MSK1901 datasheet(3/6 Pages) MSK

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APPLICATION NOTES

INITIAL SETUP

It is important to set the VOFF and VGAIN inputs to ob-

tain balanced rise/fall times during initial setup of the

MSK 1901. If the quiescent current level of VOFF is set

too low, it will slow the rise time and limit the bandwidth

of the MSK 1901.

VIDEO INPUTS

The analog inputs (±VIN) are designed to accept RS343

signals, ±0.714VPP, and will operate properly with a com-

mon mode range of ±0.5V with respect to ground. There-

fore, it is recommended that the input signal be limited

to ±1.3V with respect to ground, (signal+common

mode). Although large offsets of ±2V (with respect to

ground, signal included) can be tolerated without dam-

age to the hybrid, output linearity suffers and therefore it

is not recommended.

VGAIN

VGAIN is the DC gain (contrast) control which varies the

gain from 0 to 80V/V. The internal reference (VREF) is

available to drive this input. Normally a 5K potentiometer

is connected between VREF and GND is used to vary the

gain, but any 0-5V external DC source may be used with

some additional degredation of gain stability over tem-

perature. A 0.1µF capacitor should be connected from

the VGAIN pin to ground to improve stability.

The gain equation for the MSK 1901 is:

VLRS-VO=VIN x GM x RL

=VIN (VGAIN x 0.08) RL

The overall gain of the MSK 1901 may vary by ±20%

due to process variations of the internal components.

Temperature variations also effect gain, <150ppm/°C.

If more than one MSK 1901 is used in a system, steps

should be taken to make them track thermally (i.e. a

common heat sink). This will reduce any mismatches due

to varying temperatures.

VOFF

VOFF is the output offset (brightness) control used to

set the output quiescent current and consequently the

DC output voltage (black level). Output quiescent cur-

rent adjustment range is from several µA to 100mA nomi-

nal (80 to 130mA actual). Normally a 5K potentiometer

is connected between VREF and GND to this input, but

any 0-5V external DC source may be used. A 0.1µF ca-

pacitor should be connected from this pin to signal ground

to improve the amplifier's stability.

BLACK LEVEL

The voltage developed across the external load resis-

tor with a 0V video input to the MSK 1901 is the black

level. This voltage may be changed by adjusting the load

resistor or by varying the output quiescent current of the

MSK 1901as described in VOFF above. The black level

could also be affected by the VGAIN control voltage if the

video input has a DC component. AC coupling of the

video input will prevent this phenomenon from occuring.

BLANKING

The blank input is a TTL active high input. When active

it will disable the video input of the MSK 1901 and allow

the output to rise to approximately VLRS. If the blank in-

put rises above 3V some interaction between VOFF and

BLANK level may occur. The BLANK input is indepen-

dent of the input signal and must be tied "low" to acti-

vate the amplifier if the blanking function is not used.

VREF OUTPUT

VREF is a buffered zener reference with a nominal out-

put voltage of 5.5V ±5% which can source up to 4mA.

It is available for use in adjusting the offset and gain. If

multiple amplifiers are used for RGB amplification, they

should all share the same VREF pin from one of the hy-

brids. The VREF pin should be buffered with a unity gain

precise amplifier when driving three amplifiers for RGB

applications.

VCB

The VCB input is the base connection to the output

stage consisting of a common base, high voltage stage

and a high speed, low voltage current amplifier in a

cascode arrangement. This input requires a very stable

10V DC nominal voltage. Any AC signals at this point

will be amplified and reflected in the output. The PSRR

of the output stage is directly related to the stability of

this VCB voltage.

VIDEO OUTPUT

The video output voltage is obtained from the open

collector of a cascode circuit designed to operate with a

nominal output supply (VLRS) of +70V. VLRS must be

greater than the applied VCB voltage, but less than VCB

+65V. The output of MSK 1901 will drive loads up to

250mA when proper heat sinking is used.

Rev. A 5/02

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