Data Sheet

AD8546/AD8548

Rev. C | Page 19 of 24

COMPARATOR OPERATION

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

with feedback from its output to its inverting input. Figure 65

shows the AD8546 configured as a voltage follower with an input

voltage that is always kept at the 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.

ISY+ refers to the current flowing from the upper supply rail to the

op amp, and ISY− refers to the current flowing from the op amp

to the lower supply rail.

AD8546

1/2

A1

100kΩ

100kΩ

ISY+

+VSY

VOUT

–VSY

ISY–

A2

Figure 65. Voltage Follower Configuration

As expected, Figure 66 shows that in normal operating condition,

the total current flowing into the op amp is equivalent to the total

current flowing out of the op amp, where ISY+ = ISY− = 36 μA for

the AD8546 at VSY = 18 V.

0

5

10

15

20

25

30

35

40

0

2

4

6

8

10

12

14

16

18

VSY (V)

ISY–

ISY+

Figure 66. 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 or quad op amp is used as a comparator to save

board space and cost; however, this is not recommended.

Figure 67 and Figure 68 show the AD8546 configured as a com-

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

unused channel is configured as a buffer with the input voltage

kept at the midpoint of the power supplies.

AD8546

1/2

A1

100kΩ

100kΩ

ISY+

+VSY

VOUT

–VSY

ISY–

A2

Figure 67. Comparator Configuration A

AD8546

1/2

A1

100kΩ

100kΩ

ISY+

+VSY

VOUT

–VSY

ISY–

A2

Figure 68. Comparator Configuration B

The AD8546/AD8548 have input devices that are protected

from large differential input voltages by Diode D1 and Diode D2

(see Figure 61). These diodes consist of substrate PNP bipolar

transistors and turn on when the differential input voltage

exceeds approximately 600 mV; however, these diodes also allow

a current path from the input to the lower supply rail, resulting

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

in Figure 69, both configurations yield the same result. At 18 V

of power supply, ISY+ remains at 36 μA per dual amplifier, but

ISY− increases to 140 μA in magnitude per dual amplifier.

0

20

40

60

80

100

120

140

160

0

2

4

6

8

10

12

14

16

18

VSY (V)

ISY–

ISY+

Figure 69. Supply Current vs. Supply Voltage (AD8546 as a Comparator)