ADC10154 datasheet(19/23 Pages) NSC

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2.0 Applications Information (Continued)

The minimum value of V

REF (VREF = VREF

+ −V

REF

−) can be

quite small (see Typical Performance Characteristics) to al-

low direct conversion of transducer outputs providing less

than a 5V output span. Particular care must be taken with re-

gard to noise pickup, circuit layout and system error voltage

sources when operating with a reduced span due to the in-

creased sensitivity of the converter (1 LSB equals V

REF/2

n).

2.3 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 se-

lected assigned negative input. Input bypass capacitors

should not be used if the source resistance is greater than

Ω since they will average the AC current and cause an ef-

fective DC current to flow through the analog input source re-

sistance. 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 without

any degradation in performance.

In a true differential input stage, a signal that is common to

both “+” and “−” inputs is cancelled. For the ADC10154 and

ADC10158, 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 because it

is sampled before each decision in the SAR sequence.

Therefore, any AC common-mode signal present on the ana-

log inputs will not be completely cancelled and will cause

some conversion errors. For a sinusoid common-mode sig-

nal this error is:

error(Max) = VPEAK (2πfCM)(tC)

where f

CM is the frequency of the common-mode signal,

PEAK is its peak voltage value, and tC is the A/D’s maximum

conversion time (t

C = 22/fCLK for 10-bit plus sign resolution).

For example, for a 60 Hz common-mode signal to generate

a 1⁄4 LSB error (1.24 mV) with a 4.5 µs conversion time, its

peak value would have to be approximately 731 mV.

2.4 OPTIONAL ADJUSTMENTS

2.4.1 Zero Error

The zero error of the A/D converter relates to the location of

the first riser of the transfer function (see

Figure 1) and can

be measured by grounding the minus input and applying a

small magnitude positive or negative voltage to the plus in-

put. Zero error is the difference between actual DC input

voltage which is necessary to just cause an output digital

DS011225-21

a. Ratiometric Using the Internal Reference

DS011225-22

b. Absolute Using a 4.096V Span

FIGURE 5. Different Reference Configurations

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