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EVAL-ADCMP552BRQ Datasheet(PDF) 11 Page - Analog Devices |
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EVAL-ADCMP552BRQ Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page ADCMP551/ADCMP552/ADCMP553 Rev. 0 | Page 11 of 16 APPLICATION INFORMATION The comparators in the ADCMP55x series are very high speed devices. Consequently, high speed design techniques must be employed to achieve the best performance. The most critical aspect of any ADCMP55x design is the use of a low impedance ground plane. A ground plane, as part of a multilayer board, is recommended for proper high speed performance. Using a continuous conductive plane over the surface of the circuit board can create this, allowing breaks in the plane only for necessary signal paths. The ground plane provides a low inductance ground, eliminating any potential differences at different ground points throughout the circuit board caused by ground bounce. A proper ground plane also minimizes the effects of stray capacitance on the circuit board. It is also important to provide bypass capacitors for the power supply in a high speed application. A 1 µF electrolytic bypass capacitor should be placed within 0.5 inches of each power supply pin to ground. These capacitors reduce any potential voltage ripples from the power supply. In addition, a 10 nF ceramic capacitor should be placed as close to the power supply pins as possible on the ADCMP55x to ground. These capacitors act as a charge reservoir for the device during high frequency switching. The LATCH ENABLE input is active low (latched). If the latching function is not used, the LATCH ENABLE input pins may be left open. The internal pull-ups on the latch pins set the latch to transparent mode. If the latch is to be used, valid PECL voltages are required on the inputs for proper operation. The PECL voltages should be referenced to VCCI. Occasionally, one of the two comparator stages within the ADCMP551/ADCMP552 is not used. The inputs of the unused comparator should not be allowed to float. The high internal gain may cause the output to oscillate (possibly affecting the comparator that is being used) unless the output is forced into a fixed state. This is easily accomplished by ensuring that the two inputs are at least one diode drop apart, while also appropriately connecting the LATCH ENABLE and LATCH ENABLE inputs as described previously. The best performance is achieved with the use of proper PECL terminations. The open-emitter outputs of the ADCMP55x are designed to be terminated through 50 Ω resistors to VCCO − 2.0 V or any other equivalent PECL termination. If high speed PECL signals must be routed more than a centimeter, microstrip or stripline techniques may be required to ensure proper transition times and prevent output ringing. CLOCK TIMING RECOVERY Comparators are often used in digital systems to recover clock timing signals. High speed square waves transmitted over a distance, even tens of centimeters, can become distorted due to stray capacitance and inductance. Poor layout or improper termination can also cause reflections on the transmission line, further distorting the signal waveform. A high speed comparator can be used to recover the distorted waveform while maintaining a minimum of delay. OPTIMIZING HIGH SPEED PERFORMANCE As with any high speed comparator amplifier, proper design and layout techniques should be used to ensure optimal performance from the ADCMP55x. The performance limits of high speed circuitry can easily be a result of stray capacitance, improper ground impedance, or other layout issues. Minimizing resistance from source to the input is an important consideration in maximizing the high speed operation of the ADCMP55x. Source resistance in combination with equivalent input capacitance can cause a lagged response at the input, thus delaying the output. The input capacitance of the ADCMP55x, in combination with stray capacitance from an input pin to ground, could result in several picofarads of equivalent capacitance. A combination of 3 kΩ source resistance and 5 pF input capacitance yields a time constant of 15 ns, which is significantly slower than the 500 ps capability of the ADCMP55x. Source impedances should be significantly less than 100 Ω for best performance. Sockets should be avoided due to stray capacitance and inductance. If proper high speed techniques are used, the ADCMP55x should be free from oscillation when the comparator input signal passes through the switching threshold. COMPARATOR PROPAGATION DELAY DISPERSION The ADCMP55x has been specifically designed to reduce propagation delay dispersion over an input overdrive range of 20 mV to 1 V. Propagation delay overdrive dispersion is the change in propagation delay that results from a change in the degree of overdrive (how far the switching point is exceeded by the input). The overall result is a higher degree of timing accuracy since the ADCMP55x is far less sensitive to input variations than most comparator designs. |
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