CS5525/6/9 FAQ
Technical Brief
FAQ (FREQUENCLY ASKED QUESTIONS)
1) Do you have a four channel part?
Not at thi s time , but we have pl ans to do a mu lti-
channel produc t Q4 ‘97. We also hav e 4 digit al
output lines whic h can be used to contr ol e ithe r
switches or a multiplexer through the ADC’s serial port, thus eliminating the use of an extra port
on the system µC and a dditional opto-iso lators
in isolated applications.
2) How does the 4-bit di gital latch on a DS ADC
allow me to change channels?
The CS5525 and CS5526 as well as the CS5504
family of AD C’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 accuracy.
3) What determi nes the input sp an of the convert er?
Performing a full scale gain calibration, or modifying the reference voltage. For example, if the
reference voltage is reduced by 50% the default
input ranges sca l e 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 b and width is li mit ed to 1/2 th e sele cted output word rate due to the Nyquist theory of
sampling. Ex ampl e: With th e defaul t 15 Hz output word ra te t he a vail able sign al ban dwidth o f
the ADC is 7.5 Hz.
5) What is recommended if I need more or less
bandwidth than is provided by the on-ch ip di gi tal filter ?
Use an external clock between 30 kHz and
100 kHz to scale the digital filters corner frequency 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 serial port?
Up to 2 MHz.
7) How does the instrumentation amplifier’s chopping frequency aff ect the converte r’s input impedance and input current?
The input im pedance of the conver ter is a dynamic im pedance and de pends on whether t he
instrumentation amplifier is engaged or not. For
the lower ranges (25 mV, 55 mV, 100 mV ), t he
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 lower ranges is 1900 MW (wit h f = 256, and C =
2 pF). For the higher ranges (1 V, 2.5 V, and
5 V), the amplifier is bypassed leaving an equivalent inpu t impedance of 1/fC whe re C is 32 pF
and f is either 256 or 32,768. A typical input impedance for the hig he r range s is 120 MW (with
f = 256 and C = 2 pF).
The input cu rrent i s a dynam ic c urr ent a nd al so
depends on w hether t he instr umenta tion amp lifier is engaged or not. For the lower ranges
For further information, please contact Crystal Semiconductor
at (512) 445-7222 or 1 (800) 888-5016
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
Copyright Cirrus Logic, I nc. 1997
(All Rights Reserv ed)
NOV ‘97
DS202TB1
1
CS5525/6/9 FAQ
(25 m V, 55 mV, 100 mV), the input current is
fVosC (where Vos is the offset of the instrumentation amplifier, typically less than 40 mV,
f is the chopping frequency, either 256 or
32,768, and C is 2 pF) . A typical inpu t current
for the lower ranges is 100 pA. For the higher
ranges (1 V, 2.5 V, an d 5 V), th e inp ut current
is [(VAIN+)-(VAIN-)]fC where (VAIN+)(VAIN-) is the voltage between AIN+ and
AIN-, f is either 256 or 32,768, and C is 32 pF.
A typical input current for the highe r ranges is
µA/V.
1.2
8) When rea ding th e co nversion data I get all ze roes no matter what the analog signal is. Please
explain why.
Check the voltage between pins 19 and 20
(VREF+ and VREF-). If it is zero, the converter
will compute all zeros because the digital output word represents the ratio of the input signal
to the voltage reference.
9) Is calibration re quired to use th e converter?
When the CS5525/26 is reset, the registers are
set to known values. If the signal to be measured by the converter is within the nominal
range, the converter can perform conversions
without th e need for cal ibratio ns. Error s in the
system rem ain present w hen calibration is not
performed, h owever , this m ay b e acce ptable if
the errors ar e insig nific ant to the measu reme nt
or if the errors are removed by some other
means, such as software and registers in the microcontroller.
10) How often do I need to recali bra te?
To answer t his question one must ask: 1) What
accuracy is required fro m the A/D converter?
2) What effect s will te mper ature ch anges ha ve
upon the entire circuit, including components
outside the A/D? To obtain optimum calibration accuracy, a calibration should be performed approxi mately one mi nute after power
is applied to allow the chip to reach thermal
equilibrium.
A higher accuracy measurement requirement
will generally require calibrations more often,
because, after the initial calibration has been
performed, the converter is subject to some
drift if the operating temperature changes. Typical offse t drift and ga in drift are gi ven in the
data sheet t ables. The obse rved drift in the application circuit may be considerably greater
due to para sitic ther mocouple e ffects and gain
drift caused by the limited tempco tracking of
the externa l resi stor s. Onc e an est ima te o f drift
is determin ed for the entire ap plication circu it
(drift will usually be dominated by error sources externa l to t he conv erter), a n asse ssment o f
how it affects measurement accuracy as temperature changes can be made. Once the
amount of drift is known, you can determi ne if
a new calibration is required. A good rule of
thumb is to recalibrate the converter (or system) with every ten degrees of ambient temperature chang e.
11)What do the numbers in th e calibration registers actu ally mean ?
There are two internal read/write calibration
registers in the CS5525/26 (offset, and gain).
-24
One LSB in the offset register is 2
proportion
of the input span (bipolar span is 2 times the unipolar span). The MSB in the offset register determines if the offset that is to be trimmed is
either pos itive or negative. T he converter can
typically trim ±50% of the input span. The gain
-23
register spans fro m 0 to (2 - 2
). The decimal
equivalent meaning of the gain register is:
D = b
20+ b12-1 + b22-2 + b32-3 + ... + bN2
0
-N
where the binary numbers have a value of either
zero or one. After a gain cal ibration has been
performed, the numeric value in the gain register should not exceed the range of 0.5 to 2.0
(decimal) [400000(Hex) to FFFFFF(He x)].
2 DS202TB1
CS5525/6/9 FAQ
12) How can the gain be calibra ted if a full-scale
signal is not available?
The CS5525/26 can be gain calibrated with
some input signal o ther t han full sc ale . For ex ample, when the converter is reset, the gain reg-
ister’s calibration word is 1.0 (decimal). If a
signal representing ten percent of full scale
reads three percent less than it should, the value
in the gain register can be scaled u p by three
percent. Gain ac curacy can be impr oved if output words are av eraged while usi ng this technique. Use caution when a calibration signal
less than ful l s cale is being used. If the transfer
function of the trans du cer be ing used t o gener ate the ten percent signal happens to have a major nonlinearity at the point at which calibration
is being perform ed, i t wil l cau se the rest of t he
transfer fun ction to be incorrect.
13)Why does the offset move when the CS5525/26
with a 2.5 V reference, is calibrated several
times? What can be done to prevent this?
The CS5526 is a 20-bit ADC with inherent
Gaussian thermal and quantization noise associated with each conversion. T herefore, every
time the c onverter is c alibrated , a differen t offset calibration output has a chance of occurring.
By averaging conversions, the peak-to-peak
noise can be reduced by a factor of 1/sqrt(n)
(where n is the number of sampl es taken). The
offset register can be accessed after calibration,
and the offset uncertainty of a converter can almost be eliminated (to 1 code) by averaging.
The CS5525 (16-bits) always has 1 count of
variability, even i f averaged, because the noise
and calibration can occur at a boundary between tw o codes. If the calibration co de is on
the boundary the random noise could toggle the
offset between the two codes.
14) Is a different calibrat ion requ ired fo r ea ch gain
getting?
For maximum a cc uracy, calibra ti ons should be
performed for offset and gain for each gain setting. If a facto ry calibration is performed using
a system calibration, the offset and gain register
contents ca n be read by the system microcontroller and stored in EEPROM. These same calibration words can then be uploaded into the
offset and gain reg isters of the conv erte r when
power is re-app lied to the system, or when t he
gain range is ch anged.
15) What is the advantage of performing calibrations at lower output word rates?
Calibration s are performed at the out put word
rate selected by the WR2-WR0 bits of the configuration re gister. Si nce hi gher word rates result in conversion words with more peak-topeak noise, it is better to calibrate at lower output word rates. To minimize the digital noise
near the device, the user should wait for ea ch
calibration step to be completed before reading
or writing to the serial port.
16) How can I get the best noise performance from
the CS5525 /26?
Use the bipo lar m ode o r in cr ease t he re fe rence
voltage, since each of these increa s e th e size of
the LSB.
17) If the charge pump is engaged, how do I ensure
that the c onverte r and its externa l compone nts
are intrin s i cally safe?
Intrinsic saf ety pr ohibi ts the use o f elect rolyt ic
(or bipolar) capa citors thus limiting the use of
certain size capacitors. Although a 10
µF cap. is
recommended for the charge pump, two
µF ceramic caps in parallel can be used.
0.47
DS202TB1 3
CS5525/6/9 FAQ
18) What benefit doe s an evaluation bo ard offer?
The CDB5525 /26 eva luation board saves time
and money over prototyping. The preassembled board com e s equipped with an 8 0C51 microcontroller and a 9-pin cable to link the
evaluation board to a PC-compatible computer.
The evaluation system also includes software
which provides easy access to the internal registers of the converter and displays the convert-
er’s time do main, frequenc y domai n and no ise
histogram perfor m anc e.
19) How do the analog power supply configurations differ between the CS5525/26 and the
CS5529?
The CS5525/26 converters can be powered
from a single +5 V analog supp ly and eit her a
+5 V or +3.3 V digital supply. They have a
charge pump which uses two external diodes
and two external capacitors to generate a negative supply ( a negative bias volt age) allowing
digitization o f ground-re ferenced si gnals. The
negative bias voltage can be supplied from a
separate negative supply, if desired. The
CS5529 A/D converte r is designed to operate
from a single +5 V or a dual ±2.5 V analog supply and either a +5 V or +3 .3 V digi tal su pply.
Since the CS55 29 c an be p owered fro m a n eg ative anal og s upply, level s hifting circui try can
be eliminated when measuring ground-referenced sig n al s .
20) How do the voltage reference s differ between
the CS5525/26 and the CS5529?
The CS5529 supports differential reference
voltages up to th e analog supply allowing the
analog supply to serve as the reference voltage.
The reference’s inputs are buffered through a
coarse/fine c harge b uffer. S ince the inpu ts a re
buffered, th e dynamic curr ent is typically 8 nA
and is independent of the reference voltage.
The CS5525/26 do not have buffers yet support
a differential reference voltage up to 3.0 V.
Since their re fe rence inpu ts a ren’t buff ered the
dynamic input current is typi cally 1.2 µA/V.
21)How do the CS5525/26 and the CS5529 analog
input ranges differ?
The CS5529 has a nominal input range of
±2.5 V (with VREF=2.5 V). The analog inputs
for this range have a coarse /fine charge buffe r
which red uces the dyna mic current to typically
16 nA. The CS5525/26 have an instrumentation amplifier and a programma bl e gain amplifier which provides nominal input ranges of 25
mV, 55 mV, and 100 mV. The instrumentati on
amplifier is a chopper stabi lized amp lifier and
serves as a buffer reduc ing the dynamic input
current to typically 100 pA. In the 1 V, 2.5 V,
and 5 V ranges of the CS5525/26, the instrumentation ampl ifier is bypassed an d no buffer
exists to reduce the input current. In these ranges the input cu rre nt is typically 1.2 µA/ V .
22) How do the output word rates differ between
the CS5525/26 and the CS5529?
All three converters have eight output word
rates. The CS5529 samples at ½ the rate of the
CS5525/26. Therefore, the CS5529’s output
word rate are ½ the CS5525/26.
4 DS202TB1
• Notes •
Loading...