APPLICATION NOTES CON'T

STABILITY AND LAYOUT CONSIDERATIONS

As with all wideband devices, proper decoupling of the power

lines is extremely important. The power supplies should be by-passed

as near to pins 9 and 1 as possible with a parallel grouping of a

0.01µf ceramic disc and a 4.7µf tantalum capacitor. Wideband de-

vices are also sensitive to printed cicuit board layout. Be sure to

keep all runs as short as possible, especially those associated with

the summing junction and power lines. Circuit traces should be sur-

rounded by ground planes whenever possible to reduce unwanted

resistance and inductance. The curve below shows the relationship

between resonant frequency and capacitor value for 3 trace lengths.

FEEDBACK CAPACITANCE

Feedback capacitance is commonly used to compensate for the

"input capacitance" effects of amplifiers. Overshoot and ringing,

especially with capacitive loads, can be reduced or eliminated with

the proper value of feedback capacitance.

All capacitors have a self-resonant frequency. As capacitance in-

creases, self-resonant frequency decreases (assuming all other fac-

tors remain the same). Longer lead lengths and PC traces are other

factors that tend to decrease the self-resonant frequency. When a

feedback capacitor's self-resonant frequency falls within the fre-

quency band for which the amplifier under consideration has gain,

oscillation occurs. These influences place a practical upper limit on

the value of feedback capacitance that can be used. This value is

typically 0.5 to 5pF for the MSK 739(B).

OPTIMIZING SLEW RATE

When measuring the slew rate of the MSK 739, many external

factors must be taken into consideration to achieve best results. The

closed loop gain of the test fixture should be -1.5V/V or less with

the external feedback resistor being 499Ω Lead length on this resis-

tor must be as short as possible and the resistor should be small. No

short circuit current limit resistors should be used. (Short pin 1 to

pin 12 and pin 9 to pin 10). Pins 2,3,7 and 8 should all be shorted

directly to ground for optimum response. Since the internal feedback

resistor isn't being used, pin 4 should be shorted to pin 5. SMA

connectors are recomended for the input and output connectors to

keep external capacitances to a minimum. To compensate for input

capacitance, a small 0.5 to 5pF high frequency variable capacitor

should be connected in parallel with the feedback resistor. This ca-

pacitor will be adjusted to trim overshoot to a minimum. A 5500V/

µS slew rate limit from -10V to +10V translates to a transition time

of 2.9 nanoseconds. In order to obtain a transition time of that mag-

nitude at the output of the test fixture, the transition time of the

input must be much smaller. A rise time at the input of 500 picosec-

onds or less is sufficient. If the transition time of the input is greater

than 500 picoseconds, the following formula should be used, since

the input transition time is now affecting the measured system tran-

sition time.

WHERE:

will be calculated, not measured directly with the oscilloscope).

THE MSK 739 IS INVERTING, THEREFORE WHEN MEASURING RIS-

ING EDGE SLEW RATE:

WHEN MEASURING FALLING EDGE SLEW RATE:

LOAD CONSIDERATIONS

When determining the load an amplifier will see, the capacitive

portion must be taken into consideration. For an amplifier that slews

at 1000V/µS, each pF will require 1mA of output current.

To minimize ringing with highly capacitive loads, reduce the load

time constant by adding shunt resistance.

I=C(dV/dT)

CASE CONNECTION

The MSK 739(B) has pin 3 internally connected to the case. The

case is not electrically connected to the internal circuit. Pin 3 should

be tied to a ground plane for sheilding. For special applications,

consult factory.

4

Rev. A 4/02