Rainbow-electronics ADC12038 Manual de usuario descargar pdf (Pagina 31)

Application Hints (Continued)

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

4096 to

4095. With a

2.5V reference, as shown, 1 LSB is equal to 610 mV. 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 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. Increasing R, to 6k would allow

to be 2k.

TL/H/11354– 47

FIGURE 10. Pseudo-Differential Biasing with the Signal Source AC Coupled Directly into the ADC

An alternative method for biasing pseudo-differential opera-

tion is to use the

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.

In the circuit of

Figure 11

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 12

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 13.

One LSB

for this case is equal to (4.1V/4096)

1 mV.

TL/H/11354– 48

FIGURE 11. Alternative Pseudo-Differential Biasing

1 2 ... 26 27 28 29 30 31 32 33 34 35 36 ... 41 42

Comentarios a estos manuales

Sin comentarios

Rainbow-electronics ADC12038 Manual de usuario Pagina 31

Comentarios a estos manuales

Relacionado con productos y manuales para Sensores Rainbow-electronics ADC12038