Datasheet

VOLTS
V
REF
V
O
TIME
V
IN
V
REF
V
IN
V
O
-
+
V
-
R
PULL-UP
V
S
LM397
www.ti.com SNOS977D –MAY 2001–REVISED MARCH 2013
APPLICATION NOTES
Basic Comparators
A comparator is quite often used to convert an analog signal to a digital signal. The comparator compares an
input voltage (V
IN
) at the non-inverting pin to the reference voltage (V
REF
) at the inverting pin. If V
IN
is less than
V
REF
the output (V
O
) is low (V
OL
). However, if V
IN
is greater than V
REF
, the output voltage (V
O
) is high (V
OH
).
Refer to Figure 9.
Figure 9. Basic Comparator
Hysteresis
The basic comparator configuration may oscillate or produce a noisy output if the applied differential input is near
the comparator’s input offset voltage. This tends to occur when the voltage on the input is equal or very close to
the other input voltage. Adding hysteresis can prevent this problem. Hysteresis creates two switching thresholds
(one for the rising input voltage and the other for the falling input voltage). Hysteresis is the voltage difference
between the two switching thresholds. When both inputs are nearly equal, hysteresis causes one input to
effectively move quickly pass the other. Thus, effectively moving the input out of region that oscillation may
occur.
For an inverting configured comparator, hysteresis can be added with a three resistor network and positive
feedback. When input voltage (V
IN
) at the inverting node is less than non-inverting node (V
T
), the output is high.
The equivalent circuit for the three resistor network is R
1
in parallel with R
3
and in series with R
2
. The lower
threshold voltage V
T1
is calculated by:
V
T1
= ((V
S
R
2
) / (((R
1
R
3
) / (R
1
+ R
3
)) + R
2
)) (1)
When V
IN
is greater than V
T
, the output voltage is low. The equivalent circuit for the three resistor network is R
2
in parallel with R
3
and in series with R
1
. The upper threshold voltage V
T2
is calculated by:
V
T2
= V
S
((R
2
R
3
) / (R
2
+ R
3
)) / (R
1
+ ((R
2
R
3
) / (R
2
+ R
3
))) (2)
The hysteresis is defined as
ΔV
IN
= V
T1
– V
T2
(3)
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