Window Comparator With Independent Adjust-
ments (HA2841, HA4905)
Application Note
April 1998
AN9802
Introduction
Window comparators, or limit comparators as they are
sometimes called, determine when the input voltage is within
a pre-selected range. The output voltage usually goes to a
TTL high when the input voltage is within the pre-selected
range. A typical window comparator circuit normally consists
of two comparators with two reference adjustments: one for
the low trip level, and one for the high trip level. If the refer-
ence voltages are derived from the same resistor divider
string, hystersis can not be added without having the trip lev-
els interact. If the reference voltages are independently
derived they can drift in different directions causing the win-
dow to widen or possibly disappear.
The window comparator shown in Figure 1 circumvents
these problems with independent trip and window adjust-
ments. The HA2841 op amp (selected because it has high
speed and low input currents) adds the input signal to the
trip voltage developed by R
T
. The window width voltage R
W
is summed into the output of the op amp as an offset volt-
age. The op amp has two feedback loops each of which con-
tains a steering diode, D
1
or D
2
. The high open loop gain of
the op amp insures that one diode will always be forward
biased unless R
W
supplies the feedback current, thus when
the sum of the input voltages (closed loop gain is one) and
I
RW
passes through zero the current 鏗俹w switches from one
diode to the other. The output comparator senses the polar-
ity switch across the diodes, and the comparator output volt-
age changes.
The HA4905 comparator has inputs that can work from the
15V supplies, thus the input voltage swing is matched to the
op amp鈥檚 output voltage swing. Even though the input sec-
tion of the HA4905 is connected to 15V, the output section
can be connected from +5V to ground, thus offering a TTL
compatible output voltage.
R
T
sets the trip point anywhere between the power supplies,
and R
W
sets the window width from zero, when the center tap
voltage is zero, to about 90 percent of the input waveform.
The trip point should be set first, then the window can be set
to the desired width. Since the adjustments do not interact,
the trip point stays constant as the window width is changed.
There might be a temptation to change the comparator to an
op amp. This is poor practice for any circuit which requires
high speed performance because the op amp output would
saturate causing uncontrolled time delays. The comparator
is subject to multiple switching caused by high frequency
noise riding on slow rising input signals. The multiple switch-
ing is eliminated by the capacitor, C, which is placed across
its input leads. Effectively, C couples the high frequency
noise onto both comparator leads enabling it to be rejected
by the comparator鈥檚 common-mode rejection capability.
Notice in the waveform that there is no multiple switching
taking place.
This circuit yields a window comparator with independent
adjustments, temperature stability, wide voltage range com-
patibility, and TTL compatibility.
R
G1
10K
INPUT
R
F1
10K
+15V
R
100K
R
W
10K
+15
+15V
3
4
+5V
13
+
1
C
47pF 1/4
3 HA4905
-
9
5
+
7
6
D
1
D
2
2
V
O
HA2841
4
-
+15V
R
T
5K
-15V
2
-15V
-15
R
G2
10K
R
F2
10K
FIGURE 1. WINDOW COMPARATOR WITH INDEPENDENT
ADJUSTMENTS
FIGURE 2.
1
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Copyright
漏
Intersil Corporation 1999
INTERSIL
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