, the required dose is dependent on many factors, such as the density of the mass being x-rayed.

Figure 6 shows a 12-bit DAC being used to set the threshold voltage for the comparator. The LMP7300 has an accurate and stable 2.048-volt reference, which is amplified to 4.096 volts by amplifier A3 and is the voltage reference for the DAC, which provides the programmable threshold voltage for the LMP7300 comparator.

Another feature of the application is the use of the LM2787 and LM285-2.5 to generate a negative 0.25-volt supply voltage for amplifiers A1 and A2. This small negative voltage enables the amplifier's output to swing to zero volts and removes the output saturation voltage, near zero volts, of amplifiers A1 and A2 from rate and dose signals.

A comparator for this type of application needs to have an accurate threshold voltage that can be programmed within a range of values, to optimize the film exposure. The threshold voltage should be independent of the amount of hysteresis voltage, the value of the threshold voltage, the comparator's output saturation voltage, and the feedback resistor tolerances. A precision comparator such as the LMP7300 can provide these features. Referring to Figure 6, the LMP7300 is shown with its independent comparator function and hysteresis control.

Additionally, the positive hysteresis, which controls the USTV, and the negative hysteresis, which controls the LSTV, have independent control inputs. The significance of this is shown in Figure 7 , which shows the comparator's transfer function for the combination of input signals and hysteresis control.

(Click to enlarge image)

This comparator effectively separates the threshold voltage, VTH , which represents the desired comparison voltage, from the USTV and the LSTV. This accomplishes the task of providing an accurate signal comparison while still providing hysteresis.

The hysteresis of the LMP7300 is controlled by the voltage difference between the VREF voltage and the voltage applied to the HYSTP and the HYSTN pins. The schematics in Figure 7A and 7D show two of the possible hysteresis connections. If a hysteresis pin is connected directly to the VREF voltage, that portion of the hysteresis loop is removed. Referring to Figure 6, the amount of hysteresis used is about 20 mV:

and because the amount of hysteresis is separated from the VTH level, R5 and R6 do not need to be precision resistors. The width of the hysteresis loop can be made as wide as required without changing the value of VTH .

In conclusion, this article has highlighted how a precision comparator such as the LMP7300 can be used to overcome threshold and hysteresis interactions common in existing comparators using external feedback resistors to generate hysteresis.

About the Author
Walter Bacharowski is an amplifier-applications manager at National Semiconductor Corporation, where he has worked for 15 years. He has a bachelor's degree in electrical engineering from Cleveland State University and has had continuing education in engineering, management, marketing, and technology. His personal interests include electronics, model rocketry, and alternative-energy technology.

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