OP492GSZ Datasheet, PDF (15/20 Pages) Analog Devices – Dual/Quad Single-Supply Operational Amplifiers

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 (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.
R2
2.67kΩ
R1
2.67kΩ
1/4
OP492
V_IN
C1
C2
1µF
1µF
12V
1/4
OP492
R6
100kΩ
R3
2.67kΩ
C3
2µF
(1µF × 2)
R4
2.67kΩ
R5
1.335kΩ
(2.67k ÷ 2)
R7 ^RQ
1kΩ _8kΩ
12V
R8
100kΩ
1/4
6V
OP492
V_OUT
R9
100kΩ
+ _C4
1µF
FILTER Q ^RQ ^{(kΩ )} REJECTION (dB) VOLTAGE GAIN
0.75
1.0
40
1.33
1.00
2.0
35
1.50
1.25
3.0
30
1.60
2.50
8.0
25
1.80
5.00
18
20
1.90
10.00
38
15
1.95
NOTES
1. FOR 50Hz APPLICATION CHANGE R12 TO R4 TO 3.16kΩ
AND R5 TO 1.58kΩ (3.16kΩ ÷ 2)
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
5V
V_IN
100µF
5kΩ
1.78kΩ
2
8
0.01µF ^1/2
3 ^OP292
4
16.2kΩ
5kΩ
3300pF
6
1
0.022µF
5
1/2
OP292
7
1.1kΩ 14.3kΩ
2200pF
V_OUT
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.
R_T1
15V
10kΩ NTC
1.0µF
REF195
1µF
R1²
17.8kΩ
5V
R1²
17.8kΩ
1/2
OP292
R3
10kΩ
R4
41.2kΩ
P1
10kΩ
0°C TRIM
R5
806kΩ
P2
200Ω
R6 ^{70°C TRIM}
7.87kΩ
1/2
OP292
V_OUT
–10mV/°C
1_{RT = ALPHA THERMISTOR 13A1002-C3.}
2_{R1 = 0.1% IMPERIAL ASTRONICS M015.}
NOTES
1. ALL RESISTORS ARE 1%, 25ppm/°C EXCEPT R5 = 1%, 100ppm/°C.
Figure 41. Low Cost Linearized Thermistor Amplifier
Rev. C | Page 15 of 20