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XTR108EA Datasheet(PDF) 11 Page - Texas Instruments |
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XTR108EA Datasheet(HTML) 11 Page - Texas Instruments |
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11 / 33 page  XTR108 11 SBOS187C www.ti.com The circuit is designed for compliance with NAMUR NE43 recommendation for sensor interfaces. The limit levels are listed in Tables VII and VIII. Because of the large step sizes, units that use this feature should be checked if the value is critical. The under-scale limit circuit will override the Zero DAC level if it is set lower and there is not enough sensor offset at the PGA input. It may be necessary to disable limiting if the XTR108 is used in applications other than a 4-20mA transmitter, where the PGA output is between 0.5V and 4.5V. SENSOR FAULT DETECTION CIRCUIT To detect sensor burnout and/or short, a set of four compara- tors is connected to the inputs of the PGA. If any of the inputs are taken outside of the PGA’s common-mode range, the corresponding comparator sets a sensor fault flag that causes the PGA output to go either to the upper or lower error limit. The state of the fault condition can be read in the digital form from register 3. The direction of the analog output is set according to the “Alarm Configuration Regis- ter” (see Table X). The level of the output is produced as follows: if the over-scale/under-scale limiting is enabled, the error levels are: over-scale limit +2LSBs of the over-scale DAC, about 1mA referred to IOUT or 0.125V referred to VO, of under-scale limit –2LSBs of the under-scale DAC, about 0.4mA referred to IOUT or 0.05V referred to VO. If the over- scale/under-scale limiting is disabled, the PGA output volt- age will go to within 150mV of either positive or negative supply (VS or IRET), depending on the alarm configuration bit corresponding to the error condition. OUTPUT CURRENT AMPLIFIER + RVI RESISTOR To produce the 4-20mA output, the XTR108 uses a current amplifier with a fixed gain of 50A/A. The voltage from the PGA is converted to current by the external resistor, RVI. Pin IRET, the common potential of the circuit (substrate and local ground), is connected to the output and inverting input of the amplifier. This allows collecting all external and internal supply currents, sensor return current, and leakage currents from the different parts of the system and accounting for them in the output current. The current from RVI flows into the pin IIN that is connected to the noninverting input and therefore, is at ground potential as well. The ratio of two VOLTAGE REFERRED TO VO PIN WITH RESPECT TO IRET CURRENT REFERRED TO IOUT PIN OVERALL VZERO = VZ PROGRAM + VZ COARSE + VZ FINE IZERO = IZ PROGRAM + IZ COARSE + IZ FINE PROGRAM V V Z PROGRAM REF = 35 8 . I V R Z PROGRAM REF VI = 175 8 COARSE DAC V V N Z COARSE REF =• 80 4 13 I V R N Z COARSE REF VI =• 5 84 13 FINE DAC V VN Z FINE REF =• 80 64 12 I V R N ZFINE REF VI =• 5 864 12 NOTE: N13 and N12 are assigned decimal values of registers 13 and 12, respectively. TABLE II. Equations for Calculating Zero Output. matched internal resistors determines a current gain of this block. Note that the IOUT pin is always biased below the substrate potential. EXCITATION CURRENT DACS AND RSET RESISTOR Two matched adjustable reference current sources are avail- able for sensor excitation. The defining equations are given in Table III. Both current sources are controlled simulta- neously by the coarse and fine DACs with a pedestal. The external resistor RSET is used to convert the REF voltage into the reference current for the sensor excitation DACs. The total current output of the DACs is split, producing two references: IREF1 and IREF2. Both of the current references match very closely over the full adjustment range without mismatched differential steps. Both current reference out- puts must be within the compliance range, i.e.: one reference cannot be floated since it will change the value of the other current source. The recommended value of RSET is 12.1kΩ for use with 100 Ω RTD sensors. This generates I REF1, 2 = 492µA currents when both coarse and fine DACs are set to zero. The value of the RSET resistor can be increased if lower reference currents are required, i.e.: for 1000 Ω RTD or a bridge sensor. Similar to the Zero DACs, the outputs of the fine and coarse DAC are summed together with the pedestal IREF PROGRAM. Each of the excitation DACs has 8-bit resolution (256 steps) with 4-bit overlap between the coarse and the fine. This REFERENCE CURRENT OVERALL IREF1, 2 = IREF PROGRAM + IREF COARSE + IREF FINE PROGRAM I V R REF PROGRAM REF SET = 5 COARSE DAC I V R N REF COARSE REF SET =• 11 64 FINE DAC I V R N REF FINE REF SET =• 10 1024 NOTE: N11 and N10 are the decimal values of registers 11 and 10, respectively. TABLE III. Equations for Calculating the Values of Each Reference Current. |
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