Analog Tip; Choosing SAR vs Sigma-Delta ADCs for high dynamic range applications

October 17, 2014 // By Maithil Pachchigar, Analog Devices
High-performance data-acquisition signal chains used in industrial, instrumentation, and medical equipment require wide dynamic range and accurate measurements. The dynamic range of an ADC can be increased by adding a programmable-gain amplifier or operating multiple ADCs in parallel, using digital post-processing to average the result, but these methods can be impractical due to power, space, and cost constraints.

High-performance data-acquisition signal chains used in industrial, instrumentation, and medical equipment require wide dynamic range and accurate measurements. The dynamic range of an ADC can be increased by adding a programmable-gain amplifier or operating multiple ADCs in parallel, using digital post-processing to average the result, but these methods can be impractical due to power, space, and cost constraints.

Oversampling allows an ADC to achieve high dynamic range at low cost, while also addressing tough space, thermal, and power design challenges.

Oversampling is performed by sampling the input signal at much higher rate than the Nyquist rate (twice the signal bandwidth) to increase the signal-to-noise ratio (SNR) and effective number of bits (ENOB). When the ADC is oversampled, the quantisation noise is spread such that most of it occurs outside the bandwidth of interest, resulting in increased overall dynamic range at low frequencies. The noise outside the bandwidth of interest can be eliminated using digital post-processing as shown in Figure 1. The oversampling ratio (OSR) is the sampling rate divided by the Nyquist rate. The improvement in dynamic range (ΔDR) due to oversampling is ΔDR = log2 (OSR) × 3 dB. For example, oversampling the ADC by a factor of four provides a 6 dB increase in dynamic range, or one additional bit of resolution.