ADC10158CIN National Semiconductor, ADC10158CIN Datasheet - Page 19

• Manufacturer Part Number: ADC10158CIN
• Description: 10-Bit Plus Sign 4 s ADCs with 4- or 8-Channel MUX/ Track/Hold and Reference
• Manufacturer: National Semiconductor

2.0 Applications Information
The minimum value of V
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
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
1 k 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
REF
(V
REF
= V
REF
FIGURE 5. Different Reference Configurations
a. Ratiometric Using the Internal Reference
+
− V
b. Absolute Using a 4.096V Span
REF
(Continued)
−
) can be
REF
/2
n
).
19
the acquisition time (t
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:
V
where f
V
conversion time (t
For example, for a 60 Hz common-mode signal to generate
a
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
error
PEAK
1
⁄
4
LSB error (1.24 mV) with a 4.5 µs conversion time, its
(Max) = V
is its peak voltage value, and t
CM
is the frequency of the common-mode signal,
DS011225-21
PEAK
C
= 22/f
(2 f
A
). The negative input needs to be
CM
DS011225-22
CLK
)(t
C
for 10-bit plus sign resolution).
)
C
is the A/D’s maximum
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