AD8657 Analog Devices, AD8657 Datasheet - Page 19

• Manufacturer Part Number: AD8657
• Description: Precision, Micropower 18 V CMOS RRIO Op Amp
• Manufacturer: Analog Devices

Specifications of AD8657

• Vcc-vee: 2.7V to 18V
• Isy Per Amplifier: 18µA
• Packages: SOP,CSP
• -3db Bandwidth: 200kHz
• Slew Rate: 0.07V/µs
• Vos: 350µV
• Ib: 5pA
• # Opamps Per Pkg: 2
• Input Noise (nv/rthz): 50nV/rtHz

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
the op amp, and I
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
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
SY
+ refers to the current flowing from the upper supply rail to
Figure 68. Supply Current vs. Supply Voltage (Voltage Follower)
40
35
30
25
20
15
10
SY
5
0
+ = I
0
SY
100kΩ
100kΩ
− = 36 μA for the dual AD8657 at V
2
SY
− refers to the current flowing from the op
4
Figure 67. Voltage Follower
I
I
SY
SY
–
+
6
V
8
SY
AD8657
(V)
1/2
+V
–V
10
A1
A2
SY
SY
I
I
SY
SY
12
+
–
V
OUT
14
SY
16
= 18 V.
18
Rev. A | Page 19 of 24
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
36 μA per dual amplifier, but I
tude per dual amplifier.
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
Amps as Comparators.
Figure 71. Supply Current vs. Supply Voltage (AD8657 as a Comparator)
160
140
120
100
80
60
40
20
0
0
100kΩ
2
100kΩ
100kΩ
I
I
SY
SY
–
+
100kΩ
4
Figure 69. Comparator A
Figure 70. Comparator B
6
AN-849
AD8657
SY
8
V
1/2
− increases to 140 μA in magni-
+V
–V
SY
AD8657
A1
A2
SY
SY
Application Note Using Op
(V)
1/2
+V
–V
10
A1
A2
SY
SY
I
I
SY
SY
+
–
I
I
12
SY
SY
V
+
–
OUT
V
14
OUT
SY
+ remains at
16
AD8657
18