Editor’s note: I am pleased to bring you this month’s Signal Chain Basics #120 by Luis Chioye, Applications Engineer, SAR Data Converters, Texas Instruments
In this article, I will discuss the input range specifications of the fully-differential successive approximation register (SAR) analog-to-digital converter (ADC), as well as driver amplifier output range considerations for linear performance.
SAR ADCs can be broadly categorized by the type of input stage. There are three groups of SAR ADCs: single-ended, pseudo-differential, and fully-differential (Figure 1).
An ADC with single-ended inputs converts the analog input voltage with respect to ground (GND). The pseudo-differential SAR digitizes the differential analog input voltage (AINP-AINM) where the positive input (AINP) accepts a dynamically changing signal, and the negative input (AINM) remains at a fixed DC voltage. A fully-differential SAR also converts the differential voltage across its inputs; however, in this case both inputs change dynamically and are complementary or inverted from each other. For more information on SAR ADC types, you can refer to the TI blog, “Input considerations for SAR ADCs.”. In this article I discuss unipolar fully-differential SARs.
Fully-differential SAR ADC
Typically, high-resolution SAR ADCs incorporate a differential input structure. This structure effectively doubles the output voltage swing by a factor of two for a given voltage rail, adding an additional 6-dB improvement to the signal-to-noise ratio (SNR) at full-scale. If the positive and negative input paths are tightly routed symmetrically and the impedances are matched, the externally-coupled noise affects both conductors equally. Noise tends to get rejected by the differential stage as a common-mode signal. Furthermore, in differential systems, matching between the positive and negative input paths tends to cancel the even harmonics improving distortion. Using precision resistors (0.1% tolerance) improves matching and enhances even harmonic cancellation.
Figure 2 shows an example of input range specifications of a unipolar fully-differential SAR.
Fully-differential SAR input range specifications.
Typically, three specifications define the operating input range of the differential ADC: full-scale input, operating input voltage and common-mode voltage ranges. The converter digitizes the input signal up to the full-scale voltage where the voltage reference (VREF) defines the full-scale value. The ADC inputs must remain inside the permissible operating voltage range for the ADC to remain in its linear region.
Fully-differential ADCs have an input voltage common-mode (VCM) range specification. VCM is defined as the average voltage between the inputs (Equation 1):
Most differential input SAR ADCs prohibit the input common-mode voltage from varying more than approximately 10 percent beyond the mid-scale input (VREF/2). However, an ADC like the ADS8881 offers a unique input stage that allows VCMranges from 0V to VREF. Table 1 shows the analog input voltage range specification of this ADC.
Input range specifications of the ADS8881, a fully-differential SAR.