jl111sza National Semiconductor Corporation, jl111sza Datasheet - Page 12

• Manufacturer Part Number: jl111sza
• Description: Lm111jan Voltage Comparator
• Manufacturer: National Semiconductor Corporation

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6.
Pin connections shown are for LM111H in the H08 hermetic package
capacitive coupling from any high-level signals (such as
the output). If pins 5 and 6 are not used, they should be
shorted together. If they are connected to a trim-pot, the
trim-pot should be located, at most, a few inches away
from the LM111, and the 0.01 μF capacitor should be
installed. If this capacitor cannot be used, a shielding
printed-circuit foil may be advisable between pins 6 and
7. The power supply bypass capacitors should be located
within a couple inches of the LM111. (Some other
comparators require the power-supply bypass to be
located immediately adjacent to the comparator.)
It is a standard procedure to use hysteresis (positive
feedback) around a comparator, to prevent oscillation,
and to avoid excessive noise on the output because the
comparator is a good amplifier for its own noise. In the
circuit of Figure 2, the feedback from the output to the
positive input will cause about 3 mV of hysteresis.
However, if R
would not be reasonable to simply increase the value of
the positive feedback resistor above 510 kΩ. The circuit
S
is larger than 100Ω, such as 50 kΩ, it
FIGURE 1. Improved Positive Feedback
12
7.
8.
of Figure 3 could be used, but it is rather awkward. See
the notes in paragraph 7 below.
When both inputs of the LM111 are connected to active
signals, or if a high-impedance signal is driving the
positive input of the LM111 so that positive feedback
would be disruptive, the circuit of Figure 1 is ideal. The
positive feedback is to pin 5 (one of the offset adjustment
pins). It is sufficient to cause 1 to 2 mV hysteresis and
sharp transitions with input triangle waves from a few Hz
to hundreds of kHz. The positive-feedback signal across
the 82Ω resistor swings 240 mV below the positive
supply. This signal is centered around the nominal
voltage at pin 5, so this feedback does not add to the
V
trimmed out, using the 5 kΩ pot and 3 kΩ resistor as
shown.
These application notes apply specifically to the LM111
and LF111 families of comparators, and are applicable
to all high-speed comparators in general, (with the
exception that not all comparators have trim pins).
OS
of the comparator. As much as 8 mV of V
20142029
OS
can be