LMH7322_14 Datasheet, PDF (23/33 Pages) Texas Instruments – Dual 700 ps High Speed Comparator with RSPECL Outputs

LMH7322
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SNOSAU8H – MARCH 2007 – REVISED MAY 2011
Configuring Hysteresis for the LMH7322
The LMH7322 offers the possibility to introduce hysteresis by connecting a resistor between the RHYS pin and
the RHREF pin. This hysteresis setting resistor may vary between zero ohm and infinite. The current drawn from
the RHYS pin determines the setting of the internal reference voltage. When no resistor is present the internal
used reference voltage is set to zero (the difference between A and B level is zero, see explanation Using
Hysteresis) and no hysteresis is configured. This means the output will change state when the difference
between the positive en negative input signals crosses zero level. Connecting a resistor between the RHYS pin
and the RHREF pin produces a difference for the A and B levels which means hysteresis is introduced and the
output will change state at different levels for an up or down transition of the input signal. Due to the internal
structure a current must be drawn from the RHYS pin. This can be done by connecting a resistor to the lowest
supply voltage. In order to assure the RHYS pin is connected to the correct voltage level, and unwanted current
variations in the hysteresis level are avoided, the RHREF and VEE are connected internally within the LMH7322.
Therefore, the hysteresis resistor should only be connected between RHYS and RHREF or left open if no
hysteresis is required. To select the correct resistor for the desired hysteresis voltage see Figure 33.
70
V_CM = 300 mV
60
TEMP = 25°C
V_S = 2.7V, 5V,12V
50
V_HYS = V_(HYS+₎ - V_(HYS-₎
40
30
20
10
0
0 5 10 15 20 25 30 35
HYSTERESIS RESISTOR (k:)
Figure 33. Hysteresis Voltage vs. Hysteresis Resistor
With the use of a resistor to set the hysteresis voltage no external conditions will effect these setting as long as
they stay within the normal operating ranges. Temperature changes may cause a variation of the hysteresis
resistor dictated by the temperature coefficient of the used type. Connecting the RHYS pin to another voltage as
provided by the RHREF pin is not covered in the resistor selection plot and the designer of such a circuit must be
aware of abnormal behavior.
The Output
Output Swing Properties
The LMH7322 has differential outputs which means that both outputs have the same swing but in opposite
directions (See Figure 34). Both outputs swing around the common mode output voltage (V_O). This voltage can
be measured at the midpoint between two equal resistors connected to each output. The absolute value of the
difference between both voltages is called V_OD. The outputs cannot be held at the V_O level because of their
digital nature. They only cross this level during a transition. Due to the symmetrical structure of the circuit, both
output voltages cross at V_O regardless of whether the output changes from ‘0’ to ‘1’ or vise versa.
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