ADC12L030CIWM National Semiconductor, ADC12L030CIWM Datasheet - Page 27

• Manufacturer Part Number: ADC12L030CIWM
• Description: 3.3V Self-Calibrating 12-Bit Plus Sign Serial I/O A/D Converters with MUX and Sample/Hold
• Manufacturer: National Semiconductor

Application Hints
A/DIN1 and A/DIN2 can be assigned as the + or − input
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 digi-
tal 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 12 shows a signal AC coupled to the ADC.
This gives a digital output range of −4096 to +4095. With a
1.25V reference, as shown, 1 LSB is equal to 305 µV. 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 pe-
riods the input biasing resistor needs to be 600 or less. No-
tice though that the input coupling capacitor needs to be
Configuration
Differential
(Continued)
DS011830-40
FIGURE 11. Single-Ended Biasing
FIGURE 10.
A/DIN1 is + input
A/DIN2 is − input
27
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 600 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 ampli-
fier circuits driving the ADC as shown in Figure 13 . 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.
Configuration
Single-Ended
DS011830-41
DS011830-46
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