User Manual

Application Hints (Continued)
TL/H/11830– 46
FIGURE 9. Single-Ended Biasing
For pseudo-differential signed operation the biasing circuit
shown in
Figure 10 shows a signal AC coupled to the ADC.
This gives a digital output range of
b
4096 to
a
4095. With a
1.25V reference, as shown, 1 LSB is equal to 305 mV. Al-
though the ADC is not production tested with a 1.25V refer-
ence linearity error typically will not change more than 0.3
LSB. With the ADC set to an acquisition time of 10 clock
periods the input biasing resistor needs to be 600X or less.
Notice though that the input coupling capacitor needs to be
made fairly large to bring down the high pass corner. In-
creasing the acquisition time to 34 clock periods (with a
5 MHz CCLK frequency) would allow the 600X to increase
to 6k, which with a 1 mF coupling capacitor would set the
high pass corner at 26 Hz. The value of R1 will depend on
the value of R2.
An alternative method for biasing pseudo-differential opera-
tion is to use the
a
2.5V from the LM9140 to bias any ampli-
fier circuits driving the ADC as shown in
Figure 11. The
value of the resistor pull-up biasing the LM9140-2.5 will de-
pend upon the current required by the op amp biasing cir-
cuitry.
Fully differential operation is shown in
Figure 12. One LSB
for this case is equal to (2.5V/4096)
e
610 mV.
TL/H/11830– 47
FIGURE 10. Pseudo-Differential Biasing with the Signal Source AC Coupled Directly into the ADC
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