Microsoft LPF-00004 Manuel de l'opérateur télécharger pdf (Page 78)

Page 60 MAKING YOUR OWN ADAPTER OR ACTIVE PROBE

YBER

380, COPYRIGHT MARCH 2002, AXON INSTRUMENTS, INC.

Type J and Type K Thermocouples

ANALOG GROUND ( 10 )

INPUT ( 1 )

15 V

(

)

AD594

AD595

15 V

(

)

CONSTANT

ALUMEL

IRON

CHROME

Figure 24. Thermocouple interface. The AD594 (for type J thermocouples) and the AD595 (type K) integrated

circuits from Analog Devices allow for an extremely simple thermocouple interface. The output is 10 mV/°C.

Cold junction compensation is built in. If it is necessary to isolate the thermocouple because the thermocouple

is in contact with high voltages or because of the possibility of picking up large voltage transients on the input

leads, the 1B51 and 2B50 isolated thermocouple signal conditioners from Analog Devices can be used instead.

Amplifier Probes

10K

0.1 %

10K

0.1 %

10K

0.1 %

INPUT

2K22

Figure 25. Differential amplifier. Amplifiers U1 and U2 provide high input impedance, x10 gain, and the initial

common-mode rejection. The common-mode rejection ratio of the amplifier depends directly on how well

R4-R7 are matched. With 0.1% resistors as shown, the typical CMRR of the circuit is 76-80 dB. The gain of

U3 can be trimmed to achieve even higher CMRR values. Increasing the gain to x100 (by changing R3 from

2.22 kΩ to 202 Ω) will generally improve the CMRR by 20 dB and provide increased noise immunity.

However, the differential input range will be reduced from ±1 V to ±100 mV. For low noise and low bias

current, use low noise FET op amps for U1 and U2.

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Microsoft LPF-00004 Manuel de l'opérateur Page 78