OP292/OP492

Rev. C | Page 15 of 20

50 Hz/60 Hz SINGLE-SUPPLY NOTCH FILTER

Figure 39 shows a notch filter that achieves nearly 30 dB of

60 Hz rejection while powered by only a single 12 V supply.

The circuit also works well on 5 V systems. The filter uses a

twin-T configuration, whose frequency selectivity depends

heavily on the relative matching of the capacitors and resistors in

the twin-T section. Mylar is a good choice for the capacitors of

the twin-T, and the relative matching of the capacitors and resistors

determines the pass-band symmetry of the filter. Using 1%

resistors and 5% capacitors produces satisfactory results.

The amount of rejection and the Q of the filter is solely determined

by one resistor and is shown in the table with Figure 39. The

bottom amplifier is used to split the supply to bias the amplifier

to midlevel. The circuit can be modified to reject 50 Hz by simply

changing the resistors in the twin-T section (Rl through R4)

from 2.67 kΩ to 3.16 kΩ and by changing R5 to ½ of 3.16 kΩ. For

best results, the common value resistors can be from a resistor

array for optimum matching characteristics.

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C1

1µF

C3

2µF

(1µF × 2)

R5

1.335kΩ

(2.67k ÷ 2)

R4

2.67kΩ

C2

1µF

R6

100kΩ

8kΩ

12V

12V

R8

100kΩ

R9

100kΩ

C4

1µF

6V

R7

1kΩ

NOTES

1. FOR 50Hz APPLICATION CHANGE R12 TO R4 TO 3.16kΩ

AND R5 TO 1.58kΩ (3.16kΩ ÷ 2)

FILTER Q

0.75

1.00

1.25

2.50

5.00

10.00

1.0

2.0

3.0

8.0

18

38

REJECTION (dB)

40

35

30

25

20

15

VOLTAGE GAIN

1.33

1.50

1.60

1.80

1.90

1.95

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R3

2.67kΩ

R1

2.67kΩ

+

R2

2.67kΩ

Figure 39. Single-Supply 50 Hz/60 Hz Notch Filter

FOUR-POLE BESSEL LOW-PASS FILTER

The linear phase filter in Figure 40 is designed to roll off at a

voice-band cutoff frequency of 3.6 kHz. The four poles are

formed by two cascading stages of 2-pole Sallen-Key filters.

5V

5kΩ

5kΩ

1.78kΩ 16.2kΩ

100µF

2

3

1

8

4

6

5

7

5V

1.1kΩ 14.3kΩ

0.01µF

0.022µF

3300pF

2200pF

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Figure 40. Four-Pole Bessel Low-Pass Filter Using Sallen-Key Topology

LOW COST, LINEARIZED THERMISTOR AMPLIFIER

An inexpensive thermometer amplifier circuit can be implemented

using low cost thermistors. One such implementation is shown

in Figure 41. The circuit measures temperature over the range

of 0°C to 70°C to an accuracy of ±0.3°C as the linearization

circuit works well within a narrow temperature range. However, it

can measure higher temperatures but at a slightly reduced accuracy.

To achieve the aforementioned accuracy, the nonlinearity of the

thermistor must be corrected. This is done by connecting the

thermistor in parallel with the 10 kΩ in the feedback loop of the

first stage amplifier. A constant operating current of 281 μA is

supplied by the resistor R1 with the 5 V reference from the

REF195 such that the self-heating error of the thermistor is

kept below 0.1°C.

In many cases, the thermistor is placed some distance from the

signal conditioning circuit. Under this condition, a 0.1 μF capacitor

placed across R2 will help to suppress noise pickup.

This linearization network creates an offset voltage that is corrected

by summing a compensating current with Potentiometer P1.

The temperature dependent signal is amplified by the second

stage, producing a transfer coefficient of −10 mV/°C at the output.

To calibrate, a precision decade box can be used in place of the

thermistor. For 0°C trim, the decade box is set to 32.650 kΩ,

and P1 is adjusted until the output of the circuit reads 0 V. To

trim the circuit at the full-scale temperature of 70°C, the decade

box is then set to 1.752 kΩ, and P2 is adjusted until the circuit

reads −0.70 V.

REF195

15V

5V

1µF

17.8kΩ

17.8kΩ

10kΩ NTC

R5

806kΩ

R4

41.2kΩ

R3

10kΩ

R6

7.87kΩ

P2

200Ω

70°C TRIM

–10mV/°C

NOTES

1. ALL RESISTORS ARE 1%, 25ppm/°C EXCEPT R5 = 1%, 100ppm/°C.

P1

10kΩ

0°C TRIM

1.0µF

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Figure 41. Low Cost Linearized Thermistor Amplifier