ADCMP580/ADCMP581/ADCMP582
The hysteresis pin can also be driven by a current source.
It is biased approximately 400 mꢀ above ꢀEE and has an
internal series resistance of approximately 600 Ω.
80
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 28. If the input voltage approaches
the threshold from the negative direction, the comparator
switches from a low to a high when the input crosses +ꢀH/2.
The new switching threshold becomes −ꢀH/2. The comparator
remains in the high state until the threshold −ꢀH/2 is crossed
from the positive direction. In this manner, noise centered on
0 ꢀ input does not cause the comparator to switch states unless
it exceeds the region bounded by ꢀH/2.
70
60
50
40
30
20
10
0
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 reduce high speed performance and can even
reduce overall stability in some cases.
1
10
100
1k
10k
R
CONTROL RESISTOR (Ω)
HYS
Figure 29. Comparator Hysteresis vs. RHYS Control Resistor
MINIMUM INPUT SLEW RATE REQUIREMENT
As with many high speed comparators, a minimum slew rate
requirement must be met to ensure that the device does not
oscillate as the input signal crosses the threshold. This oscil-
lation is due in part to the high input bandwidth of the comparator
and the feedback parasitics inherent in the package. A
minimum slew rate of 50 ꢀ/μs should ensure clean output
transitions from the ADCMP58x family of comparators.
–V
2
+V
2
H
H
0V
INPUT
1
The slew rate may be too slow for other reasons. The extremely
high bandwidth of these devices means that broadband noise
can be a significant factor when input slew rates are low. There
is 120 ꢁꢀ of thermal noise generated over the bandwidth of the
comparator by the two 50 ꢂ terminations at room temperature.
With a slew rate of only 50 ꢀ/ꢁs, the inputs are inside this noise
band for over 2 ps, rendering the comparator’s jitter performance of
200 fs irrelevant. Raising the slew rate of the input signal and/or
reducing the bandwidth over which that resistance is seen at the
input can greatly reduce jitter. Devices are not characterized this
way but simply bypassing a reference input close to the package
can reduce jitter 30% in low slew rate applications.
0
OUTPUT
Figure 28. Comparator Hysteresis Transfer Function
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 ꢀEE, a variable amount
of hysteresis can be applied. Leaving the HYS pin disconnected
disables the feature, and hysteresis is then less than 1 mꢀ, as
specified. The maximum range of hysteresis that can be applied
by using this method is approximately 70 mꢀ.
Figure 29 illustrates the amount of applied hysteresis as a
function of the 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, and it would likely
degrade the jitter performance of the device.
Rev. A | Page 13 of 16
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