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SL3522 Datenblatt(PDF) 5 Page - Zarlink Semiconductor Inc |
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SL3522 Datenblatt(HTML) 5 Page - Zarlink Semiconductor Inc |
5 / 21 page 5 SL3522 APPLICATION NOTES 1) VIDEO–AMPLIFIER The SL3522 uses a single ended Video amplifier to produce a trimmable Video transfer characteristic. Both the gain (Slope) and Offset of the amplifier can be externally adjusted. a) Gain and Offset trimming (ref Applications circuits in figs 5 and 6) The Gain and Offset control is achieved by adjusting RG and RO respectively. The control is dependent upon their difference from the Trim reference resistor, RT. Adjustment of Gain has an effect on Offset, but adjustment of Offset does NOT affect the Gain. Therefore the Gain should be optimised first. The Offset should only be adjusted once the Gain has been set. Fig 7 shows the variation of Video Offset with value of RO, for a fixed value of RT and RG = 1k5Ω. Fig 8 shows the variation of Video Slope with value of RG, for a fixed value of RT and RO = 1k5Ω. The Video amplifier incorporates temperature compensation for Video gain (Slope). To ensure temperature stability for Video gain (Slope) over the operating temperature range, it is recommended that the resistors with identical temperature coefficients of resistance are used for RT and RG. The Video amplifier does NOT incorporate temperature compensation for Video Offset. Although it is recommended that a resistor with identical temperature coefficient of resistance to RT be used for RO, it may be necessary to use an additional external temperature compensating network. b) Video performance The Video–amplifier has a critically damped rise time of 16ns (10% - 90%).In order to achieve this transient performance, it is important to ensure that:- i) the resistor connected to Trim reference (pin 18), has a nominal resistance of 1.5k Ω, with a parasitic capacitance LESS than 5pF. ii) the load applied to the Video Output (pin 13) does NOT exceed 200 Ω resistance in parallel with 20pF. Also, the following decoupling should be incorporated:- i) The Video Output VCC (pin 14) should be decoupled with a 10nF capacitor to the RETURN line from the video load, connected to Video GND (pin 16), avoiding any common impedance path. ii) The Video Output Vee (pin 12) should be decoupled with a 10nF capacitor DIRECTLY to Video-Output VCC (pin 14). 2) SL3522 AS A LOG AMPLIFIER with RF output buffer disabled (pin 8 floating) If the SL3522 is to be used as a Logarithmic successive detection amplifier only, with no requirement for a limited RF Output, the RF input (pins 27 and 28) can be driven EITHER differentially or single ended from a 50 Ω source. If being used with a single ended input, the SIGNAL should be applied to pin 27 and the RETURN should be connected to pin 28, as shown in the Application circuit diagram in Fig 5. The SL3522 is VERY stable when used in this way. Although not a crucial requirement, it is recommended that the device should be mounted using a ground plane. 3) SL3522 AS A LOG/LIMITING AMPLIFIER - with RF Output-Buffer ENABLED (pin 8 connected to GND) If the SL3522 is to be used as a Limiting or Log/limiting amplifier with a requirement for a Limited RF Output signal,care is required in the layout of components and connections around the device to ensure stability. The following precautions should be observed (refer to Application circuit diagram in Fig. 6):- a) The device should be mounted on a ground plane, ensuring that the impedance between the ground plane and ALL the GND pins is kept as low as possible. If a multilayer PCB is used where the ground plane is connected to the GND pins using through-plated holes (vias), it is essential to ensure that the vias have a very low impedance. ALL supply decoupling capacitors should be RF chip capacitors whose leads should be kept as short as possible. b) The RF VEE connections (pins 3,5,7,11,20,22,24,26) should be connected to a low impedance copper plane. A two layer PCB should help to achieve this. c) The RF input (pins 27 and 28) should be driven with a balanced source impedance. One way of achieving this is to use an isolating BALUN transformer (50 Ω UNBALANCED → 50 Ω BALANCED) connected between the signal source and the RF input pins. (e.g. Mini circuits TT1–6, TO –75). The device stability is VERY sensitive to an imbalance of the differential source impedance at pins 27 and 28. Use of a transmission line BALUN though, is NOT recommended. d) The RF Output connections (pins 9 and 10) should each be loaded with matched impedances ideally 50 Ω transmission lines. The RF Output lines leading away from the device should be balanced. Driving highly reactive SWR loads is NOT recommended as these can encourage device instability, as can an imbalance of the differential load impedance at pins 9 and 10. e) The RF Output connections (pins 9 and 10) are DC coupled, and ideally the output pins should be capacitively coupled to their loads using 1nF capacitors. However the RF Outputs can drive a DC load to GND and a DC offset of approx. 400mV will exist on each RF Output pin. IT WILL NOT BE POSSIBLE TO DISABLE THE RF OUTPUT BUFFER UNDER THESE CONDITIONS. f) The RF output (pins 9 and 10) has a tendancy to limit on self noise, particularly at low ambient temperatures (-55 °C), when the RF output buffer is enabled. NOTE that this will effect the liminting range as the gain of the RF output buffer will reduce as the amount of noise limiting increases. If required the limited RF Output can be attenuated using an attenuation network as shown in fig. 9. Under these conditions the effective RF Output currents will be reduced, allowing the device to operate with a greater margin of stability.It may be possible to run the device without a BALUN transformer on the RF input if the total output impedance on the RF Output >> 50 Ω , and the attenuation components are mounted as close as possible to the RF Output connections (pins 9 and 10). The RF input connection could then be configured as in Fig 5. |
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Ähnliche Beschreibung - SL3522 |
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