adc12l032 National Semiconductor Corporation, adc12l032 Datasheet - Page 27

• Manufacturer Part Number: adc12l032
• Description: 3.3v Self-calibrating 12-bit Plus Sign Serial I/o A/d Converters With Mux And Sample/hold
• Manufacturer: National Semiconductor Corporation

Application Information
2.1 Biasing for Various Multiplexer Configurations
Figure 11 is an example of biasing the device for single-
ended operation. The sign bit is always low. The digital
output range is 0 0000 0000 0000 to 0 1111 1111 1111. One
LSB is equal to 610 µV (2.5V/4096 LSBs).
For pseudo-differential signed operation the biasing circuit
shown in Figure 13 shows a signal AC coupled to the ADC.
This gives a digital output range of −4096 to +4095. With a
1.25V reference, 1 LSB is equal to 305 µV. Although the
ADC is not production tested with a 1.25V reference, linear-
ity 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 600Ω or less. Notice though that
the input coupling capacitor needs to be made fairly large to
FIGURE 12. Alternative Pseudo-Differential Biasing
(Continued)
FIGURE 11. Single-Ended Biasing
27
bring down the high pass corner. Increasing the acquisition
time to 34 clock periods (with a 5 MHz CCLK frequency)
would allow the 600Ω to increase to 6k, which with a 1 µF
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 +1.25V from the LM4040 to bias any
amplifier circuits driving the ADC as shown in Figure 12. The
value of the resistor pull-up biasing the LM4040-2.5 will
depend upon the current required by the op amp biasing
circuitry.
Fully differential operation is shown in Figure 14. One LSB
for this case is equal to (2.5V/4096) = 610 mV.
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