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ADS1112IDGSR Datasheet(PDF) 7 Page - Texas Instruments |
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ADS1112IDGSR Datasheet(HTML) 7 Page - Texas Instruments |
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7 / 26 page  ADS1112 SBAS282D − JUNE 2003 − REVISED MARCH 2004 www.ti.com 7 absolute values of the minima and maxima are not the same; the maximum n-bit code is 2n−1 − 1, while the minimum n-bit code is −1 × 2n−1. For example, the ideal expression for output codes with a data rate of 16SPS and PGA = 2 is: Output Code + 16384 2 (V IN )) * (VIN*) 2.048V The ADS1112 outputs all codes right-justified and sign-extended. This feature makes it possible to perform averaging on the higher data rate codes using only a 16-bit accumulator. Table 2 shows the output codes for various input levels. SELF-CALIBRATION The previous expressions for the ADS1112 output code do not account for the gain and offset errors in the modulator. To compensate for these, the ADS1112 incorporates self-calibration circuitry. The self-calibration system operates continuously and requires no user intervention. No adjustments can be made to the self-calibration system, and none need to be made. The self-calibration system cannot be deactivated. The offset and gain error figures shown in the Electrical Characteristics include the effects of calibration. CLOCK OSCILLATOR The ADS1112 features an onboard clock oscillator, which drives the operation of the modulator and digital filter. The Typical Characteristics show variations in data rate over supply voltage and temperature. It is not possible to operate the ADS1112 with an external system clock. INPUT IMPEDANCE The ADS1112 uses a switched-capacitor input stage. To external circuitry, it looks roughly like a resistance. The resistance value depends on the capacitor values and the rate at which they are switched. The switching frequency is the same as the modulator frequency; the capacitor values depend on the PGA setting. The switching clock is generated by the onboard clock oscillator, so its frequency (nominally 275kHz) is dependent on supply voltage and temperature. The common-mode and differential input impedances are different. For a gain setting of the PGA, the differential input impedance is typically: 2.8M Ω/PGA The common-mode impedance also depends on the PGA setting. See the Electrical Characteristics for details. The typical value of the input impedance often cannot be neglected. Unless the input source has a low impedance, the ADS1112 input impedance may affect the measurement accuracy. For sources with high output impedance, buffering may be necessary. Bear in mind, however, that active buffers introduce noise, and also introduce offset and gain errors. All of these factors should be considered in high-accuracy applications. Because the clock oscillator frequency drifts slightly with temperature, the input impedances will also drift. For many applications, this input impedance drift can be neglected, and the expression given above for typical input impedance can be used. ALIASING If frequencies are input to the ADS1112 that exceed half the data rate, aliasing will occur. To prevent aliasing, the input signal must be bandlimited. Some signals are inherently bandlimited. For example, the output of a thermocouple, which has a limited rate of change, may nevertheless contain noise and interference components. These nuisance factors can fold back into the sampling band just as with any other signal. The ADS1112 digital filter provides some attenuation of high-frequency noise, but the digital filter Sinc1 frequency response cannot completely replace an anti-aliasing filter. For a few applications, some external filtering may be needed; in such instances, a simple RC filter will suffice. When designing an input filter circuit, remember to take into account the interaction between the filter network and the input impedance of the ADS1112. DATA RATE DIFFERENTIAL INPUT SIGNAL DATA RATE −2.048V(1) −1LSB ZERO +1LSB +2.048V 15SPS 8000H FFFFH 0000H 0001H 7FFFH 30SPS C000H FFFFH 0000H 0001H 3FFFH 60SPS E000H FFFFH 0000H 0001H 1FFFH 240SPS F800H FFFFH 0000H 0001H 07FFH (1) Differential input only; do not drive the ADS1112 inputs below −200mV. Table 2. Output Codes for Different Input Signals |
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