ADC12138CIWM datasheet(30/41 Pages) NSC

30 / 41 page

Application Hints (Continued)

The Multiplexer assignment tables for the ADC12130/2/8

(

Table 2 and Table 3) summarize the aforementioned func-

tions for the different versions of A/Ds.

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 1 mV (4.1V/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

2.5V reference, as shown, 1 LSB is equal to 610 µV. Al-

though, the ADC is not production tested with a 2.5V refer-

ence, when V

+ and V

+ are +5.0V linearity error typically

will not change more than 0.1 LSB (see the curves in the

Typical Electrical Characteristics Section). With the ADC set

to an acquisition time of 10 clock periods, the input biasing

resistor needs to be 600

Ω or less. Notice though that the in-

put coupling capacitor needs to be made fairly large to bring

down the high pass corner. Increasing the acquisition time to

34 clock periods (with a 5 MHz CCLK frequency) would al-

low the 600

Ω to increase to 6k, which with a 1 µF coupling

capacitor would set the high pass corner at 26 Hz. Increas-

ing R, to 6k would allow R

2 to be 2k.

DS012079-38

FIGURE 11. Single-Ended Biasing

ADC12138CIWM Datasheet(PDF) 30 Page - National Semiconductor (TI)