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AD8370 Datasheet(PDF) 15 Page - Analog Devices |
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AD8370 Datasheet(HTML) 15 Page - Analog Devices |
15 / 28 page AD8370 Rev. A | Page 15 of 28 APPLICATIONS BASIC CONNECTIONS Figure 44 shows the minimum connections required for basic operation of the AD8370. Supply voltages between 3.0 V and 5.5 V are allowed. The supply to the VCCO and VCCI pins should be decoupled with at least one low inductance, surface- mount ceramic capacitor of 0.1 μF placed as close as possible to the device. AD8370 67 8 23 5 1 11 10 9 15 14 16 13 12 4 SERIAL CONTROL INTERFACE 100pF 1nF 0.1 μF 100pF 0.1 μF +VS (3.0V TO 5.0V) FERRITE BEAD FERRITE BEAD 1nF 1nF 1nF 1nF BALANCED LOAD RL BALANCED SOURCE RS 2 RS 2 Figure 44. Basic Connections The AD8370 is designed to be used in differential signal chains. Differential signaling allows improved even-order harmonic cancellation and better common-mode immunity than can be achieved using a single-ended design. To fully exploit these benefits, it is necessary to drive and load the device in a balanced manner. This requires some care to ensure that the common-mode impedance values presented to each set of inputs and outputs are balanced. Driving the device with an unbalanced source can degrade the common-mode rejection ratio. Loading the device with an unbalanced load can cause degradation to even-order harmonic distortion and premature output compression. In general, optimum designs are fully balanced, although the AD8370 still provides impressive performance when used in an unbalanced environment. The AD8370 is a fine adjustment, VGA. The gain control transfer function is linear in voltage gain. On a decibel scale, this results in the logarithmic transfer functions shown in Figure 4. At the low end of the gain transfer function, the slope is steep, providing a rather coarse control function. At the high end of the gain control range, the decibel step size decreases, allowing precise gain adjustment. GAIN CODES The AD8370’s two gain ranges are referred to as high gain (HG) and low gain (LG). Within each range, there are 128 possible gain codes. Therefore, the minimum gain in the low gain range is given by the nomenclature LG0 whereas the maximum gain in that range is given by LG127. The same is true for the high gain range. Both LG0 and HG0 essentially turn off the variable transconductance stage, and thus no output is available with these codes (see Figure 26). The theoretical linear voltage gain can be expressed with respect to the gain code as AV = GainCode Vernier (1 + (PreGain − 1) MSB) where: AV is the linear voltage gain. GainCode is the digital gain control word minus the MSB (the final 7 bits). Vernier = 0.055744 V/V PreGain = 7.079458 V/V MSB is the most significant bit of the 8-bit gain control word. The MSB sets the device in either high gain mode (MSB = 1) or low gain mode (MSB = 0). For example, a gain control word of HG45 (or 10101101 binary) results in a theoretical linear voltage gain of 17.76 V/V, calculated as 45 × 0.055744 × (1 + (7.079458 − 1) × 1) Increments or decrements in gain within either gain range are simply a matter of operating on the GainCode. Six –dB gain steps, which are equivalent to doubling or halving the linear voltage gain, are accomplished by doubling or halving the GainCode. When power is first applied to the AD8370, the device is programmed to code LG0 to avoid overdriving the circuitry following it. POWER-UP FEATURE The power-up feature does not affect the GainCode, and the gain setting is preserved when in power-down mode. Powering down the AD8370 (bringing PWUP low while power is still applied to the device) does not erase or change the GainCode from the AD8370, and the same gain code is in place when the device is powered up, that is, when PWUP is brought high again. Removing power from the device all together and reapplying, however, reprograms to LG0. |
Número de pieza similar - AD8370_05 |
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Descripción similar - AD8370_05 |
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