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AD693AQ Datasheet(PDF) 11 Page - Analog Devices |
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AD693AQ Datasheet(HTML) 11 Page - Analog Devices |
11 / 12 page AD693 REV. A –11– Table II. Thermocouple Application—Cold Junction Compensation 30 mV 60 mV AMBIENT TEMP TEMP POLARITY MATERIAL TYPE TEMP RCOMP RZ RANGE RANGE + IRON J 25 ° 51.7 Ω 301K 546 °C 1035 °C – CONSTANTAN 75 ° 53.6 Ω 294K + NICKEL-CHROME 25 ° 40.2 Ω 392K 721 °C— _ NICKEL-ALUMINUM K 75 ° 42.2 Ω 374K + NICKEL-CHROME 25 ° 60.4 Ω 261K E 413 °C 787 °C – COPPER-NICKEL 75 ° 64.9 Ω 243K + COPPER 25 ° 40.2 Ω 392K T USE WITH GAIN >2 – COPPER-NICKEL 75 ° 45.3 Ω 340K Figure 19. Thermocouple Inputs with Cold Junction Compensation Table III lists the expressions required to calculate the total error. The AD693 is tested with a 250 Ω load, a 24 V loop supply Table III. RTI Contributions to Span and Offset Error RTI Contributions to Offset Error Error Source Expression for RTI Error at Zero IZE Zero Current Error IZE/XS PSRR Power Supply Rejection Ratio (|VLOOP – 24 V| + [|RL – 250 Ω| × IZ]) × PSRR CMRR Common-Mode Rejection Ratio |VCM – 3.1 V| × CMRR IOS Input Offset Current RS × IOS RTI Contributions to Span Error Error Source Expression for RTI Error at Full Scale XSE Transconductance Error VSPAN × X SE XPSRR Transconductance PSRR 1 |RL – 250 Ω| × IS × PSRR XCMRR Transconductance CMRR |VCM – 3.1 V| × VSPAN × XCMRR XNL Nonlinearity VSPAN × X NL IDIFF Differential Input Current 2 RS × I DIFF Abbreviations IZ Zero Current (usually 4 mA) IS Output span (usually 16 mA) RS Input source impedance RL Load resistance VLOOP Loop supply voltage VCM Input common-mode voltage VSPAN Input span XS Nominal transconductance in A/V 1The 4–20 mA signal, flowing through the metering resistor, modulates the power supplyvoltage seen by the AD693. The change in voltage causes a power supply rejection error that varies with the output current, thus it appears as a span error. 2The input bias current of the inverting input increases with input signal voltage. The differential input current, IDIFF, equals the inverting input current minus the noninverting input current; see Figure 2. IDIFF, flowing into an input source impedance, will cause an input voltage error that var- ies with signal. If the change in differential input current with input signal is approximated as a linear function, then any error due to source impedance may be approximated as a span error. To calculate IDIFF, refer to Figure 2 and find the value for IDIFF/ + In corresponding to the full-scale input voltage for your application. Multiply by + In max to get IDlFF. Multiply IDIFF by the source impedance to get the input voltage error at full scale. via a set of thermocouple tables referenced to °C. For example, the output of a properly referenced type J thermocouple is 60 mV when the hot junction is at 1035 °C. Table II lists the maximum measurement temperature for several thermocouple types using the preadjusted 30 mV and 60 mV input ranges. More convenient temperature ranges can be selected by deter- mining the full-scale input voltages via standard thermocouple tables and adjusting the AD693 span. For example, suppose only a 300 °C span is to be measured with a type K thermo- couple. From a standard table, the thermocouple output is 12.207 mV; since 60 mV at the signal amplifier corresponds to a 16 mA span at the output a gain of 5, or more precisely 60 mV/ 12.207 mV = 4.915 will be needed. Using a 12.207 mV span in the gain resistor formula given in “Adjusting Input Span” yields a value of about 270 Ω as the minimum from P1 to 6.2 V. Adding a 50 Ω potentiometer will allow ample adjustment range. With the connection illustrated, the AD693 will give a full-scale indication with an open thermocouple. ERROR BUDGET ANALYSIS Loop-Powered Operation specifications refer to parameters tested with the AD693 operating as a loop-powered transmitter. The specifications are valid for the preset spans of 30 mV, 60 mV and those spans in between. The section, “Components of Error,” refers to parameters tested on the individual functional blocks, (Signal Amplifier, V/I Converter, Voltage Reference, and Auxiliary Amplifier). These can be used to get an indication of device performance when the AD693 is used in local power mode or when it is adjusted to spans of less than 30 mV. |
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