Input considerations for SAR ADCs

This technical article was updated on July 23, 2020. 

Did you know that the input signal might affect how you select the best successive approximation register (SAR) analog-to-digital converter (ADC) for your application?

When we hear the word, “input,” there are several things that immediately jump into our heads – like frequency, amplitude, sinusoidal, sawtooth, etc. – all of which are relevant questions to ask when optimizing the signal conditioning.

However, one thing that many do not consider up front is the actual input type of the SAR ADC.  In this blog, I will focus on the three types of SAR inputs:  single-ended, pseudo-differential, and differential inputs, and how you can employ these in your application. In future blogs, I’ll discuss performance differences and some of the key practical considerations that must be kept in mind to obtain the optimal input performance.

Single-ended input SAR ADCs

Single-ended input is the most simple of the three input types, because the ADC has only one input. As long as the signal fed is within the range specified on input pin, the SAR digitizes the input with respect to SAR ground (see Figure 1).

Figure 1:  Single-Ended Conversion Example

While the majority of single-ended SAR ADCs can handle unipolar signals, some are designed to handle bipolar signals with amplitude (A) that can easily exceed the power supply. Some support one channel, while others can support multiple channels. One common application that uses a single-ended ADC input is supply voltage monitoring.

Here’s some additional information on the single-ended input SAR ADCs used in Figure 1:

 Pseudo-differential input SAR ADCs

Pseudo-differential SAR ADCs have two input pins; however, it is called “pseudo-differential” because a proper ADC conversion happens when one input remains at a fixed DC voltage (typically REF/2) while the other input can accept a dynamically changing input signal. The difference signal between the two inputs (AINP-AINM) is then converted into a digital code. Typically, a head room of +/-100mV is provided for variations in inputs. Figure 2 illustrates this and a unique case in which the fixed input (AINM) is tied to signal ground, making it similar to a single-ended input type.

Figure 2:  Pseudo-Differential Input Configuration

One of the most common applications that employs this configuration is shunt monitoring, where the voltage on one side of a series resistor is measured with respect to a fixed DC voltage and converted back into a current.

Examples of pseudo-differential input SAR ADCs, as used in Figure 2:

Fully differential input SAR ADCs

Fully differential input SAR ADCs accept two inputs, where one input is complementary to the other (see Figure 3).  The differential signal between the two inputs (V_DIFF = AINP – AINM) is converted.

In most differential input SARs, there is restriction on the common mode voltage (V_CM = (AINP + AINM)/2) for the ADC inputs, which translates to a fixed DC offset for both signals (typically REF/2 with tolerance of +/-100mV).

However, as is shown in Figure 3, there are newer SAR ADCs with a unique input stage that can handle common mode voltages that can vary from 0 to REF. Such an input is referred to as true-differential input.

Figure 3:  Fully Differential and True Differential Input Configuration

Fully differential SAR ADCs support bipolar inputs and/or multiple channels, similar to single-ended SAR ADCs. Applications that use transformer output employ fully differential input SARs.

Here’s more background on the fully differential input SAR ADCs used in Figure 3:

 Now that I’ve explained some of the most common SAR ADC input types, be sure to read my two-part series on how performance varies between these input types.