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ADS1224IPWTG4 Datasheet(PDF) 10 Page - Texas Instruments |
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ADS1224IPWTG4 Datasheet(HTML) 10 Page - Texas Instruments |
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10 / 33 page  ADS1224 SBAS286C − JUNE 2003 − REVISED JANUARY 2009 www.ti.com 10 TEMPERATURE SENSOR On-chip diodes provide temperature-sensing capabili- ty. By setting the TEMPEN pin high, the selected analog inputs are disconnected and the inputs to the A/D converter are connected to the anodes of two diodes scaled to 1x and 64x in current and size inside the mux, as shown in Figure 18. By measuring the difference in voltage of these diodes, temperature changes can be inferred from a baseline temperature. Typically, the difference in diode voltages is 106mV at 25 °C, with a temperature coefficient of 360 µV/°C. A similar structure is used in the MSC1210 for temperature measurement. For more information, see TI application report SBAA100, Using the MSC121x as a High-Precision Intelligent Temperature Sensor, available for download at www.ti.com. AINP AINP1 AINN1 AINP2 AINN2 AINP3 AINN3 AINP4 AINN4 8I 1I 1X 8X AINN TEMPEN AVDD MUX0 MUX1 Figure 18. Measurement of the Temperature Sensor in the Input Multiplexer VOLTAGE REFERENCE INPUTS (VREFP, VREFN) The voltage reference used by the modulator is generated from the voltage difference between VREFP and VREFN: VREF = VREFP – VREFN. The reference inputs use a structure similar to that of the analog inputs. A simplified diagram of the circuitry on the reference inputs is shown in Figure 19. The switches and capacitors can be modeled with an effective impedance of: t sample 2 16pF + 500kW where fCLK = 2MHz. AVDD Self Gain Cal AINP (1) f CLK =2MHz AINN Zeff = 500k Ω(1) VREFP VREFN AVDD ESD Protection 16pF Figure 19. Simplified Reference input Circuitry ESD diodes protect the reference inputs. To prevent these diodes from turning on, make sure the voltages on the reference pins do not go below GND by more than 100mV, and likewise, do not exceed AVDD by 100mV: GND – 100mV < (VREFP, VREFN) < AVDD + 100mV During self gain calibration, all the switches in the input multiplexer are opened, VREFN is internally connected to AINN, and VREFP is connected to AINP. The input buffer may be disabled or enabled during calibration. When the buffer is disabled, the reference pins will be driving the circuitry shown in Figure 9 during self gain calibration, resulting in increased loading. To prevent this additional loading from introducing gain errors, make sure the circuitry driving the reference pins has adequate drive capability. When the buffer is enabled, the loading on the reference pins will be much less, but the buffer will limit the |
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