APPLICATION NOTES

POWER SUPPLIES

The input stage of the MSK 1903 requires power supplies of

+20V and -10.5V for optimum performance. The negative

power supply can be increased to -12V if -10.5V is not avail-

able, 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

1903 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 +20V and -10.5V sup-

plies as well.

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

the amplifier's DC output is directly related to the high voltage

with as large a capacitor as possible to improve output stabil-

ity.

VIDEO OUTPUT

the MSK 1903 variants is determined by Vpp = (250mA) x

(Rp). The bandwidth of the amplifier 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 capacitive and the layout should

minimize capacitive coupling to ground (ie: no ground plane

under Rp).

The MSK 1903 Series is conservatively specified with low

values for Lp which yield about 5% overshoot. Additional peak-

ing can be obtained by using a high self-resonant frequency

inductor in series with the Rp pins. Since this value of induc-

tance can be very dependent on circuit layout, it is best to

determine its value by experimentation. A good starting point

is typically 0.47µH for the MSK 1903-0 and 0.0047µH for the

remaining devices.

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

connect high frequency bypass capacitors directly from pins

16 and 17 to ground.

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 1903 can be character-

ized using the following expression:

Here is a sample calculation for the MSK 1903-2:

Given information:

Rp=400

Ω (internal)

Ω) (0.9)

The expected video output would swing from approximately

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

much as ±20% due to internal high speed device variations.

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 applications such

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 1903 should be protected from

transients by connecting reverse biased ultra-low capacitance

put can also be protected from arc voltages by inserting a small

value (50-100

Ω) resistor in series with the amplifier. This resis-

tor will reduce system bandwidth along with the load capaci-

tance, but a series inductor can reduce the problem substan-

tially.

VIDEO INPUTS

including both common mode offset and signal levels. The in-

put structure of the MSK 1903 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.

SUPPLY SEQUENCING

the 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

Rev. B 3/03

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