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AD600JN Datasheet(PDF) 8 Page - Analog Devices |
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AD600JN Datasheet(HTML) 8 Page - Analog Devices |
8 / 20 page AD600/AD602 REV. A –8– APPLICATIONS The full potential of any high performance amplifier can only be realized by careful attention to details in its applications. The following pages describe fully tested circuits in which many such details have already been considered. However, as is always true of high accuracy, high speed analog circuits, the schematic is only part of the story; this is no less true for the AD600 and AD602. Appropriate choices in the overall board layout and the type and placement of power supply decoupling components are very important. As explained previously, the input grounds A1LO and A2LO must use the shortest possible connections. The following circuits show examples of time-gain control for ultrasound and for sonar, methods for increasing the output drive, and AGC amplifiers for audio and RF/IF signal process- ing using both peak and rms detectors. These circuits also illus- trate methods of cascading X-AMPs for either maintaining the optimal S/N ratio or maximizing the accuracy of the gain- control voltage for use in signal measurement. These AGC cir- cuits may be modified for use as voltage-controlled amplifiers for use in sonar and ultrasound applications by removing the detector and substituting a DAC or other voltage source for supplying the control voltage. Time-Gain Control (TGC) and Time-Variable Gain (TVG) Ultrasound and sonar systems share a similar requirement: both need to provide an exponential increase in gain in response to a linear control voltage, that is, a gain control that is “linear in dB.” Figure 11 shows the AD600/AD602 configured for a con- trol voltage ramp starting at –625 mV and ending at +625 mV for a gain-control range of 40 dB. For simplicity, only the A1 connections are shown. The polarity of the gain-control voltage may be reversed and the control voltage inputs C1HI and C1LO reversed to achieve the same effect. The gain-control voltage can be supplied by a voltage-output DAC such as the AD7242, which contains two complete DACs, operates from ±5 V supplies, has an internal reference of 3 V, and provides ±3 V of output swing. As such it is well-suited for use with the AD600/AD602, needing only a few resistors to scale the output voltage of the DACs to the levels needed by the AD600/AD602. 1 2 3 4 5 6 7 8 16 15 14 13 12 11 10 9 REF A1 A2 C1HI A1CM A1OP V+ V– A2OP A2CM C2HI C1LO A1HI A1LO GAT1 A2LO A2HI C2LO VOLTAGE-OUTPUT DAC VG CONTROL VOLTAGE, V G +625mV –625mV A1 GAIN 0dB 40dB AD600 or AD602 GAT2 +5V –5V Figure 11. The Simplest Application of the X-AMP Is as a TGC or TVG Amplifier in Ultrasound or Sonar. Only the A1 Connections Are Shown for Simplicity. Increasing Output Drive The AD600/AD602’s output stage has limited capability for negative-load driving capability. For driving loads less than 500 Ω, the load drive may be increased by about 5 mA by con- necting a 1 k Ω pull-down resistor from the output to the nega- tive supply (Figure 12). Driving Capacitive Loads For driving capacitive loads of greater than 5 pF, insert a 10 Ω resistor between the output and the load. This lowers the possi- bility of oscillation. 1 2 3 4 5 6 7 8 16 15 14 13 12 11 10 9 REF A1 A2 C1HI A1CM A1OP VPOS VNEG A2OP A2CM C2HI C1LO A1HI A1LO GAT1 GAT2 A2LO A2HI C2LO GAIN-CONTROL VOLTAGE V IN +5V 1k Ω –5V ADDED PULL-DOWN RESISTOR AD600 Figure 12. Adding a 1 k ΩPull-Down Resistor Increases the X-AMP’s Output Drive by About 5 mA. Only the A1 Con- nections Are Shown for Simplicity. Realizing Other Gain Ranges Larger gain ranges can be accommodated by cascading amplifi- ers. Combinations built by cascading two amplifiers include –20 dB to +60 dB (using one AD602), –10 dB to +70 dB (1/2 of an AD602 followed by 1/2 of an AD600), and 0 dB to 80 dB (one AD600). In multiple-channel applications, extra protection against oscillations can be provided by using amplifier sections from different packages. An Ultralow Noise VCA The two channels of the AD600 or AD602 may be operated in parallel to achieve a 3 dB improvement in noise level, providing 1 nV/ √Hz without any loss of gain accuracy or bandwidth. In the simplest case, as shown in Figure 13, the signal inputs A1HI and A2HI are tied directly together, the outputs A1OP and A2OP are summed via R1 and R2 (100 Ω each), and the control inputs C1HI/C2HI and C1LO/C2LO operate in paral- lel. Using these connections, both the input and output resis- tances are 50 Ω. Thus, when driven from a 50 Ω source and terminated in a 50 Ω load, the gain is reduced by 12 dB, so the gain range becomes –12 dB to +28 dB for the AD600 and –22 dB to +18 dB for the AD602. The peak input capability remains unaffected (1 V rms at the IC pins, or 2 V rms from an unloaded 50 Ω source). The loading on each output, with a 50 Ω load, is effectively 200 Ω, because the load current is shared between the two channels, so the overall amplifier still meets its specified maximum output and distortion levels for a 200 Ω load. This amplifier can deliver a maximum sine wave power of +10 dBm to the load. |
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