AD8611/AD8612

–7–

REV. 0

common-mode voltage range to the comparator. Note that sig-

nals much greater than 3.0 V will result in increased input

currents and may 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

the LT1016. Input bias currents to the AD8611 and LT1394 flow

out from the comparator’s 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 to

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 tempera-

ture of +85

°C.

It is not recommend to use an input signals with a fundamental

frequency above 100 MHz as the AD8611 could draw up to 20 mA

of supply current and the outputs may not settle to a definite

state. The device will return 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 should

not be connected to more than 40 TTL input logic gates or drive

less than 400

Ω of load resistance.

The AD8611 output has a typical output swing between ground

and 1 V below the positive supply voltage. Decreasing the output

load resistance to ground will lower the maximum output voltage

due to the increase in output current. Table I shows the typical

output high voltage versus load resistance to ground.

Table I. Maximum Output Voltage vs. Resistive Load

Connecting a 500

Ω–2 kΩ pull-up resistor to V+ on the output

will help increase the output voltage closer to the positive rail;

in this configuration, however, the output voltage will not reach

its maximum until at least 20 ns to 50 ns after the output voltage

switches. This is due to the R-C time constant between the pull-up

resistor and the output and load capacitances. The output pull-up

resistor will not improve propagation delay.

Optimizing High-Speed Performance

As with any high-speed comparator or amplifier, proper design and

layout should be used to ensure optimal performance from the

AD8611/AD8612. Excess stray capacitance or improper grounding

can limit the maximum performance of high-speed circuitry.

Minimizing resistance from the source to the comparator’s 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 pico-

farads 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 k

Ω 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 should be less than 1 k

Ω for best performance.

Another important consideration is the proper use of power supply

bypass capacitors around the comparator. A 1

µF bypass capacitor

should be placed within 0.5 inches of the device between each

power supply pin and ground. Another 10 nF ceramic capacitor

should be placed as close as possible to the device in parallel with

the 1

µF bypass capacitor. The 1 µF capacitor will reduce 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 differ-

ences at different ground points throughout the circuit board

caused from “ground bounce.” A proper ground plane will 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 propa-

gation delay over both comparators. These devices can easily be

replaced with the higher performance AD8611, but there are differ-

ences and it is useful to check that these ensure proper operation.

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 could be exceeded, input signals should

be shifted or attenuated to bring them into range, keeping in

mind the note about source resistance in Optimizing High-Speed

Performance.

Example: An AD8611 power from a 5 V single supply has its

noninverting input connected to 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

Output Load

V+

to Ground

(typ)

300

Ω

1.5 V

500

Ω

1.3 V

1 k

Ω

1.2 V

10 k

Ω

1.1 V

> 20 k

Ω

1.0 V