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參數(shù)資料
| 型號： | AD8612ARU |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 運動控制電子 |
| 英文描述： | Ultrafast 4 ns Single Supply Comparators |
| 中文描述： | COMPARATOR, 7000 uV OFFSET-MAX, 4.5 ns RESPONSE TIME, PDSO14 |
| 封裝： | TSSOP-14 |
| 文件頁數(shù)： | 7/12頁 |
| 文件大?。?/td> | 130K |
| 代理商： | AD8612ARU |
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
μ
A 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’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
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
–2 k
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
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
for best performance.
Another important consideration is the proper use of power supply
bypass capacitors around the comparator. A 1
μ
F 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
μ
F bypass capacitor. The 1
μ
F 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
to Ground
300
500
1 k
10 k
> 20 k
V+ V
OUT, HI
(typ)
1.5 V
1.3 V
1.2 V
1.1 V
1.0 V
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