MAX9691/MAX9692/MAX9693
Single/Dual, Ultra-Fast, ECL-Output
Comparators with Latch Enable
6 _______________________________________________________________________________________
__________ Applications Information
Layout
Because of the MAX9691/MAX9692/MAX9693s’ large
gain-bandwidth characteristic, special precautions
must be taken to use them. A PC board with a ground
plane is mandatory. Mount 0.01µF ceramic decoupling
capacitors as close to the power-supply pins as possi-
ble, and process the ECL outputs in microstrip fashion,
consistent with the load termination of 50Ω to 200Ω (for
V
T
= -2V). For low-impedance applications, microstrip
layout and terminations at the input may also be help-
ful. Pay close attention to the bandwidth of the decou-
pling and terminating components. Chip components
can be used to minimize lead inductance. Connect
GND1 and GND2 together to a solid copper ground
plane for the MAX9691/MAX9692. GND1 biases the
input gain stages, while GND2 biases the ECL output
stage. If the LE function is not used, connect the LE pin
to GND (MAX9692/MAX9693) and the complementary
LE to ECL logic high level (MAX9693 only). Do not
leave the inputs of an unused comparator floating for
the MAX9693.
Input Slew-Rate Requirements
As with all high-speed comparators, the high gain-
bandwidth product of these devices creates oscillation
problems when the input goes through the linear
region. For clean switching without oscillation or steps
in the output waveform, the input must meet certain
minimum slew-rate requirements. The tendency of the
part to oscillate is a function of the layout and source
impedance of the circuit employed. Poor layout and
larger source impedance will increase the minimum
slew-rate requirement.
Figure 1 shows a high-speed receiver application with
50Ω input and output termination. With this configura-
tion, in which a ground plane and microstrip PC board
are used, the minimum slew rate for clean output
switching is 1V/µs.
In many applications, adding regenerative feedback
will assist the input signal through the linear region,
which will lower the minimum slew-rate requirement
considerably. For example, with the addition of positive
feedback components, R
f
= 1kΩ and C
f
= 10pF, the
minimum slew-rate requirement can be reduced by a
factor of four.
As high-speed receivers, the MAX9691/MAX9692/
MAX9693 are capable of processing signals in excess
of 600MHz. Figure 2 is a 100MHz example with an
input signal level of 14mV
RMS
.
