SM73403 Datasheet, PDF (5/8 Pages) Texas Instruments – Single General Purpose Voltage Comparator

SM73403
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APPLICATION NOTES
SNOSBA0A – OCTOBER 2011 – REVISED APRIL 2013
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.
V_S
V_REF
V_IN
VOLTS
R_PULL-UP
-
V_O
+
V^-
V_O
V_REF
TIME
V_IN
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 the 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 R₁ in parallel with R₃ and in series with R₂. The lower
threshold voltage V_T1 is calculated by:
V_T1 = ((V_S R₂) / (((R₁ R₃) / (R₁ + R₃)) + R₂))
(1)
When V_IN is greater than V_T, the output voltage is low. The equivalent circuit for the three resistor network is R₂
in parallel with R₃ and in series with R₁. The upper threshold voltage V_T2 is calculated by:
V_T2 = V_S ((R₂ R₃) / (R₂ + R₃)) / (R₁ + ((R₂ R₃) / (R₂ + R₃)))
(2)
The hysteresis is defined as
ΔV_IN = V_T1 – V_T2
(3)
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