AD8541/AD8542/AD8544
Rev. G | Page 14 of 20
APPLICATIONS
NOTCH FILTER
The AD854x have 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 36 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 can be designed with only one op
amp if Q adjustment is not required. Simply remove U2 as
illustrated in Figure 37. 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 causes 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.
1/2 AD8542
5
6
7
8
3
2
4
1
1/2 AD8542
5.0
U1
V
OUT
U2
R2
2.5kΩ
R1
97.5kΩ
2.5V
REF
C
26.7nF
C
26.7nF
2.5V
REF
R/2
50kΩ
R
100kΩ
R
100kΩ
2C
53.6µF
f
0
=
f
0
=
1
2πRC
1
R1
R1 + R2
4 1 –
00935-035
V
IN
V
IN
Figure 36. 60 Hz Twin-T Notch Filter, Q = 10
C
2C
R/2
RR
7
3
2
4
6
AD8541
5.0
C
V
OUT
2.5V
REF
V
IN
00935-036
U1
Figure 37. 60 Hz Twin-T Notch Filter, Q = ∞ (Ideal)
Figure 38 is an example of the AD8544 in a notch filter circuit. The
frequency dependent negative resistance (FDNR) notch filter has
fewer critical matching requirements than the twin-T notch, where
as the Q of the FDNR is directly proportional to a single resistor R1.
Although matching component values is still important, it is also
much easier and/or less expensive to accomplish in the FDNR
circuit. For example, the twin-T notch uses three capacitors
with two unique values, whereas the FDNR circuit uses only
two capacitors, which may be of the same value. U3 is simply a
buffer that is added to lower the output impedance of the circuit.
4
1/4 AD8544
11
6
1/4 AD8544
1/4 AD8544
10
8
9
2
1
3
1/4 AD8544
12
14
13
5
7
U3
U1
U4
U2
C2
1µF
C1
1µF
R1
Q ADJUST
200Ω
R
2.61kΩ
R
2.61kΩ
R
2.61kΩ
R
2.61kΩ
V
OUT
2.5V
REF
2.5V
REF
2.5V
REF
NC
f =
1
2π LC1
L = R
2
C2
00935-037
V
IN
Figure 38. FDNR 60 Hz Notch Filter with Output Buffer
COMPARATOR FUNCTION
A comparator function is a common application for a spare op
amp in a quad package. Figure 39 illustrates ¼ of the AD8544 as a
comparator in a standard overload detection application. Unlike
many op amps, the AD854x family can double as comparators
because this op amp family has a rail-to-rail differential input
range, rail-to-rail output, and a great speed vs. power ratio.
R2 is used to introduce hysteresis. The AD854x, when used as
comparators, have 5 µs propagation delay at 5 V and 5 µs
overload recovery time.
1/4 AD8541
R1
1kΩ
V
OUT
2.5V
REF
IN
R2
1MΩ
2.5V
DC
00935-038
Figure 39. AD854x Comparator Application—Overload Detector
