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MSK1903-2 Datasheet(PDF) 3 Page - M.S. Kennedy Corporation |
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MSK1903-2 Datasheet(HTML) 3 Page - M.S. Kennedy Corporation |
3 / 6 page 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. 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. 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 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. 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. 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 5K Ω 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. The overall video output of the MSK 1903 can be character- ized using the following expression: Vpp=VHV-VOUT 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 Rp=400 Ω (internal) VHV=100VDC VHV-VOUT=(0.7V) (1V) (0.1) (400 Ω) (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. VGAIN CONTROL INPUT 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 (50-100 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 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 25 Ω resistor to ground if input offset currents are to be minimized. 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. Rev. B 3/03 3 |
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