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Signal Chain Basics #120: Designing front-end drivers for linear performance on SAR ADCs

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 ).

Figure 1

SAR ADC input types.

SAR ADC input types.

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.

Figure 2

Fully-differential SAR input range specifications.

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 VCM ranges from 0V to VREF . Table 1 shows the analog input voltage range specification of this ADC.

Table 1

Input range specifications of the ADS8881, a fully-differential SAR.

Input range specifications of the ADS8881, a fully-differential SAR.

Driving the fully-differential SAR ADC

Driving a fully-differential SAR ADC can be achieved by using a dual operational amplifier (op amp) in the inverting configuration or a fully-differential amplifier (FDA) (Figure 3 ).

Figure 3

Amplifier topologies that drive a fully-differential SAR ADC.

Amplifier topologies that drive a fully-differential SAR ADC.

One benefit of the FDA is that the output common-mode voltage can be controlled independently of the differential voltage using the VOCM input pin.

Amplifier output stages that can swing close to the complete span between negative and positive supply voltage are generally known as rail-to-rail output (RRO) stages. Most RRO stages can swing only within tens of millivolts of the rails. This output range limitation is generally specified in the datasheet as output swing (output voltage low/high). Table 2 shows the output swing specifications for the THS4551, a fully-differential amplifier.

Table 2

Output swing of the fully-differential amplifier.

Output swing of the fully-differential amplifier.

Although the amplifier functions in this region, the performance may start to degrade as the output gets closer to the rails as the amplifier open-loop gain is reduced. Therefore, it is advisable to allow extra headroom in applications that require very low distortion. Figure 4 illustrates the trade-off of differential output voltage versus distortion:

Figure 4

Differential output voltage (Vpp) versus distortion (dBc) for the fully-differential amplifier.

Differential output voltage (Vpp ) versus distortion (dBc) for the fully-differential amplifier.

In order for the amplifier to cover the complete span from negative to positive full-scale, enough headroom from the rail supplies must be provided. In many cases, the amplifier supply is larger than the reference voltage used on the system, which allows enough headroom from the positive rail. In order to reach the negative full-scale, you can connect a slightly negative supply voltage that is large enough to compensate for the headroom loss. You can refer to “Extending rail-to-rail output range for fully differential amplifiers” for a design example using asymmetrical supplies to extend range to negative full-scale.

A second technique to reach negative full-scale is to shift the VCM voltage using the FDA’s VOCM pin, provided this shift is still within the ADC’s common-mode range Figure 5 shows an example with a FDA and a SAR using analog supply of 5 V, VREF = 4.5 V. VCM is set +138 mV higher than VREF /2 to compensate for the negative swing limitation. In this case the amplifier can swing ±4.3 V (or –0.4 dB from full-scale):

Figure 5

Extending the fully-differential amplifier output range

Extending the fully-differential amplifier output range

To optimize the linear performance of a SAR acquisition system, you can refer to this article which provides an overview of the ADC’s input range specifications and explains the amplifier’s output swing considerations.

References

  1. Amit Kumbasi, Input considerations for SAR ADCs, TI Precision Hub blog, June 20, 2014.
  2. Luke LaPointe, Extending rail-to-rail output range for fully differential amplifiers, TI Designs, April 2014.
  3. James Karki, Fully-differential amplifiers, TI Application Report (SLOA054), January 2002.

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