op492gbc Analog Devices, Inc., op492gbc Datasheet - Page 12

• Manufacturer Part Number: op492gbc
• Description: Dual/quad Single Supply Operational Amplifier
• Manufacturer: Analog Devices, Inc.

OP292/OP492
A Low-Cost, Linearized Thermistor Amplifier
An inexpensive thermometer amplifier circuit can be implemented
using low-cost thermistors. One such implementation is shown in
Figure 8. 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 temperature but at a slightly reduced accuracy. To
achieve the aforementioned accuracy, the thermistor’s nonlinearity
must be corrected. This is done by connecting the thermistor in
parallel with the 10 k in the feedback loop of the first stage
amplifier. A constant operating current of 281 A is supplied by
the resistor R1 with the 5 V reference from the REF-195 such
that the thermistor’s self-heating error 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 mF capacitor
placed across R2 will help to suppress noise pickup.
This linearization network creates an offset voltage that is cor-
rected 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 k , and
P1 is adjusted until the circuit’s output reads 0 V. To trim the circuit
at the full-scale temperature of 70∞C, the decade box is then set
to 1.752 k and P2 is adjusted until the circuit reads –0.70 V.
Figure 8. A Low Cost Linearized Thermistor Amplifier
A Single-Supply Ultrasonic Clamping/Limiting Receiver
Amplifier
Figure 9 shows an ultrasonic receiver amplifier using the non-
linear impedance of low-cost diodes to effectively control the gain
for wide dynamic range. This circuit amplifies a 40 kHz ultrasonic
signal through a pair of low-cost clamping amplifiers before feeding
a band-pass filter to extract a clean 40 kHz signal for processing.
The signal is ac-coupled into the false-ground bias node by virtue of
the capacitive piezoelectric sensing element. Rather than using
an amplifier to generate a supply splitting bias, the false ground
voltage is generated by a low-cost resistive voltage divider.
Each amplifier stage provides ac gain while passing on the dc self-
bias. As long as the output signal at each stage is less than a diode’s
forward voltage, each amplifier has unrestricted gain to amplify
low level signals. However, as the signal strength increases, the
1.0 F
REF195
15V
0 C TRIM
1 F
10k
P1
5V
17.8k
R1
*
NOTES
+ = ALPHA THERMISTOR 13A1002-C3
* = 0.1% IMPERIAL ASTRONICS M015
ALL RESISTORS ARE 1%, 25 ppm/ C
EXCEPT R5 = 1%, 100 ppm/ C
10k NTC
OP292
17.8k
1/2
R1
R
41.2k
806k
T
*
R4
R5
10k
R3
7.87k
R6
OP292
1/2
70 C TRIM
200
P2
10mV/ C
V
OUT
–12–
feedback diodes begin to conduct, shunting the feedback current,
and thus reducing the gain. Although distorting the waveform,
the diodes effectively maintain a relatively constant amplitude even
with large signals that otherwise would saturate the amplifier. In
addition, this design is considerably more stable than the feed-
back type AGC.
The overall circuit has a gain range from –2 to –400, where the
inversion comes from the band-pass filter stage. Operating with
a Q of 5, the filter restores a clean, undistorted signal to the out-
put. The circuit also works well with 5 V supply systems.
Figure 9. A 40 kHz Ultrasonic Clamping/Limiting Receiver
Amplifier
Precision Single-Supply Voltage Comparator
The OP292/OP492 have excellent overload recovery characteris-
tics, making them suitable for precision comparator applications.
Figure 10 shows the saturation recovery characteristics of the
OP492. The amplifier exhibits very little propagation delay.
The amplifier compares a signal precisely to less than 0.5 mV
offset error.
Figure 10. The OP492 Has Fast Overload Recovery for
Comparator Applications
Programmable Precision Window Comparator
The OP292/OP492 can be used for precise level detection such
as in test equipment where a signal is measured within a range.
Figure 11 shows such an implementation. The threshold voltage
level is set by a pair of 12-bit DIA converters. The DACs have
serial interface thus minimizing interconnection requirements.
The DAC85 12 has a control resolution of 1 mV/bit. Thus for 5 V
supply operation, maximum DAC output is 4.095 V. However,
the OP292 will accept a maximum input of 4.0 V.
EFR-RTB40K2
RECEIVER
PANASONIC
2.21k
3Vp-p
600k
1k
12V
1M
OP492
20k
10k
15V
15V
0.01 F
390k
OP492
12V
1/4
7.5V
2k
10k
100
0.01 F
90
10
0%
100k
OP492
12V
1/4
5V
1V
0.01 F
6.04k
14k
68pF
1 F
56.2k
68pF
OP492
1/4
12V
REV. B
5µs
600k
7.5V
1M
V
OUT