ADC12062EVAL datasheet(13/20 Pages) NSC

13 / 20 page

Applications Information (Continued)

switching, reducing the op amp transient response require-

ments. The 100 pF capacitor should only be used with high

speed op amps that are unconditionally stable driving ca-

pacitive loads.

Another benefit of using a high speed buffer is improved

THD performance when using the multiplexer of the

ADC12762. The MUX on-resistance is somewhat non-linear

over input voltage, causing the RC time constant formed by

IN,RMUX, and RSW to vary depending on the input voltage.

This results in increasing THD with increasing frequency. In-

serting the buffer between the MUX OUT and the ADC IN

terminals as shown in

Figure 8 will eliminate the loading on

MUX, significantly reducing the THD of the multiplexed

system.

Correct converter operation will be obtained for input volt-

ages greater than AGND − 50 mV and less than AV

CC +50

mV. Avoid driving the signal source more than 300 mV

higher than AV

CC, or more than 300 mV below AGND. If an

analog input pin is forced beyond these voltages, the current

flowing through that pin should be limited to 25 mA or less to

avoid permanent damage to the IC. The sum of all the over-

drive currents into all pins must be less than 50 mA. When

the input signal is expected to extend more than 300 mV be-

yond the power supply limits for any reason (unknown/

uncontrollable input voltage range, power-on transients, fault

conditions, etc.) some form of input protection, such as that

shown in

Figure 9, should be used.

DS012811-30

FIGURE 7. Simplified ADC12762 Input Stage

DS012811-31

FIGURE 8. Buffering the Input with an LM6361 High Speed Op Amp

ADC12062EVAL Datasheet(PDF) 13 Page - National Semiconductor (TI)