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AD8143 Datasheet(PDF) 20 Page - Analog Devices |
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AD8143 Datasheet(HTML) 20 Page - Analog Devices |
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20 / 24 page  AD8143 Rev. 0 | Page 20 of 24 A diode is a simple example of such a clamp. Schottky diodes generally have lower clamping voltages than typical signal diodes. The clamping voltage should be larger than the largest expected signal amplitude, with enough margin to ensure that the received signal passes without being distorted. A simple way to implement a clamp is to use a number of diodes in series. The resultant clamping voltage is then the sum of the clamping voltages of individual diodes. A 1N4448 diode has a forward voltage of approximately 0.70 V to 0.75 V at typical current levels that are seen when it is being used as a clamp, and 2 pF maximum capacitance at 0 V bias. (The capacitance of a diode decreases as its reverse bias voltage is increased.) The series connection of two 1N4448 diodes, therefore, has a clamping voltage of 1.4 V to 1.5 V. Figure 43 shows how to limit the differential input voltage applied to an AD8143 amplifier to ±1.4 V to ±1.5 V (2.8 V p-p to 3.0 V p-p). Note that the resulting capacitance of the two series diodes is half that of one diode. Different numbers of series diodes can be used to obtain different clamping voltages. RT is the differential termination resistor and the series resistances, RS, limit the current into the diodes. The series resistors should be highly matched in value to preserve high frequency CMRR. POSITIVE CLAMP NEGATIVE CLAMP RS RT VIN RS – + VOUT – + Figure 43. Using Two 1N4448 Diodes in Series as a Clamp There are many other nonlinear devices that can be used as clamps. The best choice for a particular application depends upon the desired clamping voltage, response time, parasitic capacitance, and other factors. When using external differential-mode clamping, it is important to ensure that the series resistors (RS), the sum of the parasitic capacitance of the clamping devices, and the input capacitance of the AD8143 are small enough to preserve the desired signal bandwidth. Figure 44 shows a specific example of external common-mode clamping. + – VOUT V+ V+ V– V– 3 2 1 3 2 1 VIN + – RT RS RS HBAT-540C HBAT-540C Figure 44. External Common-Mode Clamping The series resistances, RS, limit the current in each leg, and the Schottky diodes limit the voltages on each input to approximately 0.3 V to 0.4 V over the positive power supply, V+ and to 0.3 V to 0.4 V below the negative power supply, V−. The maximum value of RS is determined by the required signal bandwidth, the line impedance, and the effective differential capacitance due to the AD8143 inputs and the diodes. As with the differential clamp, the series resistors should be highly matched in value to preserve high frequency CMRR. PRINTED CIRCUIT BOARD LAYOUT CONSIDERATIONS The two most important issues with regard to printed circuit board (PCB) layout are minimizing parasitic signal trace reactances in the feedback network and providing sufficient thermal relief. Excessive parasitic reactances in the feedback network cause excessive peaking in the amplifier’s frequency response and excessive overshoot in its step response due to a reduction in phase margin. Oscillation occurs when these parasitic reactances are increased to a critical point where the phase margin is reduced to zero. Minimizing these reactances is important to obtain optimal performance from the AD8143. When operating at ±12 V power, it is important to pay special attention to removing heat from the AD8143. Besides the special layout considerations previously mentioned and expounded upon in the following sections, general high speed layout practices must be adhered to when applying the AD8143. Controlled impedance transmission lines are required for incoming and outgoing signals, referenced to a ground plane. |
Similar Part No. - AD8143_15 |
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Similar Description - AD8143_15 |
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