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ADR3440 Datasheet(PDF) 3 Page - Analog Devices |
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ADR3440 Datasheet(HTML) 3 Page - Analog Devices |
3 / 9 page Circuit Note CN-0287 Rev. C | Page 3 of 9 reference resistor that is driven with the same current (i.e. a ratiometric measurement). Minimizing the leakage current through the current path is important for achieving high accuracy because the excitation current is typically only a few hundred microamps to prevent self heating. For the industrial field applications both high performance as well as protection against both high-voltage transient events and dc over-voltage conditions are important design considerations. How this Circuit Works The circuit shown in Figure 1 is designed for precision temperature measurement applications in the industrial field environment and is optimized for flexibility, performance, robustness, and cost. This circuit uses the AD7193, low noise, 24-bit sigma-delta ADC to ensure high resolution and linearity for the entire circuit. The AD5201, 33-position digital potentiometer, AD8603 op amp, and ADG702 single channel switch constitute a simple programmable current source and bias voltage buffer for the RTD and thermocouple measurements. The ADG738 routes the current source to the active RTD channel and allows wire resistance compensation for the 3-W RTD configuration. The ADT7310 digital SPI temperature sensor has ±0.8°C maximum accuracy (+5 V supply) from −40°C to +105°C and is used for cold-junction compensation for the thermocouple measurement. The ADR3440 is a low noise and high accuracy 4.096 V reference connected to REFIN1(+)/REFIN1(−) of the AD7193 for the thermocouple measurements. Analog-to-Digital Converter The AD7193 is a low noise, complete analog front end for high precision measurement applications. It contains a low noise, 24- bit sigma-delta (Σ-Δ) analog-to-digital converter (ADC). This ADC achieves high resolution, low non-linearity, and low noise performance as well as very high 50 Hz/60 Hz rejection. The data output rate can be varied from 4.7 Hz (24 bits effective resolution, Gain = 1), to 4.8 kHz (18.6 bits effective resolution, Gain = 1). The on-chip low noise PGA amplifies the small differential signal from the thermocouple or RTD with a gain programmable from 1 up to 128, thereby allowing a direct interface. The gain stage buffer has high input impedance and limits the input leakage current to ± 3 nA maximum. The gain of theAD7193 must be configured properly depending on the temperature range and type of sensors. The on-chip multiplexer allows four differential input channels to be shared with the same ADC core, saving both space and cost. Programmable Current Source for RTDs and Bias Voltage Generator Circuit for Thermocouples RTD measurements require a low noise current source that drives the RTD and a reference resistor. Thermocouple measurements, on the other hand, need a common-mode bias voltage that shifts the small thermocouple voltage into the input range of the AD7193. The circuit shown in Figure 2 meets both requirements and utilizes the AD8603 a low noise CMOS rail- to-rail input/output op amp with only 1 pA maximum input bias current and 50 μV maximum offset voltage, combined with the ADG702 single channel, CMOS low voltage 2 Ω SPST switch, and the ADG738 eight-channel matrix switch. Figure 2. External Programmable Current Source and Bias Voltage Generator With the ADG738 opened and the ADG702 closed, the AD8603 acts as a low noise, low output impedance unity-gain buffer for the thermocouple application. The voltage from the AD5201 digital potentiometer is buffered and is used for the thermocouple common-mode voltage, usually 2.5 V, which is one-half the supply voltage. The 33-position AD5201 digital potentiometer is driven with the ADR3440 low drift (5 ppm/°C) 4.096 V reference for accuracy. With the ADG738 closed and the ADG702 opened, the AD8603 generates the RTD excitation current, IEXC = VW/RREF. AD7193 REFIN2(–) REFIN2(+) AIN2 AIN1 D S ADG702 1kΩ R3 C2 D S1 ADG738 A W B AD5201 +4.096V AD8603 +5V RTD TC VW IEXC IEXC = VW RREF RREF |
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Descripción similar - ADR3440 |
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