Datasheet

ADC10731, ADC10732, ADC10734, ADC10738
SNAS081D –MAY 1999–REVISED MARCH 2013 www.ti.com
Figure 41. Absolute Using a 4.096V Span
Optional Adjustments
Zero Error
The zero error of the A/D converter relates to the location of the first riser of the transfer function (see Figure 7
Figure 8) and can be measured by grounding the minus input and applying a small magnitude voltage to the plus
input. Zero error is the difference between actual DC input voltage which is necessary to just cause an output
digital code transition from 000 0000 0000 to 000 0000 0001 and the ideal ½ LSB value (½ LSB = 1.22 mV for
V
REF
= + 2.500V).
The zero error of the A/D does not require adjustment. If the minimum analog input voltage value, V
IN
(Min), is not
ground, the effective “zero” voltage can be adjusted to a convenient value. The converter can be made to output
an all zeros digital code for this minimum input voltage by biasing any minus input to V
IN
(Min). This is useful for
either the differential or pseudo-differential input channel configurations.
Full-Scale
The full-scale adjustment can be made by applying a differential input voltage which is 1½ LSB down from the
desired analog full-scale voltage range and then adjusting the V
REF
voltage (V
REF
= V
REF
+
– V
REF
−
) for a digital
output code changing from 011 1111 1110 to 011 1111 1111. In bipolar signed operation this only adjusts the
positive full scale error.
Adjusting for an Arbitrary Analog Input
Voltage Range
If the analog zero voltage of the A/D is shifted away from ground (for example, to accommodate an analog input
signal which does not go to ground), this new zero reference should be properly adjusted first. A plus input
voltage which equals this desired zero reference plus ½ LSB is applied to selected plus input and the zero
reference voltage at the corresponding minus input should then be adjusted to just obtain the 000 0000 0000 to
000 0000 0001 code transition.
The full-scale adjustment should be made [with the proper minus input voltage applied] by forcing a voltage to
the plus input which is given by:
(3)
where V
MAX
equals the high end of the analog input range, V
MIN
equals the low end (the offset zero) of the
analog range. Both V
MAX
and V
MIN
are ground referred. The V
REF
(V
REF
= V
REF
+
− V
REF
−
) voltage is then adjusted
to provide a code change from 011 1111 1110 to 011 1111 1111. Note, when using a pseudo-differential or
differential multiplexer mode where V
REF
+ and V
REF
− are placed within the V
+
and GND range, the individual
values of V
REF
and V
REF
− do not matter, only the difference sets the analog input voltage span. This completes
the adjustment procedure.
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