ADC0804 is a commonly used 8-bit analog to digital convertor. It is a single cha

Question

ADC0804 is a commonly used 8-bit analog to digital convertor. It is a single channel IC (Fig. 1). It can take only one analog signal input. The digital outputs vary from 0 to a maximum of 255. The step size can be adjusted by setting the reference voltage at Pin-9. When this pin is not connected, the default reference voltage is the operating voltage (Vcc). The step size at 5V is 19.5 mV. (5volts/256) Therefore, for every 19.5 mV rise in the analog input, the output increases by 1 unit (1 step). To set a particular voltage level as the reference value, pin-9 is connected to half the max allowed input voltage. For example, to set a reference of 4V (Vref), Pin9 is connected to 2V (Vref /2), thereby reducing the step size to 15.6 mV. Verify this step size by making a calculation. What would the step size be if the input were to only vary from 0 to 3V but you wanted a full range of digital values on the output?

Explanation / Answer

When the Pin9 is connected to 2V(Vref/2), the reference voltage is 4V.

So the step size : 4 volts/256=15.6 mV (So the given step size is correct).

If the input voltage vary from 0 to 3V, reference votage is 3V and you should connect the Pin9 to 1.5V (Vref/2).

So the step size : 3 volts/256=11.7 mV

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