ADC12062EVAL datasheet(13/20 Pages) NSC

13 / 20 page

Applications Information (Continued)

TLH11490 – 18

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

Another benefit of using a high speed buffer is improved

THD performance when using the multiplexer of the

ADC12062 The MUX on-resistance is somewhat non-linear

over input voltage causing the RC time constant formed by

CIN RMUX and RSW to vary depending on the input voltage

This results in increasing THD with increasing frequency

Inserting the buffer between the MUX OUT and the ADC IN

terminals as shown in

Figure 8 will eliminate the loading on

RMUX significantly reducing the THD of the multiplexed sys-

tem

Correct converter operation will be obtained for input volt-

ages greater than AGND b 50 mV and less than AVCC a

50 mV Avoid driving the signal source more than 300 mV

higher than AVCC or more than 300 mV below AGND If an

analog input pin is forced beyond these voltages the cur-

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

less to avoid permanent damage to the IC The sum of all

the overdrive currents into all pins must be less than 50 mA

When the input signal is expected to extend more than

300 mV beyond the power supply limits for any reason (un-

knownuncontrollable input voltage range power-on tran-

sients fault conditions etc) some form of input protection

such as that shown in

Figure 9 should be used

TLH11490 – 19

FIGURE 9 Input Protection

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