LMH7324_14 Datasheet, PDF (20/30 Pages) Texas Instruments – Quad 700 ps High Speed Comparator with RSPECL Outputs

LMH7324
SNOSAZ2F – SEPTEMBER 2007 – REVISED JULY 2010
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In this picture there are two dotted lines A and B, both indicating the resulting level at which the comparator
output will switch over. Assume that for this situation the input signal is connected to the negative input and the
switching level (V_REF) to the positive input. The LMH7324 has a built-in hysteresis voltage that is fixed at
approximately 20 mV_PP. The input level of Figure 27 starts much lower than the reference level and this means
that the state of the input stage is well defined with the inverting input much lower than the non-inverting input.
As a result the output will be in the high state. Internally the switching level is at A, with the input signal sloping
up, this situation remains until V_IN crosses level A at t = 1. Now the output toggles, and the internal switching
level is lowered to level B. So before the output has the possibility to toggle again, the difference between the
inputs is made sufficient to have a stable situation again. When the input signal comes down from high to low,
the situation is stable until level B is reached at t = 0. At this moment the output will toggle back, and the circuit is
back in the starting situation with the inverting input at a much lower level than the non-inverting input. In the
situation without hysteresis, the output will toggle exactly at V_REF. With hysteresis this happens at the internally
introduced levels A and B, as can be seen in Figure 27. If the levels A and B change, due to a change in the
built-in hysteresis voltage depending of e.g. temperature variations, then the timing of t = 0 and t = 1 will also
vary. The variation of the hysteresis voltage over temperature is very low and ranges from 22 mV to 23 mV at 5V
Supply over a temperature variation of -25 °C to 125 °C (see Figure 28). When designing a circuit be aware of
this effect. Introducing hysteresis will cause some time shift between output and input (e.g. duty cycle variations),
but will eliminate undesired switching of the output.
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29
V_S = 5V
28
V_CM = 300 mV
27
26
25
24
23
22
21
20
-50 -25 0 25 50 75 100 125
TEMPERATURE (°C)
Figure 28. Hysteresis Voltage over Temperature
THE OUTPUT
Output Swing Properties
The LMH7324 has differential outputs, which means that both outputs have the same swing but in opposite
directions. (See Figure 29.) 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.
Output Q
V_OH
V_OD
V_O
Output Q
V_OL
Figure 29. Output Swing
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