AD8541/AD8542/AD8544

Rev. F | 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

5.0V

U1

U2

R2

2.5kΩ

R1

97.5kΩ

C

26.7nF

C

26.7nF

R/2

50kΩ

R

100kΩ

R

100kΩ

2C

53.6µF

1

2πRC

1

R1

R1 + R2

4 1 –

Figure 36. 60 Hz Twin-T Notch Filter, Q = 10

C

2C

R/2

RR

7

3

2

4

6

AD8541

5.0V

C

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

NC

f =

1

2π LC1

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

R2

1MΩ

Figure 39. AD854x Comparator Application—Overload Detector