Datasheet AMP04 (Analog Devices) - 10

AMP04 APPLICATION CIRCUITS
output. Note that a 0 volt output is also the negative output swing
Low Power Precision Single Supply RTD Amplifier
limit of the AMP04 powered with a single supply. Therefore, be Figure 11 shows a linearized RTD amplifier that is powered sure to adjust R3 to first cause the output to swing positive and from a single 5 volt supply. However, the circuit will work up to then back off until the output just stops swinging negatively. 36 volts without modification. The RTD is excited by a 100 µA constant current that is regulated by amplifier A (OP295). The Next, set the LINEARITY ADJ potentiometer to the midrange. 0.202 volts reference voltage used to generate the constant current Substitute an exact 247.04 Ω resistor (equivalent to 400°C is divided down from the 2.500 volt reference. The AMP04 ampli- temperature) in place of the RTD. Adjust the FULL-SCALE fies the bridge output to a 10 mV/°C output coefficient. potentiometer for a 4.000 volts output. Finally substitute a 175.84 Ω resistor (equivalent to 200°C
R9
temperature), and adjust the LINEARITY ADJ potentiometer
50 5V
for a 2.000 volts at the output. Repeat the full-scale and the
C3 R8 R10 R3
half-scale adjustments as needed.
0.1 F 383 BALANCE 100 FULL-SCALE R1 500 R2 ADJ
When properly calibrated, the circuit achieves better than
26.7k 26.7k 7 3
±
1 C1
0.5°C accuracy within a temperature measurement range
8 0.47 F
from 0°C to 400°C.
AMP04 VOUT 6 2 RTD 5 Precision 4-20 mA Loop Transmitter with Noninteractive Trim 0 4.00V 100 1 4 R4 (0 C TO 400 C)
Figure 12 shows a full bridge strain gage transducer amplifier
100 1/2 A
circuit that is powered off the 4-20 mA current loop. The AMP04
OP295 5V
amplifies the bridge signal differentially and is converted to a
R7 6 8 2 3 121k 7 1/2
current by the output amplifier. The total quiescent current
B OP295 5
drawn by the circuit, which includes the bridge, the amplifiers,
0.202V 50k R5 4 R6
and the resistor biasing, is only a fraction of the 4 mA null
1.02k 11.5k LINEARITY
current that flows through the current-sense resistor R
ADJ.
SENSE.
2.5V 6 RSENSE (@1/2 FS)
The voltage across RSENSE feeds back to the OP90’s input,
1k OUT 2 REF43 IN 5V
whose common-mode is fixed at the current summing reference
C2 GND
voltage, thus regulating the output current.
0.1 F 4
With no bridge signal, the 4 mA null is simply set up by the
NOTES: ALL RESISTORS 0.5%, 25 PPM/ C
50 kΩ NULL potentiometer plus the 976 kΩ resistors that
ALL POTENTIOMETERS 25 PPM/ C
inject an offset that forces an 80 mV drop across RSENSE. At a Figure 11. Precision Single Supply RTD Thermometer 50 mV full-scale bridge voltage, the AMP04 amplifies the Amplifier voltage-to-current converter for a full-scale of 20 mA at the The RTD is linearized by feeding a portion of the signal back to output. Since the OP90’s input operates at a constant 0 volt the reference circuit, increasing the reference voltage as the common-mode voltage, the null and the span adjustments do temperature increases. When calibrated properly, the RTD’s not interact with one another. Calibration is simple and easy nonlinearity error will be canceled. with the NULL adjusted first, followed by SPAN adjust. The entire circuit can be remotely placed, and powered from the To calibrate, either immerse the RTD into a zero-degree ice 4-20 mA 2-wire loop. bath or substitute an exact 100 Ω resistor in place of the RTD. Then adjust bridge BALANCE potentiometer R3 for a 0 volt
U3 5.00V 4mA NULL REF02 6 2 OUT N 3500 STRAIN 2.49k GAGE BRIDGE GND 0.22 F 50k 1N4002 4 50mV 7 3 1 FS 5k 976k 0.1 F U1 8 10-TURN 97.6k 3 7 AMP04 2k 6 U2 6 5% 2 5 B OP90 T1P29A 20mA 4 2 SPAN HP 4 5082-2810 220pF +VS 100k 13.3k 12V TO 36V 5% R R SENSE LOAD 4-20mA UNLESS OTHERWISE SPECIFIED, ALL RESISTORS 1% 20 100 15.8k OR BETTER POTENTIOMETER < 50 PPM/ C INULL + ISPAN
Figure 12. Precision 4-20 mA Loop Transmitter Features Noninteractive Trims –10– REV. C