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Window Comparator With Independent Adjust-

ments (HA2841, HA4905)

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

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-

the op amp insures that one diode will always be forward

the sum of the input voltages (closed loop gain is one) and

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’s 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.

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’s 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.

HA2841

10K

10K

5K

+15V

-15V

10K

-15V

+15V

3

2

4

7

6

10K

+

-

47pF

3

4

INPUT

-15

+15

+5V

R

100K

+15V

5

9

2

13

HA4905

1/4

+

-

10K

C

FIGURE 1. WINDOW COMPARATOR WITH INDEPENDENT

ADJUSTMENTS

1

FIGURE 2.

Application Note

April 1998

AN9802