AD8611/AD8612 Data Sheet
Rev. B | Page 10 of 20
APPLICATIONS INFORMATION
OPTIMIZING HIGH SPEED PERFORMANCE
As with any high speed comparator or amplifier, proper design
and layout of the AD8611/AD8612 must ensure optimal
performance. Excess stray capacitance or improper grounding
can limit the maximum performance of high speed circuitry.
Minimizing resistance from the source to the comparator input
is necessary to minimize the propagation delay of the circuit.
Source resistance in combination with the equivalent input
capacitance of the AD8611/AD8612 creates an R-C filter that
could cause a lagged voltage rise at the input to the comparator.
The input capacitance of the AD8611/AD8612 in combination
with stray capacitance from an input pin to ground results in
several picofarads of equivalent capacitance. Using a surface-mount
package and a minimum of input trace length, this capacitance
is typically around 3 pF to 5 pF. A combination of 3 kΩ source
resistance and 3 pF of input capacitance yields a time constant
of 9 ns, which is slower than the 4 ns propagation delay of the
AD8611/AD8612. Source impedances must be less than 1 kΩ
for best performance.
Another important consideration is the proper use of power-
supply-bypass capacitors around the comparator. A 1 μF bypass
capacitor must be placed within 0.5 inches of the device between
each power supply pin and ground. Another 10 nF ceramic
capacitor must be placed as close as possible to the device in
parallel with the 1 μF bypass capacitor. The 1 μF capacitor reduces
any potential voltage ripples from the power supply, and the 10 nF
capacitor acts as a charge reservoir for the comparator during
high frequency switching.
A continuous ground plane on the PC board is also recommended
to maximize circuit performance. A ground plane can be created
by using a continuous conductive plane over the surface of the
circuit board, only allowing breaks in the plane for necessary
traces and vias. The ground plane provides a low inductive current
return path for the power supply, thus eliminating any potential
differences at various ground points throughout the circuit board
caused from ground bounce. A proper ground plane can also
minimize the effects of stray capacitance on the circuit board.
UPGRADING THE LT1394 AND LT1016
The AD8611 single comparator is pin-for-pin compatible with
the LT1394 and LT1016 and offers an improvement in propagation
delay over both comparators. These devices can easily be replaced
with the higher performance AD8611; however, there are differ-
ences, so it is useful to ensure that the system still operates properly.
The five major differences between the AD8611 and the LT1016
include input voltage range, input bias currents, propagation delay,
output voltage swing, and power consumption. Input common-
mode voltage is found by taking the average of the two voltages
at the inputs to the comparator.
The LT1016 has an input voltage range from 1.25 V above the
negative supply to 1.5 V below the positive supply. The AD8611
input voltage range extends down to the negative supply voltage
to within 2 V of V+. If the input common-mode voltage is
exceeded, input signals must be shifted or attenuated to bring
them into range, keeping in mind the note about source resistance
in the Optimizing High Speed Performance section.
For example, an AD8611 powered from a 5 V single supply has
its noninverting input connected to a 1 V peak-to-peak, high
frequency signal centered around 2.3 V and its inverting input
connected to a fixed 2.5 V reference voltage. The worst-case
input common-mode voltage to the AD8611 is 2.65 V. This is
well below the 3.0 V input common-mode voltage range to the
comparator. Note that signals much greater than 3.0 V result in
increased input currents and can cause the comparator to
operate more slowly.
The input bias current to the AD8611 is 7 μA maximum over
temperature (−40°C to +85°C). This is identical to the maximum
input bias current for the LT1394, and half of the maximum I
B
for the LT1016. Input bias currents to the AD8611 and LT1394
flow out from the comparator inputs, as opposed to the LT1016
whose input bias current flows into its inputs. Using low value
resistors around the comparator and low impedance sources
will minimize any potential voltage shifts due to bias currents.
The AD8611 is able to swing within 200 mV of ground and
within 1.5 V of positive supply voltage. This is slightly more
output voltage swing than the LT1016. The AD8611 also uses
less current than the LT1016—5 mA as compared to 25 mA of
typical supply current.
The AD8611 has a typical propagation delay of 4 ns, compared
with the LT1394 and LT1016, whose propagation delays are
typically 7 ns and 10 ns, respectively.
MAXIMUM INPUT FREQUENCY AND OVERDRIVE
The AD8611 can accurately compare input signals up to 100 MHz
with less than 10 mV of overdrive. The level of overdrive required
increases with ambient temperature, with up to 50 mV of
overdrive recommended for a 100 MHz input signal and an
ambient temperature of +85°C.
It is not recommend to use an input signal with a fundamental
frequency above 100 MHz because the AD8611 could draw up
to 20 mA of supply current and the outputs may not settle to a
definite state. The device returns to its specified performance
once the fundamental input frequency returns to below 100 MHz.
OUTPUT LOADING CONSIDERATIONS
The AD8611 can deliver up to 10 mA of output current without
increasing its propagation delay. The outputs of the device must
not be connected to more than 40 TTL input logic gates or drive
less than 400 Ω of load resistance.
