LM7131 datasheet(12/23 Pages) NSC

12 / 23 page

Notes on Performance Curves and Datasheet Limits (Continued)

DRIVING TYPE 1175 FLASH A/D LOADS

The circuits in

Figures 6, 7, 8, 9, 10, 11 show a LM7131 in a

voltage follower configuration driving the passive equivalent

of a typical flash A/D input. Note that there is a slight ringing

on the output, which can affect accurate analog-to-digital

conversion. In these graphs, we have adjusted the ringing to

be a little larger than desirable in order to better show the

settling time. Most settling times at low gain are about 75 ns

to < 1% of final voltage. The ringing can be reduced by add-

ing a low value (approximately 500

Ω) feedback resistor from

the output to the inverting input and placing a small (pico-

farad range) capacitor across the feedback resistor. See

Fig-

ure 9 and Figure 10 for schematics and respective perfor-

mance curves for flash A/D driving at A

V = +5 with and

withouta2pF feedback capacitor.

See section on feedback compensation. Ringing can also be

reduced by placing an isolation resistor between the output

and the analog-to-digital converter input — see sections on

driving capacitive loads and analog-to-digital converters.

Please refer to

Figures 6, 7, 8, 9, 10, 11 for schematics of

test setups for driving flash A/D converters.

DS012313-12

Numbers in parentheses are measured

fixture capacitances w/o DUT and load.

FIGURE 4. Cable Driver 75' RG-59

DS012313-13

Numbers in parentheses are measured

fixture capacitances w/o DUT and load.

FIGURE 5. Cable Driver Gain of 10 A

V = +10

LM7131 Datasheet(PDF) 12 Page - National Semiconductor (TI)