Rainbow Electronics ADC10738 - Manual (Page 22)

Applications Hints
(Continued)
3 0 APPLICATIONS INFORMATION
3 1 Multiplexer Configuration
The design of these converters utilizes a sampled-data
comparator structure which allows a differential analog in-
put to be converted by the successive approximation rou-
tine
The actual voltage converted is always the difference be-
tween an assigned ‘‘a’’ input terminal and a ‘‘b’’ input ter-
minal The polarity of each input terminal or pair of input
terminals being converted indicates which line the converter
expects to be the most positive
A unique input multiplexing scheme has been utilized to pro-
vide multiple analog channels The input channels can be
software configured into three modes differential single-
ended or pseudo-differential
Figure 12 illustrates the three
modes using the 4-channel MUX of the ADC10734 The
eight inputs of the ADC10738 can also be configured in any
of the three modes The single-ended mode has CH0 – CH3
assigned as the positive input with COM serving as the neg-
ative input In the differential mode the ADC10734 channel
inputs are grouped in pairs CH0 with CH1 and CH2 with
CH3 The polarity assignment of each channel in the pair is
interchangeable Finally in the pseudo-differential mode
CH0 – CH3 are positive inputs referred to COM which is now
a pseudo-ground This pseudo-ground input can be set to
any potential within the input common-mode range of the
converter The analog signal conditioning required in trans-
ducer-based data acquisition systems is significantly simpli-
fied with this type of input flexibility One converter package
can now handle ground-referred inputs and true differential
inputs as well as signals referred to a specific voltage
The analog input voltages for each channel can range from
50 mV below GND to 50 mV above V
a e DV a e AV a
without degrading conversion accuracy If the voltage on an
unselected channel exceeds these limits it may corrupt the
reading of the selected channel
3 2 Reference Considerations
The voltage difference between the V
REF
a and V
REF
b in-
puts defines the analog input voltage span (the difference
between V
IN (Max) and V IN (Min)) over which 1023 positive
and 1024 negative possible output codes apply
The value of the voltage on the V
REF
a or V
REF
b inputs
can be anywhere between AV a a 50 mV and b50 mV so
long as V REF a is greater than V REF b The
ADC10731 2 4 8 can be used in either ratiometric applica-
tions or in systems requiring absolute accuracy The refer-
ence pins must be connected to a voltage source capable
of driving the minimum reference input resistance of 5 kX
The internal 2 5V bandgap reference in the
ADC10731 2 4 8 is available as an output on the V
REF
Out
pin To ensure optimum performance this output needs to
be bypassed to ground with 100 mF aluminum electrolytic or
tantalum capacitor The reference output can be unstable
with capacitive loads greater than 100 pF and less than
100 mF Any capacitive loading less than 100 pF and
greater than 100 mF will not cause oscillation
Lower
output noise can be obtained by increasing the output ca-
pacitance A 100 mF capacitor will yield a typical noise floor
of 200 nV
0Hz The pseudo-differential and differential mul-
tiplexer modes allow for more flexibility in the analog input
voltage range since the ‘‘zero’’ reference voltage is set by
the actual voltage applied to the assigned negative input
pin
In a ratiometric system (Figure 13a) the analog input volt-
age is proportional to the voltage used for the A D refer-
ence This voltage may also be the system power supply so
V
REF a can also be tied to AV a This technique relaxes the
stability requirements of the system reference as the analog
input and A D reference move together maintaining the
same output code for a given input condition
For absolute accuracy (Figure 13b) where the analog input
varies between very specific voltage limits the reference pin
can be biased with a time- and temperature-stable voltage
source that has excellent initial accuracy The LM4040
LM4041 and LM185 references are suitable for use with the
ADC10731 2 4 8
The minimum value of V REF (V REF e V REF a – V REF b ) can
be quite small (see Typical Performance Characteristics) to
allow direct conversion of transducer outputs providing less
than a 5V output span Particular care must be taken with
regard to noise pickup circuit layout and system error volt-
age sources when operating with a reduced span due to the
increased sensitivity of the converter (1 LSB equals V
REF
1024)
3 3 The Analog Inputs
Due to the sampling nature of the analog inputs at the clock
edges short duration spikes of current will be seen on the
selected assigned negative input Input bypass capacitors
should not be used if the source resistance is greater than
1 kX since they will average the AC current and cause an
effective DC current to flow through the analog input source
resistance An op amp RC active lowpass filter can provide
both impedance buffering and noise filtering should a high
impedance signal source be required Bypass capacitors
may be used when the source impedance is very low with-
out any degradation in performance
In a true differential input stage a signal that is common to
both
‘‘a’’ and ‘‘b’’ inputs is canceled For the
ADC10731 2 4 8 the positive input of a selected channel
pair is only sampled once before the start of a conversion
during the acquisition time (t
A ) The negative input needs to
be stable during the complete conversion sequence be-
cause it is sampled before each decision in the SAR se-
quence Therefore any AC common-mode signal present
on the analog inputs will not be completely canceled and
will cause some conversion errors For a sinusoid common-
mode signal this error is
V
ERROR
(max) e V
PEAK
(2 q f
CM
) (t
C
)
where f CM is the frequency of the common-mode signal
V PEAK is its peak voltage value and t C is the A D’s conver-
sion time (t C e 12 f CLK ) For example for a 60 Hz com-
mon-mode signal to generate a LSB error (0 61 mV) with
a 4 8 ms conversion time its peak value would have to be
approximately 337 mV
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