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MX7705 Datasheet(PDF) 19 Page - Maxim Integrated Products |
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MX7705 Datasheet(HTML) 19 Page - Maxim Integrated Products |
19 / 34 page ![]() Increasing the gain increases the resolution of the ADC (LSB size decreases), but reduces the differential input voltage range. Calculate 1 LSB in unipolar mode using the following equation: where VREF = VREF+ - VREF-. For a gain of one and VREF = 2.5V, the full-scale volt- age in unipolar mode is 2.5V and 1 LSB ≈ 38.1µV. For a gain of four, the full-scale voltage in unipolar mode is 0.625V (VREF / GAIN) and 1 LSB ≈ 9.5µV. The differen- tial input voltage range in this example reduces from 2.5V to 0.625V, and the resolution increases, since the LSB size decreased from 38.1µV to 9.5µV. Calculate 1 LSB in bipolar mode using the following equation: where VREF = VREF+ - VREF-. Unipolar and Bipolar Modes The B/U bit in the setup register (Table 9) configures the MX7705 for unipolar or bipolar transfer functions. Figures 4 and 5 illustrate the unipolar and bipolar trans- fer functions, respectively. In unipolar mode, the digital output code is straight binary. When AIN+ = AIN-, the outputs are at zero scale, which is the lower endpoint of the transfer func- tion. The full-scale endpoint is given by AIN+ - AIN- = VREF / GAIN, where VREF = VREF+ - VREF-. In bipolar mode, the digital output code is in offset binary. Positive full scale is given by AIN+ - AIN- = +VREF / GAIN and negative full scale is given by AIN+ - AIN- = -VREF / GAIN. When AIN+ = AIN-, the outputs are at zero scale, which is the midpoint of the bipolar transfer function. When the MX7705 is in buffered mode, the absolute and common-mode analog input voltage ranges reduce to between (GND + 50mV) and (VDD - 1.5V). The differential input voltage range is not affected in buffered mode. 1 65 536 2 (, ) LSB V GAIN REF =× 1 65 536 (, ) LSB V GAIN REF = 16-Bit, Low-Power, 2-Channel, Sigma-Delta ADC ______________________________________________________________________________________ 19 Table 5. Input Sampling Capacitor vs. Gain GAIN INPUT SAMPLING CAPACITOR (CSAMP) (pF) 1 3.75 2 7.5 415 8–128 30 01 2 3 0000 0000 0000 0000 0000 0000 0000 0001 0000 0000 0000 0010 0000 0000 0000 0011 FULL-SCALE TRANSITION 1 LSB = VREF (GAIN) (65,536) 65,535 65,533 DIFFERENTIAL INPUT VOLTAGE (LSB) 1111 1111 1111 1101 1111 1111 1111 1110 1111 1111 1111 1111 1111 1111 1111 1100 VREF / GAIN Figure 4. MX7705 Unipolar Transfer Function 1 LSB = VREF (GAIN) (65,536) 0+1 -1 0000 0000 0000 0000 0000 0000 0000 0001 0000 0000 0000 0010 +32,767 +32,765 1111 1111 1111 1101 1111 1111 1111 1110 1111 1111 1111 1111 1000 0000 0000 0000 1000 0000 0000 0001 0111 1111 1111 1111 x 2 -32,768 -32,766 DIFFERENTIAL INPUT VOLTAGE (LSB) VREF / GAIN VREF / GAIN Figure 5. MX7705 Bipolar Transfer Function |
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