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MAX1460 Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX1460 Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 20 page Low-Power, 16-Bit Smart ADC 8 _______________________________________________________________________________________ analog front-end settings when the sensor’s character- istics are unknown. Use the tabulated values (Tables 1 and 2) if the peak sensor excursions are known. See the Test System Interface section for details on writing these analog front-end bits. The PGA gain and the CO are very stable, but are not accurate. Manufacturing variances on the gain and off- set of the MAX1460 analog front-end superposition the residual sensor errors, and are later removed during final calibration. For example, suppose the sensor’s sensitivity is +10mV/V with an offset of -12mV/V. Let the supply volt- age be +5V. The full scale (-FS) output of the sensor is then +5V(-12mV/V) = -60mV; +FS is then +5V (-12mV/V + 10mV/V) = -10mV. Following through the flowchart, the PGA gain setting is +3 (gain = 93V/V) and the CO correction setting is +1 (+25mV RTI) - (Referred-to Input). The coarsely corrected -FS input to the ADC is (-60mV + 25mV)93 = -3.255V. The +FS input to the ADC is (-10mV + 25mV)93 = +1.395V. The input range of the ADC is ±VDD. Thus the maximum and minimum digitized sensor signals become -3.255 / 5 = -0.651 and +1.395 / 5 = +0.279. Notice that the bridge multiplies the signal by VDD and the ADC divides the signal by VDD. Thus, the system is ratiometric and not dependent on the DC value of VDD. The ADC output clips to ±1.0 when input values exceed ±VDD. The best signal-to-noise ratio (SNR) is achieved when the ADC input is within ±85% of VDD (Figure 2). The MAX1460 includes an internal temperature-sensing bridge allowing the MAX1460 temperature to be used as a proxy for the sensor temperature. For this reason, the MAX1460 must be mounted in thermal proximity to the sensor. The output of the temperature-sensing bridge is also corrected by a 3-bit coarse-offset DAC and processed by the ADC. The selection of the Temperature Sensor Offset (TSO) bits in the configura- tion register should be made so that the digitized tem- perature signal is as close to 0.0 as possible at midscale temperature. This is done to maximize the dynamic range of the thermal-calibration coefficients (Table 3). -0.010 -0.006 -0.008 -0.002 0.004 0.002 0 0.004 0.008 0.006 0.010 -100 -60 -40 -20 -80 0 20 40 80 60 100 SENSOR SIGNAL INPUT OR ADC INPUT/OUTPUT RANGE (%) Figure 2a. Analog Front-End INL (typical) -4 -3 0 -1 -2 1 2 3 4 -100 -20 -40 -80 -60 0 20 40 60 80 100 SENSOR SIGNAL INPUT OR ADC INPUT RANGE (%) Figure 2b. Analog Front-End Differential Nonlinearity (DNL) (typical) 0 1.0 0.5 1.5 3.0 3.5 2.5 2.0 4.0 -100 -60 -40 -20 0 -80 20 40 60 80 100 SENSOR SIGNAL INPUT OR ADC INPUT/OUTPUT RANGE (%) Figure 2c. Analog Front-End Noise Standard Deviation of the Samples (typical) |
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