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AD8131AR Folha de dados(PDF) 10 Page - Analog Devices |
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AD8131AR Folha de dados(HTML) 10 Page - Analog Devices |
10 / 12 page REV. 0 AD8131 –10– R R R RR k IN dm G F GF , . = − ×+ ( ) = 1 2 1 125 Ω The circuit’s input impedance is effectively higher than it would be for a conventional op amp connected as an inverter because a fraction of the differential output voltage appears at the inputs as a common-mode signal, partially bootstrapping the voltage across the input resistor RG. Input Common-Mode Voltage Range in Single Supply Applications The AD8131 is optimized for level-shifting “ground” referenced input signals. For a single-ended input this would imply, for example, that the voltage at –DIN in Figure 37 would be zero volts when the amplifier’s negative power supply voltage (at V–) was also set to zero volts. Setting the Output Common-Mode Voltage The AD8131’s VOCM pin is internally biased at a voltage approximately equal to the midsupply point (average value of the voltages on V+ and V–). Relying on this internal bias will result in an output common-mode voltage that is within about 25 mV of the expected value. In cases where more accurate control of the output common- mode level is required, it is recommended that an external source, or resistor divider (made up of 10 k Ω resistors), be used. Driving a Capacitive Load A purely capacitive load can react with the pin and bondwire inductance of the AD8131 resulting in high frequency ringing in the pulse response. One way to minimize this effect is to place a small resistor in series with the amplifier’s outputs as shown in Figure 23. APPLICATIONS Twisted-Pair Line Driver The AD8131 has on-chip resistors that provide for a gain-of- two without any external parts. Several on-chip resistors are trimmed to ensure that the gain is accurate, the common-mode rejection is good, and the output is well balanced. This makes the AD8131 very suitable as a single-ended-to-differential twisted-pair line driver. Figure 38 shows a circuit of an AD8131 driving a twisted-pair line, like a Category 3 or Category 5 (Cat3 or Cat5), that are already installed in many buildings for telephony and data com- munications. The characteristic impedance of such transmission lines is usually about 100 Ω. The outstanding balance of the AD8131 output will minimize the common-mode signal and there- fore the amount of EMI generated by driving the twisted pair. The two resistors in series with each output terminate the line at the transmit end. Since the impedances of the outputs of the AD8131 are very low, they can be thought of as a short circuit, and the two terminating resistors form a 100 Ω termination at the transmit end of the transmission line. The receive end is directly terminated by a 100 Ω resistor across the line. This back-termination of the transmission line divides the out- put signal by two. The fixed gain of two of the AD8131 will create a net unity gain for the system from end to end. In this case, the input signal is provided by a signal generator with an output impedance of 50 Ω. This is terminated with a 49.9 Ω resistor near +D IN of the AD8131. The effective parallel resistance of the source and termination is 25 Ω. The 24.9 Ω resistor from –DIN to ground matches the +DIN source impedance and minimizes any dc and gain errors. If +DIN is driven by a low-impedance source over a short dis- tance, such as the output of an op amp, then no termination resistor is required at +DIN. In this case, the –DIN can be directly tied to ground. +3 V Supply Differential A-to-D Driver Many newer A-to-D converters can run from a single +3 V supply, which can save significant system power. In order to increase the dynamic range at the analog input, they have differ- ential inputs, which doubles the dynamic range with respect to a single-ended input. An added benefit of using a differential input is that the distortion can be improved. The low distortion and ability to run from a single +3 V supply make the AD8131 suited as an A-to-D driver for some 10-bit, single supply applications. Figure 39 shows a schematic for a circuit for an AD8131 driving an AD9203, a 10-bit, 40 MSPS A-to-D converter. The common mode of the AD8131 output is set at midsupply by the voltage divider connected to VOCM, and ac bypassed with a 0.1 µF capacitor. This provides for maximum dynamic range between the supplies at the output of the AD8131. The 110 Ω resistors at the AD8131 output, along with the shunt capacitors form a one pole, low-pass filter for lowering noise and antialiasing. Figure 40 shows an FFT plot that was taken from the combined devices at an analog input frequency of 2.5 MHz and a 40 MSPS sampling rate. The performance of the AD8131 compares very favorably with a center-tapped transformer drive, which has typically been the best way to drive this A-to-D converter. The AD8131 has the advantage of maintaining dc performance, which a transformer solution cannot provide. Unity-Gain, Single-Ended-to-Differential Driver If it is not necessary to offset the output common-mode volt- age (via the VOCM pin), then the AD8131 can make a simple unity-gain single-ended-to-differential amplifier that does not require any external components. Figure 41 shows the schematic for this circuit. Referring to Figure 2, when –DIN is left floating, there is 100 percent feedback of +OUT to –IN via the internal feedback resistor. This contrasts with the typical gain-of-two operation where –DIN is grounded and one third of the +OUT is fed back to –IN. The result is a closed-loop differential gain of one. Upon careful observation, it can be seen that only +DIN and VOCM are referenced to ground. It is the case that the ground voltage at VOCM is the reference for this circuit. In this unity gain configuration, if a dc voltage is applied to VOCM to shift the common-mode voltage, a differential dc voltage will be created at the output, along with the common-mode voltage change. Thus, this configuration cannot be used when it is desired to offset the common-mode voltage of the output with respect to the input at +DIN. |
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