ADC10831 datasheet(22/30 Pages) NSC

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Applications Hints (Continued)

30 APPLICATIONS INFORMATION

31 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 ADC10834 The

eight inputs of the ADC10838 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 ADC10834 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 Vb to 50 mV above Va e DVa e AVa

without degrading conversion accuracy If the voltage on an

unselected channel exceeds these limits it may corrupt the

reading of the selected channel

32 Reference Considerations

The voltage difference between the VREFa and VREFb in-

puts defines the analog input voltage span (the difference

between VIN(Max) and VIN(Min)) over which 1023 positive

and 1024 negative possible output codes apply

The value of the voltage on the VREFa or VREFb inputs

can be anywhere between AVa a 50 mV and GND

50 mV so long as VREFa is greater than VREFb The

ADC10831248 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

25V

bandgap

reference

the

ADC10831248 is available as an output on the V

REFOut

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

In a ratiometric system

(Figure 13a) the analog input volt-

age is proportional to the voltage used for the AD refer-

ence This voltage may also be the system power supply so

VREFa can also be tied to AVa This technique relaxes the

stability requirements of the system reference as the analog

input and AD 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

ADC10831248

The minimum value of VREF (VREF e VREFa –VREFb) 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 VREF

1024)

33 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

1kX 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

canceled

For

the

ADC10831248 the positive input of a selected channel

pair is only sampled once before the start of a conversion

during the acquisition time (tA) 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

VERROR(max) e VPEAK (2 q fCM)(tC)

where fCM is the frequency of the common-mode signal

VPEAK is its peak voltage value and tC is the AD’s conver-

sion time (tC e 12fCLK) For example for a 60 Hz com-

mon-mode signal to generate a

LSB error (061 mV) with

a 48 ms conversion time its peak value would have to be

approximately 337 mV

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