Cirrus Logic CS5529 - Manual

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For further information, please contact Crystal Semiconductor
at (512) 445-7222 or 1 (800) 888-5016
Copyright  Cirrus Logic, Inc. 1997
(All Rights Reserved)
Cirrus Logic, Inc.
Crystal Semiconductor Products Division
P.O. Box 17847, Austin, Texas 78760
(512) 445 7222 FAX: (512) 445 7581
http://www.crystal.com
CS5525/6/9 FAQ
Technical Brief
FAQ (FREQUENCLY ASKED QUESTIONS)
1) Do you have a four channel part?
Not at this time, but we have plans to do a multi-
channel product Q4 ‘97. We also have 4 digital
output lines which can be used to control either
switches or a multiplexer through the ADC’s se-
rial port, thus eliminating the use of an extra port
on the system µC and additional opto-isolators
in isolated applications.
2) How does the 4-bit digital latch on a DS ADC
allow me to change channels?
The CS5525 and CS5526 as well as the CS5504
family of ADC’s are designed to settle in one
conversion cycle. This means a mux can be
switched from channel-to-channel with every
conversion while maintaining resolution and ac-
curacy.
3) What determines the input span of the convert-
er?
Performing a full scale gain calibration, or mod-
ifying the reference voltage. For example, if the
reference voltage is reduced by 50% the default
input ranges scale by one half. Example:
Vref = 2.5 V, Vin = 25 mV to 5 V and
Vref = 1.25 V, Vin = 12.5 mV to 2.5 V.
4) How does the output word rate affect the ADC’s
bandwidth?
The input bandwidth is limited to 1/2 the select-
ed output word rate due to the Nyquist theory of
sampling. Example: With the default 15 Hz out-
put word rate the available signal bandwidth of
the ADC is 7.5 Hz.
5) What is recommended if I need more or less
bandwidth than is provided by the on-chip digi-
tal filter?
Use an external clock between 30 kHz and
100 kHz to scale the digital filters corner fre-
quency accordingly. Example: Using a 3x clock
= 3x32.768 kHz = 3 x the word rate = 3 x
3.76 Hz to 3 x 202 Hz = 11.28 Hz to 606 Hz.
6) How fast can the converter shift data from its se-
rial port?
Up to 2 MHz.
7) How does the instrumentation amplifier’s chop-
ping frequency affect the converter’s input im-
pedance and input current?
The input impedance of the converter is a dy-
namic impedance and depends on whether the
instrumentation amplifier is engaged or not. For
the lower ranges (25 mV, 55 mV, 100 mV), the
instrumentation amplifier is engaged setting the
input impedance to 1/fC (where C is 2 pF, and f
is the chopping frequency, either 256 or
32,768). A typical input impedance for the low-
er ranges is 1900 MW (with f = 256, and C =
2 pF). For the higher ranges (1 V, 2.5 V, and
5 V), the amplifier is bypassed leaving an equiv-
alent input impedance of 1/fC where C is 32 pF
and f is either 256 or 32,768. A typical input im-
pedance for the higher ranges is 120 MW (with
f = 256 and C = 2 pF).
The input current is a dynamic current and also
depends on whether the instrumentation ampli-
fier is engaged or not. For the lower ranges
NOV ‘97
DS202TB1