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AD8519AR Datasheet(PDF) 9 Page - Analog Devices |
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AD8519AR Datasheet(HTML) 9 Page - Analog Devices |
9 / 12 page REV. C AD8519/AD8529 –9– 500mV 50 s VS = 2.5V AVCC = 1 TA = +25 C CL = 100pF TPC 19. Large Signal Transient Response APPLICATIONS INFORMATION Maximum Power Dissipation The maximum power that can be safely dissipated by the AD8519/AD8529 is limited by the associated rise in junction temperature. The maximum safe junction temperature is 150 ∞C for these plastic packages. If this maximum is momentarily exceeded, proper circuit operation will be restored as soon as the die temperature is reduced. Operating the product in the “overheated” condition for an extended period can result in permanent damage to the device. Precision Full-Wave Rectifier Slew rate is probably the most underestimated parameter when designing a precision rectifier. Yet without a good slew rate large glitches will be generated during the period when both diodes are off. Let’s examine the operation of the basic circuit (shown in Figure 1) before considering slew rate further. U1 is set up to have two states of operation. D1 and D2 diodes switch the output between the two states. State one is an inverter with a gain of +1, and state two is a simple unity gain buffer where the output is equal to the value of the virtual ground. The virtual ground is the potential present at the noninverting node of the U1. State one is active when VIN is larger than the virtual ground. D2 is on in this condition. If VIN drops below virtual ground, D2 turns off and D1 turns on. This causes the output of U1 to simply buffer the virtual ground and this configuration is state two. So, the function of U1, which results from these two states of operation, is a half wave inverter. The U2 function takes the inverted half wave at a gain of two and sums it into the original VIN wave, which outputs a rectified full wave. VV V OUT IN IN =- < - 20 1 This type of rectifier can be very precise if the following electrical parameters are adhered to: 1. All passive components should be of tight tolerance, 1% resistors and 5% capacitors. 2. If the application circuit requires high impedance (i.e., direct sensor interface), then a FET amplifier is probably a better choice than the AD8519. 3. An amp such as the AD8519, which has a great slew rate specification, will yield the best result because the circuit involves switching. AD8519 D1 1N914 R6 5k R1 10k VIN R4 10k R2 10k D2 1N914 R3 4.99k R5 10k VOUT R7 3.32k AD8519 U2 U1 VIRTUAL GROUND = VCC 2 NODE A Figure 1. Precision Full-Wave Rectifier Switching glitches are caused when D1 and D2 are both momen- tarily off. This condition occurs every time the input signal is equal to the virtual ground potential. When this condition occurs, the U1 stage is taken out of the VOUT equation and VOUT is equal to VIN ¥ R5 ¥ (R4 R1 + R2 + R3). Note that Node A should be VIN inverted or virtual ground, but in this condition Node A is simply tracking VIN. Given a sine wave input centered around virtual ground, glitches are generated at the sharp negative peaks of the rectified sine wave. If the glitches are hard to notice on an oscilloscope, raise the frequency of the sine wave until they become apparent. The size of the glitches is proportional to the input frequency, the diode turn-on potential (0.2 V or 0.65 V), and the slew rate of the op amp. R6 and R7 are both necessary to limit the amount of bias current related voltage offset. Unfortunately, there is no “perfect” value for R6 because the impedance at the inverting node is altered as D1 and D2 switch. Therefore, there will also be some unresolved bias current related offset. To minimize this offset, use lower value resistors or choose a FET amplifier if the optimized offset is still intolerable. The AD8519 offers a unique combination of speed versus power ratio at 2.7 V single supply, small size (SC70 and SOT-23), and low noise that make it an ideal choice for most high volume and high precision rectifier circuits. 10 Microphone Preamp Meets PC99 Specifications This circuit, while lacking a unique topology, is anything but featureless when an AD8519 is used as the op amp. This preamp gives 20 dB gain over a frequency range of 20 Hz to 20 kHz and is fully PC99 compliant in all parameters including THD+N, dynamic range, frequency range, amplitude range, crosstalk, and so on. Not only does this preamp comply with the PC99 specifi- cations, it far surpasses them. In fact, this preamp has a VOUT noise of around 100 dB, which is suitable for most professional 20-bit audio systems. Referred to input noise is 120 dB. At 120 dB THD+N in unity gain, the AD8519 is suitable for 24-bit profes- sional audio systems. In other words, the AD8519 will not be the limiting performance factor in audio systems despite its small size and low cost. |
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