ADCMP580BCP(RevPrA) 데이터 시트보기 (PDF) - Analog Devices

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ADCMP580BCP
(Rev.:RevPrA)

Ultrafast SiGe Voltage Comparator

Analog Devices 

Preliminary Technical Data

COMPARATOR HYSTERESIS

Adding hysteresis to a comparator is often desirable in a noisy

environment or when the differential inputs are very small or

slow moving. The transfer function for a comparator with

hysteresis is shown in Figure 18. If the input voltage approaches

the threshold from the negative direction, the comparator

switches from a low to a high when the input crosses +VH/2.

The new switching threshold becomes −VH/2. The comparator

remains in the high state until the threshold −VH/2 is crossed

coming from the positive direction. In this manner, noise

centered on 0 V input does not cause the comparator to switch

states unless it exceeds the region bounded by ±VH/2.

The customary technique for introducing hysteresis into a

comparator uses positive feedback from the output back to the

input. A limitation of this approach is that the amount of

hysteresis varies with the output logic levels, resulting in

hysteresis that is not symmetric about the threshold. The

external feedback network can also introduce significant

parasitics that will reduce high speed performance, and can

even reduce overall stability in some cases.

The ADCMP58x family of comparators offers a programmable

hysteresis feature that can significantly improve the accuracy

and stability of the desired hysteresis. By connecting an external

pull-down resistor from the HYS pin to VEE, a variable amount

of hysteresis can be applied. Leaving the HYS pin disconnected

disables the feature and hysteresis is then less than 1 mV as

specified. The maximum range of hysteresis that can be applied

by using this method is approximately ±25 mV.

Figure 19 illustrates the amount of applied hysteresis as a

function of external resistor value. The advantage of applying

hysteresis in this manner is improved accuracy, stability, and

reduced component count. An external bypass capacitor is not

required on the HYS pin because it would likely degrade the

jitter performance of the device.

ADCMP580/ADCMP581/ADCMP582

–VH

+VH

2

0V

2

INPUT

1

0

OUTPUT

Figure 18. Comparator Hysteresis Transfer Function of the

ADCMP580/ADCMP581

Figure 19. Comparator Hysteresis vs. RHYS Control Resistor

MINIMUM INPUT SLEW RATE REQUIREMENT

As with all high speed comparators, a minimum slew rate must

be met to ensure that the device does not oscillate when the

input signal crosses the threshold. This oscillation is due in part

to the high input bandwidth of the comparator and the

feedback parasitics inherent in the package. Analog Devices

recommends a minimum slew rate of 50 V/µs to ensure a clean

output transition from the ADCMP58x family of comparators

unless hysteresis is programmed as discussed previously.

Rev. PrA | Page 11 of 16

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