AD8541/AD8542/AD8544
55
1,000
V
S
= 2.7V AND 5V
A
V
= 1
800 T
A
= 25 C
900
700
V
S
= 5V
A
V
= 1
MARKER SET @ 10kHz
MARKER READING: 37.6 V/ Hz
T
A
= 25 C
200mV/DIVISION
SUPPLY CURRENT/AMPLIFIER – A
50
V
S
= 5V
45
40
V
S
= 2.7V
35
30
25
IMPEDANCE –
600
500
400
300
200
100
20
55
35 15
5 25 45 65 85 105 125 145
TEMPERATURE – C
0
1k
10k
100k
1M
FREQUENCY – Hz
10M
100M
0
5
10
15
FREQUENCY – kHz
20
25
TPC 28. Supply Current per
Amplifier vs. Temperature
TPC 29. Closed-Loop Output
Impedance vs. Frequency
TPC 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
mA
of current per amplifier.
This is much less than the 200
mA
to 700
mA
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 per-
formance can be added at the same current drain.
Higher Output Current
drift will cause 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 mismatch 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
mA.
Even 1 V from the supply rail, the AD854x amplifiers can provide
30 mA, sourcing or sinking.
Sourcing and sinking are 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/AD8532/AD8534
parts, with output currents to 250 mA. Information on these
parts is available from your Analog Devices representative,
and data sheets 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
2pRC
4 1
[
]
5.0V
R
3
2
7
2.5V
REF
The AD854x family of parts has 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.
Voltage 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.
V
IN
Figure 1. 60 Hz Twin-T Notch Filter, Q = 10
R
AD8541
4
6
V
OUT
2C
APPLICATIONS
Notch Filter
The AD8542 has very high open-loop gain (especially with a
supply voltage below 4 V), which makes it useful for active filters
of all types. For example, Figure 1 illustrates the AD8542 in the
classic 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 illus-
trated in Figure 2. 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
REV. D
–9–
2.5V
REF
C
R/2
C
Figure 2. 60 Hz Twin-T Notch Filter, Q =
•
(Ideal)
Figure 3 shows another example of the AD8542 in a notch
filter circuit. The FNDR notch filter has fewer critical
matching requirements than the Twin-T Notch and for the
FNDR Q is directly proportional to a single resistor R1.
While matching component values is still important, it is also
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