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ADC08231CIN View Datasheet(PDF) - National ->Texas Instruments

Part NameDescriptionManufacturer
ADC08231CIN 8-Bit 2ms Serial I/O A/D Converters with MUX, Reference, and Track/Hold National-Semiconductor
National ->Texas Instruments National-Semiconductor
ADC08231CIN Datasheet PDF : 24 Pages
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Functional Description (Continued)
4 0 THE ANALOG INPUTS
The most important feature of these converters is that they
can be located right at the analog signal source and through
just a few wires can communicate with a controlling proces-
sor with a highly noise immune serial bit stream This in itself
greatly minimizes circuitry to maintain analog signal accura-
cy which otherwise is most susceptible to noise pickup
However a few words are in order with regard to the analog
inputs should the input be noisy to begin with or possibly
riding on a large common-mode voltage
The differential input of these converters actually reduces
the effects of common-mode input noise a signal common
to both selected ‘‘a’’ and ‘‘b’’ inputs for a conversion
(60 Hz is most typical) The time interval between sampling
the ‘‘a’’ input and then the ‘‘b’’ input is of a clock peri-
od The change in the common-mode voltage during this
short time interval can cause conversion errors For a sinus-
oidal common-mode signal this error is
 J0 5
Verror(max) e VPEAK(2qfCM) fCLK
where fCM is the frequency of the common-mode signal
VPEAK is its peak voltage value
and fCLK is the A D clock frequency
For a 60Hz common-mode signal to generate a LSB er-
ror ( 5mV) with the converter running at 250kHz its peak
value would have to be 6 63V which would be larger than
allowed as it exceeds the maximum analog input limits
Source resistance limitation is important with regard to the
DC leakage currents of the input multiplexer While operat-
ing near or at maximum speed bypass capacitors should
not be used if the source resistance is greater than 1kX
The worst-case leakage current of g1mA over temperature
will create a 1mV input error with a 1kX source resistance
An op amp RC active low pass filter can provide both im-
pedance buffering and noise filtering should a high imped-
ance signal source be required
5 0 OPTIONAL ADJUSTMENTS
5 1 Zero Error
The zero of the A D does not require adjustment If the
minimum analog input voltage value VIN(MIN) is not ground
a zero offset can be done The converter can be made to
output 0000 0000 digital code for this minimum input voltage
by biasing any VIN (b) input at this VIN(MIN) value This
utilizes the differential mode operation of the A D
The zero error of the A D converter relates to the location
of the first riser of the transfer function and can be mea-
sured by grounding the VIN (b) input and applying a small
magnitude positive voltage to the VIN (a) input Zero error
is the difference between the actual DC input voltage which
is necessary to just cause an output digital code transition
from 0000 0000 to 0000 0001 and the ideal LSB value
( LSB e 9 8mV for VREF e 5 000VDC)
5 2 Full Scale
A full-scale adjustment can be made by applying a differen-
tial input voltage which is 1 LSB down from the desired
analog full-scale voltage range and then adjusting the mag-
nitude of the VREFIN input for a digital output code which is
just changing from 1111 1110 to 1111 1111 (See figure enti-
tled ‘‘Span Adjust 0V s VIN s 3V’’) This is possible only
with the ADC08234 and ADC08238 (The reference is inter-
nally connected to VREFIN of the ADC08231)
5 3 Adjusting for an Arbitrary Analog Input
Voltage Range
If the analog zero voltage of the A D is shifted away from
ground (for example to accommodate an analog input sig-
nal which does not go to ground) this new zero reference
should be properly adjusted first A VIN (a) voltage which
equals this desired zero reference plus LSB (where the
LSB is calculated for the desired analog span using 1 LSB
e analog span 256) is applied to selected ‘‘a’’ input and
the zero reference voltage at the corresponding ‘‘b’’ input
should then be adjusted to just obtain the 00HEX to 01HEX
code transition
The full-scale adjustment should be made with the proper
VIN (b) voltage applied by forcing a voltage to the VIN (a)
input which is given by
( VIN (a) fs adj e VMAX b 1 5
(VMAX b VMIN)
256
where
VMAX e the high end of the analog input range
and
VMIN e the low end (the offset zero) of the analog range
(Both are ground referenced )
The VREFIN (or VCC) voltage is then adjusted to provide a
code change from FEHEX to FFHEX This completes the ad-
justment procedure
17
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