LMH7324_14 Datasheet, PDF (9/30 Pages) Texas Instruments – Quad 700 ps High Speed Comparator with RSPECL Outputs

LMH7324
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SNOSAZ2F – SEPTEMBER 2007 – REVISED JULY 2010
APPLICATION INFORMATION
INTRODUCTION
The LMH7324 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 LMH7324 enable data transmission rates up to several Gigabits per second (Gbps). The LMH7324
inputs have a common mode voltage range that extends 200 mV below the negative supply voltage thus allowing
ground sensing when used with a single supply. The rise and fall times of the LMH7324 are about 150 ps, while
the propagation delay time is about 700 ps. The LMH7324 can operate over the supply voltage range of 5V to
12V, while using single or dual supply voltages. This is a flexible way to interface between several high speed
logic families. Several configurations are described in the section INTERFACE BETWEEN LOGIC FAMILIES.
The outputs are referenced to the positive V_CCO supply rail. The supply current is 17 mA at 5V (per comparator,
load current excluded.) The LMH7324 is offered in a 32-Pin WQFN package. This small package is ideal where
space is an important issue.
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 13, both inputs are connected to these
diodes. Protection against excessive supply voltages is provided by two power clamps per comparator: one
between the V_CCI and the V_EE and one between the V_CCO and the V_EE.
V_CCI
V_CCI
V_CCI
V_CCI
V_CCO
IN+
IN-
V_EE
V_EE
V_EE
Power
Clamp
2X per Comparator
Figure 13. Equivalent Input Circuitry
The output stage of the LMH7324 is built using two emitter followers, which are referenced to the V_CCO. (See
Figure 14.) Each of the output transistors is active when a current is flowing through any external output resistor
connected to a lower supply rail. Activating the outputs is done by connecting the emitters to a termination
voltage which lies 2V below the V_CCO. In this case a termination resistor of 50Ω can be used and a transmission
line of 50Ω can be driven. Another method is to connect the emitters through a resistor to the most negative
supply by calculating the right value for the emitter current in accordance with the datasheet tables. Both
methods are useful, but they each have good and bad aspects.
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