ADCMP580 AD [Analog Devices], ADCMP580 Datasheet - Page 11

• Manufacturer Part Number: ADCMP580
• Description: Ultrafast SiGe Voltage Comparator
• Manufacturer: AD [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 +V
The new switching threshold becomes −V
remains in the high state until the threshold −V
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 ±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 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 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 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.
EE
H
/2. The comparator
, a variable amount
H
/2 is crossed
H
/2.
H
/2.
Rev. PrA | Page 11 of 16
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.
Figure 18. Comparator Hysteresis Transfer Function of the
Figure 19. Comparator Hysteresis vs. R
ADCMP580/ADCMP581/ADCMP582
0
–V
2
H
ADCMP580/ADCMP581
0V
OUTPUT
HYS
+V
2
H
Control Resistor
INPUT
1