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User's Guide
SNOA494A–September 2007–Revised May 2013
AN-1683 LMH7324 High Speed Comparator Evaluation
Board
1 General Description
This board is designed to demonstrate the LMH7324 quad comparator with RSPECL outputs. It will
facilitate the evaluation of the LMH7324 configured as a window detector. The board detects the level of
the incoming signal and presents the outcome in a 3-bit presentation. One bit indicates that the signal is
below the lowest window level, another bit indicates that the signal is above the highest window level, and
the third bit indicates that the incoming signal is just between both set levels. All three outputs are fed to
SMA connectors mounted at the edge of the board. The impedance of the output track is 50Ω which
makes it easy to connect these signals to any scope or analyzer by the use of a 50Ω coaxial cable. Each
comparator of the LMH7324 has individual positive supplies for the input and output circuits. The negative
supply is common for all input and output circuitry. This setup will work with a supply of ±2.5V as a
minimum supply, with the window voltage centered at ground. If a setup with only one positive supply
voltage is used, jumper J1 (see Figure 7) has to be placed between both positive supply connections. To
examine the possibility of two separate supplies for the input and the output stage the jumper has to be
removed and an extra supply has to be connected.
2 Basic Operation
2.1 Reference Levels
The circuit is built around the four comparators of one LMH7324. Two reference levels are created using
four resisters and two capacitors (R3, R6, R7, R9 and C9, C12 see Figure 7) The ‘ref high’ level is a
positive voltage referred to the ground level and the ‘ref low’ level is a negative voltage referred to ground.
The input connector (con2) is also referenced to ground which means that any AC signal at the input will
vary around the ground level, which is in the center of the reference levels.
2.2 Comparators
The comparators B and C form the window detector, while the comparator A is a level detector indicating
that the input voltage exceeds the ‘ref high’ voltage in the positive direction. The comparator D is a level
detector indicating that the input signal exceeds the ‘ref low’ voltage in the negative direction. The outputs
are connected to a 50Ω connector via a 50Ω track. All three outputs are ‘active low’ as can be seen in
Table 1.
Table 1. Four Comparators Output
V
IN
QA QB QC QD
High 0 1 0 1
In Window 1 0 0 1
Low 1 0 1 0
The window detector output is formed by the OR-function of combining both Q outputs of comparators B
and C. Outputs which have an ECL (Emitter Coupled Logic) structure can be wired together to form an
OR function. The overall truth table is shown in Table 2:
1
SNOA494A–September 2007–Revised May 2013 AN-1683 LMH7324 High Speed Comparator Evaluation Board
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Copyright © 2007–2013, Texas Instruments Incorporated
Verzeichnis
- ・ Blockdiagramm on Seite 6
- ・ Beschreibung der Funktionen on Seite 1
- ・ Anwendungsbereich on Seite 9
