| 摘要 |
PURPOSE:To improve the resolution to an analog input signal low in level by applying a different upper limit reference voltage respectively to plural A/D converters having a prescribed bit to which an analog signal is inputted in common. CONSTITUTION:The input analog signal V1 is inputted in common to each data input terminal Vin of 8-bit A/D converters ADC1, 2, a voltage Vr is fed to an upper limit reference voltage terminal Vref of the ADC1 and a Vr/4 is fed to a terminal Vref pf the ADC2 while being divided by resistors 32, 33. When the signal V1 is within a range of Vr/4-Vr, an H level is outputted to an overflow terminal OF of the ADC2 and a digital signal of the ADC1 is outputted to terminals OD2-OD9 from the buffer 11. The output of a tri-stage buffer 12 is at a high impedance and terminals D0, D1 are fixed to logical 0 by pull-down resistors 40, 41. When the signal V1 is less than the Vr/4, the output of the buffer 11 shows a high impedance, the buffer 12 gives a digital signal of the ADC2 to the terminals OD0-OD7 and the level of the OD8,OD9 are fixed to logical 0 by resistors 42, 43.
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