MSK707H Datasheet, PDF (4/6 Pages) M.S. Kennedy Corporation – ULTRA-ACCURATE/HIGH SLEW RATE INVERTING OPERATIONAL AMPLIFIER

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.
OPTIMIZING SLEW RATE
When measuring the slew rate of the MSK 707, 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 4 should all be shorted
directly to ground for optimum response. Since the internal feedback
resistor isn't being used, pin 8 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.
TA=√TB²+TC²
WHERE:
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 707.
TA=Transition time measured at output jack on MSK 707 test card.
TB=Transition time measured at input jack on MSK 707 test card.
TC=Actual output transition time of MSK 707(note that this quantity
will be calculated, not measured directly with the oscilloscope).
THE MSK 707 IS INVERTING, THEREFORE WHEN MEASURING RIS-
ING EDGE SLEW RATE:
TA=Rise time measured at output
TB=Fall time measured at input
TC=Actual rise time of output
WHEN MEASURING FALLING EDGE SLEW RATE:
TA=Fall time measured at output
TB=Rise time measured at input
TC=Actual fall time of output
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 707 has pin 3 and 7 internally connected to the case.
Pin 3 and 7 should be tied to a ground plane for sheilding. For special
applications, consult factory.
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PRELIMINARY Rev. - 2/04