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ADS1245 Datasheet(PDF) 7 Page - Texas Instruments |
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ADS1245 Datasheet(HTML) 7 Page - Texas Instruments |
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7 / 23 page  ADS1245 SBAS287A − JUNE 2003 − REVISED SEPTEMBER 2003 www.ti.com 7 OVERVIEW The ADS1245 is an ADC comprised of a 3rd-order modulator followed by a digital filter. The modulator measures the differential input signal VIN = (AINP – AINN) against the differential reference VREF = (VREFP – VREFN). Figure 15 shows a conceptual diagram. The differential reference is scaled internally so that the full-scale input range is ±2VREF. The digital filter receives the modulator signal and provides a low-noise digital output. The filter also sets the frequency response of the converter and provides 50Hz and 60Hz rejection while settling in a single conversion cycle. A two-wire serial interface indicates conversion completion and provides the user with the output data. Modulator Digital Filter and Serial Interface Σ 2 VREFP VREFN V REF DRDY/DOUT SCLK CLK 2V REF Σ V IN AINN AINP X1 X1 Figure 15. Conceptual Diagram of the ADS1245 ANALOG INPUTS (AINP, AINN) The input signal to be measured is applied to the input pins AINP and AINN. The ADS1245 features a low-drift chopper-stabilized buffer to achieve very high input impedance. The input impedance can be modeled by resistors, as shown in Figure 16. The impedance scales inversely with fCLK frequency. For example, if the frequency of fCLK is reduced by a factor of two, the impedances ZeffA and ZeffB will double. Zeff A = 280GΩ Zeff B = 3.46GΩ Zeff A = 280GΩ f CLK =2.4576MHz. AINP AINN AVDD/2 AVDD/2 Figure 16. Effective Analog Input Impedances The ADS1245 accepts differential input signals, but can also measure unipolar signals. Note that the analog inputs (listed in the Electrical Characteristics table as Absolute Input Range) must remain between GND + 0.1V to AVDD − 1.25V. Exceeding this range will degrade linearity and result in performance outside specified limits. 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. A simplified diagram of the circuitry on the reference inputs is shown in Figure 17. The switches and capacitors can be modeled with an effective impedance equal to: t SAMPLE 2 25pF + 1MW for f CLK + 2.4576MHz 25pF S 2 S 1 S 1 AVDD VREFP VREFN AVDD ESD Protection ON OFF ON S 1 S 2 OFF t SAMPLE = 128/fCLK Figure 17. Simplified Reference Input Circuitry The ADS1245 is specified for operation with VREF = 1.25V, resulting in a full−scale input value of ±2.5V. However, the buffered analog inputs can accept voltages within the range of 0.10V to 3.75V, resulting in a maximum VIN of ±3.65V. Input voltages can be accurately measured over this entire range if a voltage reference of 1.825V is pro- vided. In any case, digital output codes will clip to the full scale value if the absolute input voltage range exceeds 2VREF. |
Número de pieza similar - ADS1245 |
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Descripción similar - ADS1245 |
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