Input Considerations for SAR ADCs

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How might an input signal affect the selection of the best successive approximation register (SAR) analog-to-digital converter (ADC) for an application?

When we hear the word “input,” several things immediately come to mind: frequency, amplitude, sine waves, sawtooth waves, etc. All of these are relevant issues to consider when optimizing signal conditioning.

However, one thing many people don’t 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) and how to use them in an application. In a future blog, I will also discuss the performance differences and some important practical considerations that must be taken into account to obtain optimal input performance.

Single-ended input SAR ADC

The single-ended input is the simplest of the three input types because the ADC has only one input. As long as the fed signal is within the range specified by the input pin, the SAR will digitize the input with respect to the SAR ground (see Figure 1).

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Figure 1 : Single-ended conversion example

Although most single-ended SAR ADCs can process unipolar signals, some can be used to process bipolar signals whose amplitude (A) can easily exceed the power supplies. Some support one channel, while others support multiple channels. A common application using single-ended ADC inputs is power supply voltage monitoring.

Here is more information about the single-ended input SAR ADC used in Figure 1:

Part Number	Resolution	Adoption rate
ADS8568	16 bit	500 kSPS
ADS8517	16 bit	200 kSPS
ADS8528	12 bits	650 kSPS
ADS7866	12 bits	200 kSPS
ADS7867	10 digits	280 kSPS
ADS7868	8-bit	280 kSPS

Pseudo-differential input SAR ADC

A pseudo differential SAR ADC has two input pins, but is called "pseudo differential" because proper ADC conversions occur when one input is held at a fixed DC voltage (typically REF/2) while the other input can accept a dynamically changing input signal. The differential signal between the two inputs (AINP-AINM) can then be converted to a digital code. A +/-100mV headroom is typically provided for input variation. Figure 2 shows this input and a unique case where the fixed input (AINM) can be tied to signal ground, making it similar to a single-ended input type.

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Figure 2 : Pseudo-differential input configuration

One of the most common applications using this configuration is current shunt monitoring, where the voltage on one side of a series resistor is not only measured against a fixed DC voltage, but it is also converted back into a current.

Example of a pseudo differential input SAR ADC used in Figure 2:

Part Number	Resolution	Sampling rate
ADS8319	16 bit	500 kSPS
ADS8317	16 bit	250 kSPS
ADS8339	16 bit	250 kSPS
ADS8324	14-bit	50 kSPS

Fully Differential Input SAR ADC

A fully differential input SAR ADC accepts two sets of inputs, one of which is the complement of the other (see Figure 3 ) . The differential signal between these two sets of inputs (V _DIFF = AINP – AINM) is converted.

In most differential input SARs, there is a limit on the common-mode voltage at the ADC input (V _CM = (AINP + AINM)/2), which translates to a fixed DC offset of the two signals (typically REF/2 with a tolerance of +/-100mV).

However, some newer SAR ADCs offer a special input stage that can handle a common-mode voltage that can vary between 0 and REF, as shown in Figure 3. This input is called a true differential input.

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Figure 3 : Fully differential input configuration

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

Here is more information about the fully differential input SAR ADC used in Figure 3:

Part Number	Resolution	Sampling rate
ADS8881	18-bit	1 MSPS
ADS8861	16 bit	1000 kSPS
ADS8318	16 bit	500 kSPS
ADS8323	16 bit	500 kSPS