MSK1911 Datasheet, PDF (3/6 Pages) M.S. Kennedy Corporation – HIGH PERFORMANCE, HIGH VOLTAGE VIDEO DISPLAY DRIVER

APPLICATION NOTES
POWER SUPPLIES
The input stage of the MSK 1911 requires power supplies of
+10V 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 1µF) connected
as close to the amplifiers pins as possible. The MSK 1911 has
internal 0.01µF capacitors that also improve high frequency
performance. Additionally, it is also recommended to put 0.1µF
decoupling capacitors on the +10V and -10.5V supplies as
well.
The high voltage power supply (+VHV) is connected to the
amplifiers output stage and must be kept as stable as possible.
The internal Rp is connected to +VHV and as such, the amplifi-
ers DC output is directly related to the high voltage value. The
+VHV pin of the hybrid should be decoupled to ground with as
large a capacitor as possible to improve output stability.
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
inputs until +VCC is turned off.
VIDEO INPUTS
The video input signals should be kept below ±2VMAX total
including both common mode offset and signal levels. The
input structure of the MSK 1911 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.
OUTPUT PROTECTION
The output pin of the MSK 1911 can be protected from tran-
sients by connecting reversed biased ultra-low capacitance di-
odes 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 re-
sistor will reduce system bandwidth along with the load capaci-
tance, but a series inductor can reduce the problem substan-
tially.
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 100V/V. The VGAIN input should be connected to the VREF
pin through a 5kΩ pot to ground. For convenient stable gain
adjustment, 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 connect-
ing a D/A converter to the VGAIN pin. However, some tempera-
ture tracking performance may be lost when using an external
DC voltage source other than VREF for gain adjustment.
VIDEO OUTPUT
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 bandwidth of the amplifier largely de-
pends on both Rp and Lp. With +VHV set to +70V and total
Rp=200Ω (internal), the device is capable of approximately
62Vpp total output swing.
Hybrid pin 13 is directly connected to Rp. Additional exter-
nal resistance can be added to reduce power dissipation, but
slower transition times will result. If an additional resistor is
used, it must be low capacitance and the layout should mini-
mize capacitive coupling to ground (ie: no ground plane under
Rp).
The MSK 1911 is conservatively specified with low values
for external Lp which yield about 5% overshoot. Additional
peaking can be obtained by using a high self-resonant frequency
inductor in series with +VHV pin. 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 1911.
If external resistors or inductors are not used, be sure to
connect high frequency bypass capacitors directly from pin 13
to ground.
The overall video output of the MSK 1911 can be charac-
terized using the following expression:
Vpp=VHV-VOUT
VHV-VOUT=(VIN)(VGAIN)(Rp)(0.08)
Here is a sample calculation for the MSK1911:
Given information
VIN=0.7V
VGAIN=1VDC
Rp=200Ω (internal)
VHV=70VDC
VHV-VOUT=(0.7V)(1V)(.08)(200Ω)
VHV-VOUT=11.2Vpp Nominal
The expected video output would swing from approximately
+70V to +58.8V assuming that VOFF=0V. This calculation
should be used as a nominal result because the overall gain
may vary as much as ±10% due to internal high speed device
variations. Changing ambient conditions can also effect the
video gain of the amplifier slightly. 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.
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Rev. D 8/00