Rainbow Electronics ADC0838 - Manual (Page 15)

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 period
The change in the common-mode voltage during this short
time interval can cause conversion errors For a sinusoidal
common-mode signal this error is
V
error
(max)e V
PEAK
(2 q f
CM
)
0 5
f
CLK
J
where f CM is the frequency of the common-mode signal
V PEAK is its peak voltage value
and f
CLK
is the A D clock frequency
For a 60 Hz common-mode signal to generate a LSB
error ( 5 mV) with the converter running at 250 kHz its
peak value would have to be 6 63V which would be larger
than allowed as it exceeds the maximum analog input limits
Due to the sampling nature of the analog inputs short spikes
of current enter the ‘‘a’’ input and exit the ‘‘b’’ input at the
clock edges during the actual conversion These currents
decay rapidly and do not cause errors as the internal com-
parator is strobed at the end of a clock period Bypass ca-
pacitors at the inputs will average these currents and cause
an effective DC current to flow through the output resist-
ance of the analog signal source Bypass capacitors should
not be used if the source resistance is greater than 1 kX
This source resistance limitation is important with regard to
the DC leakage currents of input multiplexer as well The
worst-case leakage current of
g 1 mA over temperature will
create a 1 mV input error with a 1 kX source resistance An
op amp RC active low pass filter can provide both imped-
ance buffering and noise filtering should a high impedance
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 V
IN(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 V
IN
(b) input at this V
IN(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 V
IN
(b) input and applying a small
magnitude positive voltage to the V
IN
(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
( LSBe9 8 mV for V
REF e 5 000 V
DC
)
5 2 Full-Scale
The full-scale adjustment can be made by applying a differ-
ential input voltage which is 1
LSB down from the desired
analog full-scale voltage range and then adjusting the mag-
nitude of the V
REF
input (or V
CC
for the ADC0832) for a
digital output code which is just changing from 1111 1110 to
1111 1111
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 V
IN (a) voltage which
equals this desired zero reference plus LSB (where the
LSB is calculated for the desired analog span using
1 LSBe 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 00
HEX to
01
HEX
code transition
The full-scale adjustment should be made with the proper
V
IN (b) voltage applied by forcing a voltage to the V IN (a)
input which is given by
V
IN
(a) fs adj e V
MAX b 1 5
(V
MAX b V
MIN
)
256
(
where
V MAX e the high end of the analog input range
and
V MIN e the low end (the offset zero) of the analog
range
(Both are ground referenced )
The V
REF
(or V
CC
) voltage is then adjusted to provide a
code change from FE HEX to FF HEX This completes the ad-
justment procedure
6 0 POWER SUPPLY
A unique feature of the ADC0838 and ADC0834 is the inclu-
sion of a zener diode connected from the V
a terminal to
ground which also connects to the V CC terminal (which is
the actual converter supply) through a silicon diode as
shown in
Figure 3 (See Note 3)
TL H 5583 – 11
FIGURE 3 An On-Chip Shunt Regulator Diode
15