APPLICATION NOTES

POWER SUPPLIES

The input stage of the MSK 1922 requires power supplies of

+15V and -10.5V for optimum operation. The negative power

supply can be increased to -12V if -10.5V is not available, but

additional power dissipation will cause the internal temperature

to rise. Both low voltage power supplies should be effectively

decoupled with tantalum capacitors (at least 4.7µF) connected

as close to the amplifier's pins as possible. The MSK 1922 has

internal 0.01µF capacitors that also improve high frequency

performance. In any case, it is also recommended to put 0.1µF

decoupling capacitors on the +15V and -10.5V supplies as

well.

amplifier's output stage and must be kept as stable as possible.

DC output is directly related to the high voltage value. The

large a capacitor as possible to improve output stability.

SUPPLY SEQUENCING

other DC control inputs.

If power supply sequencing is not

possible, the time difference between each supply should be

less than five milliseconds. If the DC control signals are being

output, reverse biased diodes should be connected from each

VIDEO OUTPUT

MSK 1922 is determined by (Rp). The bandwidth of the ampli-

fier largely depends on both Rp and Lp.

Hybrid pins 16 and 17 are directly connected to Rp. Addi-

tional external resistance can be added to reduce power dissi-

pation, but slower transition times will result. If an additional

resistor is used, it must be low capacitance and the layout

should minimize capacitive coupling to ground (ie: no ground

plane under Rp).

The MSK 1922 is specified with no external Lp which yields

about 10% overshoot. Additional peaking can be obtained by

using a high self-resonant frequency inductor in series with pins

16 & 17. Since this value of inductance can be very dependent

on circuit layout, it is best to determine its value by experimen-

tation. A good starting point is typically 0.0047µH.

If external resistors or inductors are not used, be sure to

connect high frequency bypass capacitors directly from pins

16 and 17 to ground.

Rev. D 4/02

3

user to vary the video gain. By simply applying a DC voltage

through a 5K

Ω pot to ground. For convenient stable gain adjust-

ment, a 0.1µF bypass capacitor should be connected near the

Digital gain control can be accomplished by connecting a D/A

ing performance may be lost when using an external DC voltage

The overall video output of the MSK 1922 can be character-

ized using the following expression:

(or)

Here is a sample calculation for the MSK 1922:

Given information

Rp=150

Ω (internal)

Ω)(0.09)

The expected video output would swing from approximately

should be used as a nominal result because the overall gain may

vary as much as ±20% due to internal high speed device varia-

tions. Changing ambient conditions can also effect the video

gain of the amplifier by as much as 150 PPM/°C. It is wise to

connect all video amplifiers to a common heat sink to maximize

thermal tracking when multiple amplifiers are used in applica-

outputs should be shared by all three amplifiers. This voltage

should be buffered with a suitable low drift op-amp for best

tracking performance.

OUTPUT PROTECTION

The output pin of the MSK 1922 should be protected from

transients by connecting reversed biased ultra-low capacitance

The

output can also be protected from arc voltages by inserting a

small value (25-50

Ω) resistor in series with the amplifier. This

resistor will reduce system bandwidth along with the load ca-

pacitance, but a series inductor can reduce the problem sub-

stantially.

VIDEO INPUTS

including both common mode offset and signal levels.

The

input structure of the MSK 1922 was designed for ±0.714Vpp

RS343 signals.

If either input is not used it should be con-

nected directly to the analog ground or through a 25

Ω resistor

to ground if input offset currents are to be minimized.