ad8465 Analog Devices, Inc., ad8465 Datasheet - Page 12

• Manufacturer Part Number: ad8465
• Description: Rail-to-rail, Very Fast, 2.5 V To 5.5 V, Single-supply Lvds Comparator
• Manufacturer: Analog Devices, Inc.

AD8465
The AD8465 comparator offers a programmable hysteresis
feature that significantly improves accuracy and stability.
Connecting an external pull-down resistor or a current source
from the LE/HYS pin to GND varies the amount of hysteresis
in a predictable and stable manner. Leaving the LE/HYS pin
disconnected or driving it high removes hysteresis. The maxi-
mum hysteresis that can be applied using this pin is approximately
160 mV. Figure 19 illustrates the amount of hysteresis applied as
a function of external resistor value. Figure 10 illustrates hysteresis
as a function of current.
The hysteresis control pin appears as a 1.25 V bias voltage
seen through a series resistance of 70 kΩ ± 20% throughout the
hysteresis control range. The advantages of applying hysteresis
in this manner are improved accuracy, improved stability, reduced
component count, and maximum versatility. An external bypass
capacitor is not recommended on the LE/HYS pin because it
would likely degrade the jitter performance of the device and
impair the latch function. As described in the Using/Disabling
the Latch Feature section, hysteresis control need not compro-
mise the latch function.
250
200
150
100
50
0
50
V
CC
100
Figure 19. Hysteresis vs. R
= 5.5V
150
V
CC
HYSTERESIS RESISTOR (kΩ)
= 2.5V
200
250
300
HYS
Control Resistor
350
400
450
500
Rev. 0 | Page 12 of 16
CROSSOVER BIAS POINTS
Rail-to-rail inputs of this type, in both op amps and comparators,
have a dual front-end design. Certain devices are active near the
V
mined point in the common-mode range, a crossover occurs.
At this point, normally V
reverses and there are changes in measured offset voltages and
currents.
MINIMUM INPUT SLEW RATE REQUIREMENT
With the rated load capacitance and normal good PCB design
practice, as discussed in the Optimizing Performance section,
these comparators should be stable at any input slew rate with
no hysteresis. Broadband noise from the input stage is observed
in place of the violent chattering seen with most other high
speed comparators. With additional capacitive loading or
poor bypassing, oscillation is observed. This oscillation is
due to the high gain bandwidth of the comparator in combina-
tion with feedback parasitics in the package and PCB. In many
applications, chattering is not harmful.
CCI
rail and others are active near the V
CCI
/2, the direction of the bias current
EE
rail. At some predeter-