ADC12H034 datasheet(31/41 Pages) NSC

31 / 41 page

Application Hints (Continued)

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, 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 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 600

Ω to increase to

6k, which with a 1 µF coupling capacitor would set the high

pass corner at 26 Hz. Increasing R, to 6k would allow R

2 to

be 2k.

An alternative method for biasing pseudo-differential opera-

tion is to use the +2.5V 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.

In the circuit of Figure 13 some voltage range is lost since

the amplifier will not be able to swing to +5V and GND with

a single +5V supply. Using an adjustable version of the

LM4041 to set the full scale voltage at exactly 2.048V and a

lower grade LM4040D-2.5 to bias up everything to 2.5V as

shown in Figure 14 will allow the use of all the ADC’s digital

output range of −4096 to +4095 while leaving plenty of head

room for the amplifier.

Fully differential operation is shown in Figure 15. One LSB

for this case is equal to (4.1V/4096) = 1 mV.

01135447

FIGURE 12. Pseudo-Differential Biasing with the Signal Source AC Coupled Directly into the ADC

ADC12H034 Datasheet(PDF) 31 Page - National Semiconductor (TI)