APPLICATION NOTES

ADJUSTABLE HIGH VOLTAGE POWER SUPPLY

The high voltage power supply of the MSK 690 can be ad-

justed from +40 volts to +75 volts. To minimize device power

much as possible without causing output signal clipping. The

This will ensure that the transistion times are not degraded due

to the output transistor temporarily going into saturation.

FEEDBACK CAPACITANCE

The gain range of the MSK 690 is ±5V/V to ±100V/V. When

configured for low closed loop gains in the range of ±5V/V to

±25V/V, a small 0.5pF to 2.0pF adjustable capacitor should

be placed in parallel with the feedback resistor. This capacitor

can be adjusted to tailor overshoot and minimize ringing de-

pending on the load. For closed loop gains greater than ±25V/

V the user may omit this capacitor without any loss in circuit

stability. See the table below labeled "recommended compo-

nent values" and the typical connection diagram for compo-

nent selection vs. closed loop gain.

CASE CONNECTION

The case of the MSK 690 is internally connected to pin five

of the package. This pin can be left as a no connect but it is

recommended that the user connect this pin to ground to re-

duce noise and improve overall circuit stability.

DECOUPLING AND LAYOUT

Since the MSK 690 is a high voltage amplifier, it is com-

monly used in high gain configurations. Consequently, any

noise introduced into the system through the power supplies

will be amplified by the system gain. It is therefore imperative

that proper power supply decoupling and printed circuit card

layout guidelines are adhered to. Each power supply should be

effectively decoupled with a parallel combination of capacitors

as shown in the Typical Inverting Connection Diagram. These

capacitors should be connected as close as possible to the pack-

age pins and lead lengths must be kept to a minimum. On the

printed circuit card, the input and output traces should be kept

apart whenever possible to avoid localized feedback. The power

supply lines should be kept as wide as possible to keep their

effective impedance down thereby minimizing pickup.

SOURCE RESISTOR SELECTION

When driving reactive loads, such as the effective capaci-

tance of a cathode ray tube, local oscillations may often occur

in the output transistors of the op-amp. To minimize these

oscillations, an output source resistor may be added in series

with the amplifier output and the capacitive load as seen in the

figure below. This R-C combination acts as a snubber network

that lowers the high frequency bandwidth. The source resistor

is typically in the 10 ohm to 100 ohm range. In exchange for

increased overall circuit stability, a minor reduction in amplifier

bandwidth may occur. The following formula may be used to

approximate the frequency at which the zero will occur on the

open loop plot due to the addition of the isolation resistor.

TYPICAL INVERTING CONNECTION DIAGRAM

-5V/V

-10V/V

-50V/V

-100V/V

2.7KΩ

2.7KΩ

25KΩ

50KΩ

Cf

0.5-2.0pF

0.5-2.0pF

N/A

N/A

510Ω

270Ω

500Ω

500Ω

RECOMMENDED COMPONENT VALUES

499Ω

249Ω

495Ω

499Ω

SOURCE RESISTOR CONNECTION

Rev. A 8/00

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