LMH7322 Datasheet, PDF (11/33 Pages) Texas Instruments – Dual 700 ps High Speed Comparator with RSPECL Outputs

LMH7322
www.ti.com
SNOSAU8I – MARCH 2007 – REVISED MARCH 2013
APPLICATION INFORMATION
Introduction
The LMH7322 is a high speed comparator with RS(P)ECL (Reduced Swing Positive Emitter Coupled Logic)
outputs, and is compatible with LVDS (Low Voltage Differential Signaling) if V_CCO is set to 2.5V. The use of
complementary outputs gives a high level of suppression for common mode noise. The very fast rise and fall
times of the LMH7322 enable data transmission rates up to several Gigabits per second (Gbps). The LMH7322
inputs have a common mode voltage range that extends 200 mV below the negative supply voltage thus allowing
ground sensing in case of single supply. The rise and fall times of the LMH7322 are about 160 ps, while the
propagation delay time is about 700 ps. The LMH7322 can operate over the full supply voltage range of 2.7V to
12V, while using single or dual supply voltages. This is a very useful feature because it provides a flexible way to
interface between several high speed logic families. Several setups are shown in the application information
section “Interface Between Logic Families”. The outputs are referenced to the positive V_CCO supply rail. The
supply current is 23 mA at 5V (per comparator, load current excluded.) The LMH7322 is available in a 24-Pin
WQFN package.
The following topics will be discussed in this application section.
• Input and output topology
• Specification definitions
• Propagation delay and dispersion
• Hysteresis and oscillations
• Output
• Applying transmission lines
• PCB layout
Input & Output Topology
All input and output pins are protected against excessive voltages by ESD diodes. These diodes are conducting
from the negative supply to the positive supply. As can be seen in Figure 17, both inputs are connected to these
diodes. Further protection of the inputs is provided by the two resistors of 250Ω, in conjunction with the string of
anti-parallel diodes connected between both bases of the input stage. This combination of resistors and diodes
reduces excessive input voltages over the input stage, but is low enough to maintain switching speed to the
output signal.
Protection against excessive supply voltages is provided by a power clamp between V_CC and GND.
When using this part be aware of situations in which the differential input voltage level is such that these diodes
are conducting. In this case the input current is raised far above the normal value stated in the datasheet tables
because input current is flowing through the bypass diode string between both inputs.
VCCI
VCCI
VCCI
VCCI
VCCO
250:
IN+
250:
IN-
VEE
VEE
Power
Clamp
2X
VEE
PART A
VEE
PART B
Figure 17. Equivalent Input Circuitry
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