APPLICATION NOTES
POWER SUPPLIES
VIDEO INPUTS
The video input signals should be kept below 2VMAX total,
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 25Ω resistor
to ground if input offset currents are to be minimized.
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.
OUTPUT PROTECTION
The output pin of the MSK 1903 should be protected from
transients by connecting reverse biased ultra-low capacitance
diodes from the output pin to both -VHV and ground. The out-
put can also be protected from arc voltages by inserting a small
value (50-100Ω) 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.
The high voltage power supply (-VHV) is connected to the
amplifier's output stage and must be kept as stable as possible.
The internal or external Rp is connected to -VHV and as such,
the amplifier's DC output is directly related to the high voltage
value. The -VHV pins of the hybrid should be decoupled to ground
with as large a capacitor as possible to improve output stabil-
ity.
VGAIN CONTROL INPUT
The VGAIN control (contrast) input is designed to allow the
user to vary the video gain. By simply applying a DC voltage
from 0V to VREF, the video gain can be linearly adjusted from 0
to 80V/V. The VGAIN input should be connected to the VREF pin
through a 5KΩ pot to ground. For convenient stable gain adjust-
ment, a 0.1µF bypass capacitor should be connected near the
VGAIN input pin to prevent output instability due to noisy sources.
Digital gain control can be accomplished by connecting a D/A
converter to the VGAIN pin. However, some temperature track-
ing performance may be lost when using an external DC voltage
source other than VREF for gain adjustment.
SUPPLY SEQUENCING
The power supply sequence is -VHV, VCC, VEE followed by
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
generated from a low impedance source other than the VREF
output, reverse biased diodes should be connected from each
input (VGAIN, VOFF) to the +VCC pin. This will protect the in-
puts until +VCC is turned on.
The overall video output of the MSK 1903 can be character-
ized using the following expression:
VIDEO OUTPUT
Vpp=VHV-VOUT
When power is first applied and VIN=VGAIN=VOFF=0V, the
output will be practically at the -VHV rail voltage. The output
voltage is a function of the value of Rp and also the VGAIN and
VOFF DC inputs. The maximum output voltage swing for any of
the MSK 1903 variants is determined by Vpp = (250mA) x
(Rp). The bandwidth of the amplifier largely depends on both
Rp and Lp.
VHV-VOUT=(VIN) (VGAIN) (0.1) (Rp) (0.9)
Here is a sample calculation for the MSK 1903-2:
Given information:
VIN=0.7V
VGAIN=1VDC
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).
Rp=400Ω (internal)
VHV=100VDC
VHV-VOUT=(0.7V) (1V) (0.1) (400Ω) (0.9)
VHV-VOUT=25.2V Nominal
The expected video output would swing from approximately
-80V to -54.8V assuming that VOFF=0V. This calculation should
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
as RGB systems. Additionally, only one of the VREF outputs should
be shared by all three amplifiers. This voltage should be buffered
with a suitable low drift op-amp for best tracking performance.
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.
Rev. B 3/03
3
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