Analog Input Voltage

The Analog Input Voltage (A_IN) is the voltage to be measured and converted into a digital value. The input voltage should always be less than V_REF to avoid saturation of the ADC. The input voltage range is also called as conversion range.

In single-ended input⁽¹⁾, the ADC sampling and conversion is performed only on one analog input signal. The basic block diagram of a single-ended input is as shown below.

Figure 1. Single-ended Input

In differential input, the difference in voltage of two analog inputs is applied to the ADC module. This can be done either directly or after performing some amplification using suitable gain stages. Differential conversions are usually operated in signed mode, where the MSB of the output code acts as the sign bit. When using differential mode, offset error can be measured easily by setting up the positive and negative input on the same pin and the offset can be measured directly as the ADC does not require the Ground (GND) level as a reference.

For example, when two Analog Inputs are provided, A_IN1 = 1.5V & A_IN2 = 0.5V, the digital value of differential output (A_IN1 - A_IN2 = 1V) is expected.

Figure 2. Differential Input

Most ADCs can operate only using a positive input voltage. The ADCs whose A_IN falls in the positive range (0 < A_IN < V_REF) are called as Unipolar ADC. ADCs capable of accepting both positive and negative input voltages are called as Bipolar ADC⁽¹⁾. For example, in a Unipolar ADC, if the V_REF = 2V, then 0 < A_IN < 2V should always be maintained to get expected output.