cs5317 Cirrus Logic, Inc., cs5317 Datasheet - Page 6

• Manufacturer Part Number: cs5317
• Description: 16-bit, 20 Khz Oversampling A/d Converter
• Manufacturer: Cirrus Logic, Inc.

GENERAL DESCRIPTION
The CS5317 functions as a complete data conver-
sion subsystem for voiceband signal processing.
The A/D converter, sample/hold, voltage refer-
ence, and much of the antialiasing filtering are
performed on-chip. The CS5317’s serial interface
offers its 16-bit, 2’s complement output in a for-
mat
industry-standard micro’s and DSP’s.
The CS5317 also includes a phase-locked loop
that simplifies the converter’s application in sys-
tems which require sampling to be locked to an
external signal source. The CS5317 continuously
samples its analog input at a rate set by an exter-
nal clock source. On-chip digital filtering, an
integral part of the delta-sigma ADC, processes
the data and updates the 16-bit output register at
up to 20 kHz. The CS5317 can be read at any rate
up to 20 kHz.
The CS5317 is a CS5316 with an on-chip sam-
pling clock generator. As such, it replaces the
CS5316 and should be considered for all new de-
signs. In addition, a CS5316 look-alike mode is
included, allowing a CS5317 to be dropped into a
CS5316 socket.
THEORY OF OPERATION
The CS5317 utilizes the delta-sigma technique of
executing low-cost, high-resolution A/D conver-
sions. A delta-sigma A/D converter consists of
two basic blocks: an analog modulator and a digi-
tal filter.
Conversion
The analog modulator consists of a 1-bit A/D
converter (that is, a comparator) embedded in an
analog negative feedback loop with high open-
loop gain. The modulator samples and converts
the analog input at a rate well above the band-
width of interest (2.5 MHz for the CS5317). The
6
which
easily
interfaces
with
modulator’s 1-bit output conveys information in
the form of duty cycle. The digital filter then
processes the 1-bit signal and extracts a high
resolution output at a much lower rate (that is,
16-bits at a 20 kHz word rate with a 5 kHz input
bandwidth).
An elementary example of a delta-sigma A/D
converter is a conventional voltage-to-frequency
converter and counter. The VFC’s 1-bit output
conveys information in the form of frequency (or
duty-cycle), which is then filtered (averaged) by
the counter for higher resolution. In comparison,
the CS5317 uses a more sophisticated multi-order
modulator and more powerful FIR filtering to ex-
tract higher word rates, much lower noise, and
more useful system-level filtering.
Filtering
At the system level, the CS5317’s digital filter
can be modeled exactly like an analog filter with
a few minor differences. First, digital filtering re-
sides behind the A/D conversion and can thus
reject noise injected during the conversion proc-
ess (i.e. power supply ripple, voltage reference
noise, or noise in the ADC itself). Analog filtering
cannot.
Also, since digital filtering resides behind the
A/D converter, noise riding unfiltered on a near-
full-scale input could potentially saturate the
ADC. In contrast, analog filtering removes the
noise before it ever reaches the converter. To ad-
dress this issue, the CS5317’s analog modulator
and digital filter reserve headroom such that the
device can process signals with 100mV "excur-
sions" above full-scale and still output accurately
converted and filtered data. Filtered input signals
above full-scale still result in an output of all
ones.
An Application Note called "Delta Sigma Over-
view" contains more details on delta-sigma
conversion and digital filtering.
CS5317
DS27F4