Feedback Resistor Values and Feedback

Compensation

Using large values of feedback resistances (roughly 2k) with

low gains (such gains of 2) will result in degraded pulse re-

sponse and ringing. The large resistance will form a pole

with the input capacitance of the inverting input, delaying

feedback to the amplifier. This will produce overshoot and

ringing. To avoid this, the gain setting resistors should be

values of feedback resistors can be used.

Overshoot and ringing of the LM7131 can be reduced by

adding a small compensation capacitor across the feed back

resistor. For the LM7131 values in pF to tens of pF range are

useful initial values. Too large a value will reduce the circuit

bandwidth and degrade pulse response.

Since the small stray capacitance from the circuit layout,

other components, and specific circuit bandwidth require-

ments will vary, it is often useful to select final values based

on prototypes which are similar in layout to the production

circuit boards.

Reflections

The output slew rate of the LM7131 is fast enough to pro-

duce reflected signals in many cables and long circuit traces.

For best pulse performance, it may be necessary to termi-

nate cables and long circuit traces with their characteristic

impedance to reduce reflected signals.

Reflections should not be confused with overshoot. Reflec-

tions will depend on cable length, while overshoot will de-

pend on load and feedback resistance and capacitance.

When determining the type of problem, often removing or

drastically shortening the cable will reduce or eliminate re-

flections. Overshoot can exist without a cable attached to the

op amp output.

Driving Flash A/D Converters (Video Converters)

The LM7131 has been optimized to drive flash analog to

digital converters in a +5V only system. Different flash A/D

converters have different voltage input ranges. The LM7131

has enough gain-bandwidth product to amplify standard

video level signals to voltages which match the optimum in-

put range of many types of A/D converters.

For example, the popular 1175 type 8-bit flash A/D converter

has a preferred input range from 0.6V to 2.6V. If the input

signal has an active video range (excluding sync levels) of

approximately 700 mV, a circuit like the one in

Figure 13 can

be used to amplify and drive an A/D. The 10 µF capacitor

blocks the DC components, and allows the + input of the

LM7131 to be biased through R clamp so that the minimum

output is equal to V

circuit is determined as follows:

Output Signal Range = 2.6V (V top) = 0.6V (V bottom) =

2.0V

Gain = Output Signal Range/Input Signal = 2.857 =

2.00/0.700

Gain = (R

R isolation and C

based on the A/D input capacitance and the desired pulse

response of the system. The nominal values of 33

Ω and 5.6

pF shown in the schematic may be a useful starting point,

however, signal levels, A/D converters, and system perfor-

mance requirements will require modification of these val-

ues.

The isolation resistor, R isolation should be placed close to

the output of the LM7131, which should be close to the A/D

input for best results.

R clamp is connected to a voltage level which will result in

the bottom of the video signal matching the Vrb level of the

A/D converter. This level will need to be set by clamping the

black level of the video signal. The clamp voltage will depend

on the level and polarity of the video signal. Detecting the

sync signal can be done by a circuit such as the LM1881

Video Sync Separator.

Note: This is an illustration of a conceptual use of the LM7131, not a com-

plete design. The circuit designer will need to modify this for input pro-

tection, sync, and possibly some type of gain control for varying signal

levels.

Some A/D converters have wide input ranges where the

lower reference level can be adjusted. With these convert-

ers, best distortion results are obtained if the lower end of the

output range is about 250 mV or more above the V− input of

the LM7131 more. The upper limit can be as high as 4.0V

with good results.

www.national.com

17