Data Sheet OP292/OP492
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 (R1 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.
1/4
OP492
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
V
IN
V
OUT
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
R
Q
(kΩ )
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
1/4
OP492
1/4
OP492
R3
2.67kΩ
R1
2.67kΩ
R
Q
+
00310-039
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
V
IN
V
OUT
1.1kΩ 14.3kΩ
0.01µF
0.022µF
3300pF
2200pF
1/2
OP292
1/2
OP292
00310-040
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
R1
2
17.8kΩ
R1
2
17.8kΩ
R
T
1
10kΩ NTC
R5
806kΩ
R4
41.2kΩ
R3
10kΩ
R6
7.87kΩ
P2
200Ω
70°C TRIM
V
OUT
–10mV/°C
NOTES
1. ALL RESISTORS ARE 1%, 25ppm/°C EXCEPT R5 = 1%, 100ppm/°C.
1
R
T
= ALPHA
THERMISTOR 13A1002-C3.
2
R1 = 0.1% IMPERIAL ASTRONICS M015.
P1
10kΩ
0°C TRIM
1.0µF
1/2
OP292
1/2
OP292
00310-041
Figure 41. Low Cost Linearized Thermistor Amplifier
Rev. D | Page 15 of 20
