ADCMP580_05 AD [Analog Devices], ADCMP580_05 Datasheet - Page 13

• Manufacturer Part Number: ADCMP580_05
• Description: Ultrafast SiGe Voltage Comparators
• Manufacturer: AD [Analog Devices]

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 +V
The new switching threshold becomes −V
remains in the high state until the threshold −V
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 ±V
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.
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 V
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 29 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 and it would likely degrade the jitter
performance of the device.
Figure 28. Comparator Hysteresis Transfer Function
0
–V
2
H
0V
OUTPUT
H
/2.
+V
2
EE
H
H
/2. The comparator
, a variable amount
INPUT
H
1
/2 is crossed
H
/2.
Rev. 0 | Page 13 of 16
The hysteresis pin may also be driven by a current source.
It is biased approximately 400 mV above V
internal series resistance of approximately 600 Ω.
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 V/μs should ensure clean output
transitions from the ADCMP58x family of comparators.
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 μV of thermal noise generated over the comparator’s
bandwidth by the two 50 Ω terminations at room temperature.
With a slew rate of only 50 V/μs, the inputs will be 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. We do
not characterize the devices this way, but simply bypassing a
reference input close to the package can reduce jitter 30% in
low slew rate applications.
80
70
60
50
40
30
20
10
Figure 29. Comparator Hysteresis vs. R
0
ADCMP580/ADCMP581/ADCMP582
0
10
RESISTOR (Ω)
100
HYS
Control Resistor
EE
and has an
1k
10k