Datasheet OP191, OP291, OP491 (Analog Devices) - 19

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OP191/OP291/OP491 APPLICATIONS INFORMATION SINGLE 3 V SUPPLY, INSTRUMENTATION SINGLE-SUPPLY RTD AMPLIFIER AMPLIFIER
The circuit in Figure 66 uses three op amps of the OP491 to The OP291 low supply current and low voltage operation develop a bridge configuration for an RTD amplifier that make it ideal for battery-powered applications, such as the operates from a single 5 V supply. The circuit takes advantage of instrumentation amplifier shown in Figure 65. The circuit uses the OP491 wide output swing range to generate a high bridge the classic two op amp instrumentation amplifier topology, with excitation voltage of 3.9 V. In fact, because of the rail-to-rail four resistors to set the gain. The equation is simply that of a output swing, this circuit works with supplies as low as 4.0 V. noninverting amplifier, as shown in Figure 65. The two resistors Amplifier A1 servos the bridge to create a constant excitation labeled R1 should be closely matched both to each other and to current in conjunction with the AD589, a 1.235 V precision the two resistors labeled R2 to ensure good common-mode reference. The op amp maintains the reference voltage across rejection performance. Resistor networks ensure the closest the parallel combination of the 6.19 kΩ and 2.55 MΩ resistors, matching as well as matched drifts for good temperature which generate a 200 μA current source. This current splits stability. Capacitor C1 is included to limit the bandwidth and, evenly and flows through both halves of the bridge. Thus, therefore, the noise in sensitive applications. The value of this 100 μA flows through the RTD to generate an output voltage capacitor should be adjusted depending on the desired closed- based on its resistance. A 3-wire RTD is used to balance the line loop bandwidth of the instrumentation amplifier. The RC resistance in both 100 Ω legs of the bridge to improve accuracy. combination creates a pole at a frequency equal to 1/(2π × R1C1). If AC-CMRR is critical, then a matched capacitor to C1
GAIN = 274 200Ω
should be included across the second resistor labeled R1.
10 TURNS 5V 26.7kΩ 26.7kΩ 3V A3 1/4 8 100Ω VOUT A2 OP491 + 5 1/2 RTD 1/4 V 7 IN OP291 VOUT 2.55MΩ 100Ω OP491 6 4 – 3 1/2 365Ω 365Ω 100kΩ OP291 1 6.19kΩ A1 2 1/4 100kΩ R1 R2 R2 R1 OP491 0.01pF ALL RESISTORS 1% OR BETTER AD589
69
C1
0
37.4kΩ
70 4- 0
V R1
4-
OUT = (1 + ) = VIN 100pF
29
R2
29 00 00 Figure 65. Single 3 V Supply Instrumentation Amplifier
5V
Figure 66. Single-Supply RTD Amplifier Because the OP291 accepts rail-to-rail inputs, the input common-mode range includes both ground and the positive Amplifier A2 and Amplifier A3 are configured in the two op supply of 3 V. Furthermore, the rail-to-rail output range ensures amp instrumentation amplifier topology described in the Single the widest signal range possible and maximizes the dynamic 3 V Supply, Instrumentation Amplifier section. The resistors are range of the system. Also, with its low supply current of chosen to produce a gain of 274, such that each 1°C increase in 300 μA/device, this circuit consumes a quiescent current of temperature results in a 10 mV change in the output voltage, for only 600 μA yet still exhibits a gain bandwidth of 3 MHz. ease of measurement. A 0.01 μF capacitor is included in parallel with the 100 kΩ resistor on Amplifier A3 to filter out any A question may arise about other instrumentation amplifier unwanted noise from this high gain circuit. This particular RC topologies for single-supply applications. For example, a combination creates a pole at 1.6 kHz. variation on this topology adds a fifth resistor between the two inverting inputs of the op amps for gain setting. While that topology works well in dual-supply applications, it is inherently inappropriate for single-supply circuits. The same could be said for the traditional three op amp instrumentation amplifier. In both cases, the circuits simply cannot work in single-supply situations unless a false ground between the supplies is created. Rev. E | Page 19 of 24 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION PIN CONFIGURATIONS TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ELECTRICAL SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION INPUT OVERVOLTAGE PROTECTION OUTPUT VOLTAGE PHASE REVERSAL OVERDRIVE RECOVERY APPLICATIONS INFORMATION SINGLE 3 V SUPPLY, INSTRUMENTATION AMPLIFIER SINGLE-SUPPLY RTD AMPLIFIER A 2.5 V REFERENCE FROM A 3 V SUPPLY 5 V ONLY, 12-BIT DAC SWINGS RAIL-TO-RAIL A HIGH-SIDE CURRENT MONITOR A 3 V, COLD JUNCTION COMPENSATED THERMOCOUPLE AMPLIFIER SINGLE-SUPPLY, DIRECT ACCESS ARRANGEMENT FOR MODEMS 3 V, 50 HZ/60 HZ ACTIVE NOTCH FILTER WITH FALSE GROUND SINGLE-SUPPLY, HALF-WAVE, AND FULL-WAVE RECTIFIERS OUTLINE DIMENSIONS ORDERING GUIDE