AD8541/AD8542/AD8544

Rev. E | Page 13 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 35 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 36. 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

C2

53.6µF

1

2πRC

1

R1

R1 + R2

4 1 –

Figure 35. 60 Hz Twin-T Notch Filter, Q = 10

C

2C

R/2

RR

7

3

2

4

6

AD8541

5.0V

C

Figure 36. 60 Hz Twin-T Notch Filter, Q = ∞ (Ideal)

Figure 37 is an example of the AD8544 in a notch filter circuit.

The frequency dependent negative resistance (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 much easier and/or less expensive to

accomplish in the FNDR circuit. For example, the twin-T notch

uses three capacitors with two unique values, whereas the

FNDR 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 37. FNDR 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 38 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 38. AD854x Comparator Application—Overload Detector