Datasheet

ADC10731, ADC10732, ADC10734, ADC10738
SNAS081D –MAY 1999–REVISED MARCH 2013 www.ti.com
The internal 2.5V bandgap reference in the ADC10731/2/4/8 is available as an output on the VREFOut pin. To
ensure optimum performance this output needs to be bypassed to ground with 100 μF aluminum electrolytic or
tantalum capacitor. The reference output can be unstable with capacitive loads greater than 100 pF and less
than 100 μF. Any capacitive loading less than 100 pF and greater than 100 μF will not cause oscillation. Lower
output noise can be obtained by increasing the output capacitance. A 100 μF capacitor will yield a typical noise
floor of
(1)
The pseudo-differential and differential multiplexer 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 40), the analog input voltage is proportional to the voltage used for the A/D
reference. This voltage may also be the system power supply, so V
REF
+ can also be tied to AV
+
. 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 41), 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
= V
REF
+–V
REF
−) 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 voltage sources when operating with a reduced
span due to the increased sensitivity of the converter (1 LSB equals V
REF
/1024).
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 kΩ 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 without any degradation in performance.
In a true differential input stage, a signal that is common to both “+” and “−” 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 because it is sampled before each decision in the SAR sequence. 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) = V
PEAK
(2 π f
CM
) (t
C
) (2)
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
conversion time (t
C
= 12/f
CLK
). For example, for a 60 Hz common-mode signal to generate a ¼ LSB error (0.61
mV) with a 4.8 μs conversion time, its peak value would have to be approximately 337 mV.
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