Datasheet

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.
I
SY
+ refers to the current flowing from the upper supply rail to
the op amp, and I
SY
− refers to the current flowing from the op
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,
where, I
SY
+ = I
SY
− = 36 μA for the dual AD8657 at V
SY
= 18 V.
AD8657
1/2
A1
100kΩ
100kΩ
I
SY
+
+V
SY
V
OUT
–V
SY
I
SY
–
A2
08804-066
Figure 67. Voltage Follower
0
5
10
15
20
25
30
35
40
0 2 4 6 8 10 12 14 16 18
I
SY
PER DUAL AMPLIFIER (µA)
V
SY
(V)
I
SY
–
I
SY
+
08804-067
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
yield the same result. At 18 V of power supply, I
SY
+ remains at
36 μA per dual amplifier, but I
SY
− increases to 140 μA in magni-
tude per dual amplifier.
AD8657
1/2
A1
100kΩ
100kΩ
I
SY
+
+V
SY
V
OUT
–V
SY
I
SY
–
A2
08804-068
Figure 69. Comparator A
AD8657
1/2
A1
100kΩ
100kΩ
I
SY
+
+V
SY
V
OUT
–V
SY
I
SY
–
A2
08804-069
Figure 70. Comparator B
0
20
40
60
80
100
120
140
160
0 2 4 6 8 10 12 14 16 18
I
SY
pER DUAL AMPLIFIER (µA)
V
SY
(V)
I
SY
–
I
SY
+
08804-070
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.