APPLICATION NOTES
POWER SUPPLIES
The input stage of the MSK 1933 requires power supplies of
+20V 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 1933 has
internal 0.01µF capacitors that also improve high frequency
performance. It is also recommended to put 0.1µF decoupling
capacitors on the +20V and -10.5V supplies as well. Since
the output stage is returned to +20V internally, all of the out-
put current will flow through this supply pin.
The high voltage power supply (-V
HV
) is connected to the
amplifier's output stage and must be kept as stable as possible.
The internal or external Rp is connected to -V
HV
and as such,
the amplifier's DC output is directly related to the high voltage
value. The -V
HV
pins of the hybrid should be decoupled to
ground with as large a capacitor as possible to improve output
stability.
VIDEO INPUTS
The video input signals should be kept below ±2V
MAX
total,
including both common mode offset and signal levels. The
input structure of the MSK 1933 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 1933 should be protected from
transients by connecting reversed biased ultra-low capacitance
diodes from the output pin to both -V
HV
and ground. The out-
put can also be protected from arc voltages by inserting a small
value (25-50Ω) 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.
V
GAIN
CONTROL INPUT
The V
GAIN
control (contrast) input is designed to allow the
user to vary the video gain. By simply applying a DC voltage
from 0V to V
REF
, the video gain can be linearly adjusted from 0
to 195V/V (MSK 1933-2). The V
GAIN
input should be connected
to the V
REF
pin through a 5KΩ pot to ground. For convenient
stable gain adjustment, a 0.1µF bypass capacitor should be con-
nected near the V
GAIN
input pin to prevent output instability due
to noisy sources. Digital gain control can be accomplished by
connecting a D/A converter to the V
GAIN
pin. However, some
temperature tracking performance may be lost when using an
external DC voltage source other than V
REF
for gain adjustment.
The bandwidth of the V
GAIN
input is approximately 1MHz.
The overall video output of the MSK 1933 can be character-
ized using the following expression:
Vpp=V
HV
-V
OUT
V
HV
-V
OUT
=(V
IN
)(V
GAIN
)(Rp)(0.09)
(or)
Voltage Gain=V
OUT
/V
IN
=(V
GAIN
)(Rp)(0.09)
Here is a sample calculation for the MSK 1932-2:
Given information
V
IN
=0.7V
V
GAIN
=1VDC
Rp=400Ω (internal)
V
HV
=-80VDC
V
HV
-V
OUT
=(0.7V)(1V)(400Ω)(0.09)
V
HV
-V
OUT
=25.2V Nominal
The expected video output would swing from approximately
-80V to -54.8V assuming that V
OFF
=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 V
REF
outputs
should be shared by all three amplifiers. This voltage should be
buffered with a suitable low drift op-amp for best tracking per-
formance.
Rev. A 8/00
SUPPLY SEQUENCING
The power supply sequence is V
HV
, V
CC
, V
EE
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 V
REF
output, reverse biased diodes should be connected from each
input (V
GAIN
, V
OFF
) to the V
CC
pin. This will protect the inputs
until V
CC
is turned on.
VIDEO OUTPUT
When power is first applied and V
IN
=V
GAIN
=V
OFF
=0V, the
output will be practically at the -V
HV
rail voltage. The output
voltage is a function of the value of Rp and also the V
GAIN
and
V
OFF
DC inputs. The maximum output voltage swing for any of
the MSK 1933 variants is determined by Vpp = (250mA) x
(Rp). The bandwidth of the amplifier largely depends on both
Rp and Lp.
Hybrid pins 12 and 13 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 1933 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 pins 12 & 13. 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 1933-0 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
12 and 13 to ground for the devices that contain an internal
Rp.
3
MSK [ M.S. KENNEDY CORPORATION ]
