AD8541/AD8542/AD8544
55
SUPPLY CURRENT/AMPLIFIER – A
1,000
V
S
= +2.7V AND +5V
A
V
= +1
800 T
A
= +25 C
900
V
S
= +5V
50
45
40
V
S
= +2.7V
35
30
25
IMPEDANCE –
700
600
500
400
300
200
100
V
S
= +5V
A
V
= +1
MARKER SET @ 10kHz
MARKER READING: 37.6 V/ Hz
T
A
= +25 C
20
55
35 15
5 25 45 65 85 105 125 145
TEMPERATURE – C
0
1k
200mV/DIVISION
0
10k
100k
1M
FREQUENCY – Hz
10M
100M
5
10
15
FREQUENCY – kHz
20
25
Figure 28. Supply Current per
Amplifier vs. Temperature
Figure 29. Closed-Loop Output
Impedance vs. Frequency
Figure 30. Voltage Noise
NOTES ON THE AD854x AMPLIFIERS
The AD8541/AD8542/AD8544 amplifiers are improved perfor-
mance general purpose operational amplifiers. Performance has
been improved over previous amplifiers in several ways.
Lower Supply Current for 1 MHz Gain Bandwidth
The AD854x series typically uses 45 microamps of current per
amplifier. This is much less than the 200
µA
to 700
µA
used in
earlier generation parts with similar performance. This makes
the AD854x series a good choice for upgrading portable designs
for longer battery life. Alternatively, additional functions and
performance can be added at the same current drain.
Higher Output Current
the circuit to no longer attenuate at the ideal notch frequency.
To achieve desired performance, 1% or better component
tolerances or special component screens are usually required.
One method to desensitize the circuit-to-component mis-
match is to increase R2 with respect to R1, which lowers Q. A
lower Q increases attenuation over a wider frequency range,
but reduces attenuation at the peak notch frequency.
5.0V
R
100k
C2
53.6 F
R/2
50k
2.5V
REF
C
26.7nF
C
26.7nF
R2
2.5k
5
6
R1
97.5k
1
R1
R1+R2
R
100k
3
2
8
U1
4
1/2 AD8542
1
V
OUT
At +5 V single supply, the short circuit current is typically 60
µA.
Even 1 V from the supply rail, the AD854x amplifiers can provide
30 mA, sourcing or sinking.
Sourcing and sinking is strong at lower voltages, with 15 mA
available at +2.7 V, and 18 mA at 3.0 V. For even higher output
currents, please see the Analog Devices AD8531/AD853/AD8534
parts, with output currents to 250 mA. Information on these
parts is available from your Analog Devices representative, and
datasheets are available at the Analog Devices website at
www.analog.com.
Better Performance at Lower Voltages
1/2 AD8542
7
U2
f
0
=
f
0
=
1
2πRC
4 1
[
]
5.0V
R
3
2
7
2.5V
REF
Figure 31. 60 Hz Twin-T Notch Filter, Q = 10
The AD854x family parts have been designed to provide better ac
performance, at 3.0 V and 2.7 V, than previously available parts.
Typical gain-bandwidth product is close to 1 MHz at 2.7 V. Volt-
age gain at 2.7 V and 3.0 V is typically 500,000. Phase margin is
typically over +60°C, making the part easy to use.
APPLICATIONS
Notch Filter
R
AD8541
4
6
V
OUT
V
IN
2C
The AD8542 has very high open loop gain (especially with supply
voltage below 4 V), which makes it useful for active filters of all
types. For example, Figure 31 illustrates the AD8542 in the clas-
sic Twin-T Notch Filter design. The Twin-T Notch is desired for
simplicity, low output impedance and minimal use of op amps. In
fact, this notch filter may be designed with only one op amp if Q
adjustment is not required. Simply remove U2 as illustrated in
Figure 32. However, a major drawback to this circuit topology is
ensuring that all the Rs and Cs closely match. The components
must closely match or notch frequency offset and drift will cause
REV. A
–9–
2.5V
REF
C
R/2
C
Figure 32. 60 Hz Twin-T Notch Filter, Q =
∞
(Ideal)
Figure 33 diagrams another example of the AD8542 in a
notch filter circuit. The FNDR notch filter has several
unique features as compared to the Twin-T Notch including:
less critical matching requirements; Q is directly proportional
to a single resistor R1. While matching component values is
still important, it is also much easier and/or less expensive to
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