REV. B

OP292/OP492

–12–

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.

REF195

15V

5V

1 F

R1

*

17.8k

R1

*

17.8k

10k NTC

R5

806k

R4

41.2k

R3

10k

R6

7.87k

P2

200

70 C TRIM

10mV/ C

NOTES

+ = ALPHA THERMISTOR 13A1002-C3

* = 0.1% IMPERIAL ASTRONICS M015

ALL RESISTORS ARE 1%, 25 ppm/ C

EXCEPT R5 = 1%, 100 ppm/ C

P1

10k

0 C TRIM

1.0 F

1/2

OP292

1/2

OP292

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

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.

12V

600k

1M

RECEIVER

PANASONIC

EFR-RTB40K2

12V

390k

10k

0.01 F

7.5V

12V

100k

10k

0.01 F

14k

0.01 F

6.04k

68pF

1 F

12V

600k

1M

7.5V

56.2k

68pF

1/4

OP492

1/4

OP492

1/4

OP492

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.

10

90

100

0%

1V

5µs

5V

2k

15V

15V

20k

2.21k

1k

3Vp-p

OP492

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.