Autoranging Circuit

To extend the measurement range of an available ADC(analog to digital conveter) we can use this autoranging. If it implemented on multiplexed input, this circuit will give best result. To adjust an ADC’s full-scale range we use a system autoranging that use LTC1257  because it is appropriate for system autoranging.

During the conversion process, U2 and U3 receive the full and zero scale codes, respectively, that correspond to a selected multiplexer channel. For example, let channel 2’s span begin at 2V and end at 4.5V. When a host processor wants a conversion of channel 2’s input signal, it first sends code that sets the output of U2 to 2V and U3 to 4.5V, fixing the span to 2.5V. The processor then sends data to the LTC1296 selecting channel 2. The processor next clocks the LTC1296 and reads the data generated during the conversion of the 3.5VP-P signal applied to channel 2. As other multiplexer channels are selected the DAC outputs are changed to match their spans.

The reference voltage still give us a problem, because it is general to have have input signals with different full-scale and zero -scale magnitude requirements. To solve this problem we can matching the reference voltage span to each multiplexer input. However this method give us some disadvantage, including inputs that do not span the same range will not generate all codes, reducing the ADC’s effective resolution. [Circuit’s schematic diagram source: Linear Technology Application Notes]