AD8657

Rev. A | Page 19 of 24

COMPARATOR OPERATION

Op amps are designed to operate in a closed-loop configuration

with feedback from its output to its inverting input. Figure 67

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 68, 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

100k

Ω

100k

Ω

A2

Figure 67. Voltage Follower

0

5

10

15

20

25

30

35

40

0

2

4

6

8

10

12

14

16

18

Figure 68. 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 69 and Figure 70 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 has input

devices that are protected from large differential input voltages

by Diode D1 and Diode D2 (refer to Figure 63). These diodes

consist of substrate PNP bipolar transistors, and conduct whenever

the differential input voltage exceeds approximately 600 mV; how-

ever, 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 71, both configurations

tude per dual amplifier.

AD8657

1/2

A1

100k

Ω

100k

Ω

A2

Figure 69. Comparator A

AD8657

1/2

A1

100k

Ω

100k

Ω

A2

Figure 70. Comparator B

0

20

40

60

80

100

120

140

160

0

2

4

6

8

10

12

14

16

18

Figure 71. 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.