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AD8039 bảng dữ liệu(PDF) 9 Page - Analog Devices |
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AD8039 bảng dữ liệu(HTML) 9 Page - Analog Devices |
9 / 12 page REV. B AD8038/AD8039 –9– FREQUENCY – MHz –90 –80 –70 –60 –50 –40 –30 –20 –10 0 0.1 1000 1.0 10 100 TPC 37. AD8038 Input-Output Isolation (G = +2, RL = 2 k Ω, VS = ±5V LAYOUT, GROUNDING, AND BYPASSING CONSIDERATIONS Disable The AD8038 in the SOIC-8 package provides a disable feature. This feature disables the input from the output (see TPC 37 for input-output isolation) and reduces the quiescent current from typically 1 mA to 0.2 mA. When the DISABLE node is pulled below 4.5 V from the positive supply rail, the part becomes disabled. In order to enable the part, the DISABLE node needs to be pulled up to above 2.5 V below the positive rail. Power Supply Bypassing Power supply pins are actually inputs, and care must be taken so that a noise-free stable dc voltage is applied. The purpose of bypass capacitors is to create low impedances from the sup- ply to ground at all frequencies, thereby shunting or filtering a majority of the noise. Decoupling schemes are designed to minimize the bypassing impedance at all frequencies with a parallel combination of capacitors. 0.01 µF or 0.001 µF (X7R or NPO) chip capacitors are critical and should be as close as possible to the amplifier package. Larger chip capacitors, such as the 0.1 µF capacitor, can be shared among a few closely spaced active components in the same signal path. A 10 µF tantalum capacitor is less critical for high frequency bypassing and, in most cases, only one per board is needed at the supply inputs. Grounding A ground plane layer is important in densely packed PC boards to spread the current minimizing parasitic inductances. However, an understanding of where the current flows in a circuit is critical to implementing effective high speed circuit design. The length of the current path is directly proportional to the magnitude of parasitic inductances, and thus the high frequency impedance of the path. High speed currents in an inductive ground return will create an unwanted voltage noise. The length of the high frequency bypass capacitor leads are most critical. A parasitic inductance in the bypass grounding will work against the low impedance created by the bypass capacitor. Place the ground leads of the bypass capacitors at the same physical location. Because load currents flow from the supplies as well, the ground for the load impedance should be at the same physical location as the bypass capacitor grounds. For the larger value capacitors, which are intended to be effective at lower frequencies, the current return path distance is less critical. Input Capacitance Along with bypassing and ground, high speed amplifiers can be sensitive to parasitic capacitance between the inputs and ground. A few pF of capacitance will reduce the input impedance at high frequencies, in turn increasing the amplifiers’ gain, causing peaking of the frequency response, or even oscillations if severe enough. It is recommended that the external passive components that are connected to the input pins be placed as close as possible to the inputs to avoid parasitic capacitance. The ground and power planes must be kept at a distance of at least 0.05 mm from the input pins on all layers of the board. Output Capacitance To a lesser extent, parasitic capacitances on the output can cause peaking of the frequency response. There are two methods to minimize this effect. 1. Put a small value resistor in series with the output to isolate the load capacitor from the amp’s output stage; see TPCs 7, 8, 22, and 23. 2. Increase the phase margin with higher noise gains or add a pole with a parallel resistor and capacitor from –IN to the output. Input-to-Output Coupling The input and output signal traces should not be parallel to minimize capacitive coupling between the inputs and outputs, avoiding any positive feedback. APPLICATIONS Low Power ADC Driver 8 1 0.1 F10 F +5V 0.1 F 10 F 7 0.1 F 10 F –5V 3 2 6 5 4 AD8039 1k 1k 1k 1k VINA VINB REF 50 50 AD9203 1k 1k 1k 1k VIN 0V 3V 2.5V Figure 3. Schematic to Drive AD9203 with the AD8039 Differential A/D Driver The AD9203 is a low power (125 mW on a 5 V supply) 40 MSPS 10-bit converter. This represents a breakthrough in power/speed for ADCs. As such, the low power, high performance AD8039 is an appropriate choice of amplifier to drive it. In low supply voltage applications, differential analog inputs are needed to increase the dynamic range of the ADC inputs. Differential driving can also reduce second and other even-order distortion products. The AD8039 can be used to make a dc-coupled, single-ended-to-differential driver for one of these ADCs. Figure 3 is a schematic of such a circuit for driving an AD9203, a 10-bit, 40 MSPS ADC. |
Số phần tương tự - AD8039 |
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Mô tả tương tự - AD8039 |
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