IN+

IN-

250:

250:

VEE

VCCI

VCCI

VCCI

VCCO

Power

Clamp

2X

VCCI

VEE

VEE

PART A

VEE

PART B

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 VCCO 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 VCCO 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 250

Ω, 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 VCC 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.

Figure 17. Equivalent Input Circuitry

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