Texas Instruments SLAU039, User Manual

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Special Considerations When Using Sigma-Delta Converters
1-4
1.3 Special Considerations When Using Sigma-Delta Converters
Sigma-delta analog-to-digital converters typically consist of an analog modu-
lator (fourth order in the case of the AD50) followed by a digital filter section.
The modulator contains a 1-bit ADC (a comparator) which produces a 1-bit
wide data stream, which is applied to the input of the digital filter. It is also ap-
plied to the input of a 1-bit DAC, the output of which is fed back to the input of
the modulator. The 1-bit ADC is clocked much faster than the desired output
sampling frequency (F
S
), (64
×
F
S
for the AD50) and the large amount of quan-
tization noise generated is uniformly spread over a wide range of frequencies.
A noise-shaping filter within the analog modulator reduces the noise in the
pass-band, increasing it elsewhere. A low-pass digital filter then removes the
unwanted high frequency quantization noise and the signal is resampled at the
desired output frequency.
The sigma-delta DAC in the AD50 consists of a digital interpolating filter oper-
ating at 256
×
F
S
followed by a 1-bit DAC and a second-order modulator. The
digital filter removes most of the image frequencies, that would otherwise be
present at the output of a conventional DAC. This avoids the need for a high-or-
der analog low-pass reconstruction filter. Separate sin(x)/x compensation is
not needed, as this is inherent in the DAC architecture.
Sigma-delta converters have several advantages and a few disadvantages
relative to other types of ADC which are summarized below.
1.3.1 Advantages
-
Sigma-delta conversion is inherently linear because there is no reference
resistor chain as in flash or successive approximation converters. This re-
sults in extremely low distortion.
-
Inherent monotonicity
-
No missing codes
-
Antialias filtering is inherent within sigma-delta converters. This greatly
simplifies their application, because only a simple external RC filter is re-
quired at the input of the ADC to achieve the necessary alias rejection.
-
The digital filters which form an integral part of the ADC and DAC are usu-
ally of the finite impulse response type, which gives a linear phase charac-
teristic with high stability.
-
The cutoff frequency of the digital filters automatically tracks the sampling
rate.
-
DAC anti-image filtering is greatly simplified because an internal digital fil-
ter attenuates image frequencies. A simple analog filter is sufficient to re-
move HF noise.
-
No need for sin(x)/x compensation.
-
The sigma-delta architecture is compatible with dense, low-cost, digital IC
processes.