Data Sheet

AD8657/AD8659

Rev. B | Page 21 of 24

COMPARATOR OPERATION—AD8657

An op amp is designed to operate in a closed-loop configuration

with feedback from its output to its inverting input. Figure 72

shows the AD8657 configured as a voltage follower with an input

voltage that is always kept at midpoint of the power supplies.

The same configuration is applied to the unused channel. A1 and

A2 indicate the placement of ammeters to measure supply current.

amp to the lower supply rail. As shown in Figure 73, as expected

in normal operating condition, the total current flowing into the

op amp is equivalent to the total current flowing out of the op amp,

AD8657

1/2

A1

100kΩ

100kΩ

A2

Figure 72. Voltage Follower

0

5

10

15

20

25

30

35

40

0

2

4

6

8

10

12

14

16

18

Figure 73. Supply Current vs. Supply Voltage (Voltage Follower)

In contrast to op amps, comparators are designed to work in an

open-loop configuration and to drive logic circuits. Although

op amps are different from comparators, occasionally an unused

section of a dual op amp is used as a comparator to save board

space and cost; however, this is not recommended.

Figure 74 and Figure 75 show the AD8657 configured as a com-

parator, with 100 kΩ resistors in series with the input pins. Any

unused channels are configured as buffers with the input voltage

kept at the midpoint of the power supplies. The AD8657/AD8659

have input devices that are protected from large differential

input voltages by Diode D1 and Diode D2 (refer to Figure 68).

These diodes consist of substrate PNP bipolar transistors, and

conduct whenever the differential input voltage exceeds approxi-

mately 600 mV; however, these diodes also allow a current path

from the input to the lower supply rail, thus resulting in an

increase in the total supply current of the system. As shown in

Figure 76, both configurations yield the same result. At 18 V of

increases to 140 μA in magnitude per dual amplifier.

AD8657

1/2

A1

100kΩ

100kΩ

A2

Figure 74. Comparator A

AD8657

1/2

A1

100kΩ

100kΩ

A2

Figure 75. Comparator B

0

20

40

60

80

100

120

140

160

0

2

4

6

8

10

12

14

16

18

Figure 76. Supply Current vs. Supply Voltage (AD8657 as a Comparator)

Note that 100 kΩ resistors are used in series with the input of

the op amp. If smaller resistor values are used, the supply current of

the system increases much more. For more details on op amps as

comparators, refer to the AN-849 Application Note Using Op

Amps as Comparators.