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AD602AQ データシート(PDF) 10 Page - Analog Devices |
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AD602AQ データシート(HTML) 10 Page - Analog Devices |
10 / 28 page AD600/AD602 Rev. E | Page 10 of 28 THEORY OF OPERATION The AD600/AD602 have the same general design and features. They comprise two fixed gain amplifiers, each preceded by a voltage-controlled attenuator of 0 dB to 42.14 dB with independent control interfaces, each having a scaling factor of 32 dB per volt. The AD600 amplifiers are laser trimmed to a gain of 41.07 dB (×113), providing a control range of −1.07 dB to +41.07 dB (0 dB to +40 dB with overlap). The AD602 amplifiers have a gain of 31.07 dB (×35.8) and provide an overall gain of −11.07 dB to +31.07 dB (−10 dB to +30 dB with overlap). The advantage of this topology is that the amplifier can use negative feedback to increase the accuracy of its gain. In addition, because the amplifier does not have to handle large signals at its input, the distortion can be very low. Another feature of this approach is that the small-signal gain and phase response, and thus the pulse response, are essentially independent of gain. Figure 21 is a simplified schematic of one channel. The input attenuator is a 7-stage R-2R ladder network, using untrimmed resistors of nominally R = 62.5 Ω, which results in a characteristic resistance of 125 Ω ± 20%. A shunt resistor is included at the input and laser trimmed to establish a more exact input resistance of 100 Ω ± 2%, which ensures accurate operation (gain and HP corner frequency) when used in conjunction with external resistors or capacitors. PRECISION PASSIVE INPUT ATTENUATOR GATING INTERFACE SCALING REFERENCE GAT1 A1OP A1CM C1HI C1LO A1HI A1LO VG R-2R LADDER NETWORK GAIN CONTROL INTERFACE RF2 2.24kΩ (AD600) 694Ω (AD602) RF1 20Ω FIXED-GAIN AMPLIFIER 41.07dB (AD600) 31.07dB (AD602) 500Ω 0dB –6.02dB –12.04dB –18.06dB –22.08dB –30.1dB –36.12dB –42.14dB 62.5Ω Figure 21. Simplified Block Diagram of Single Channel of the AD600/AD602 The nominal maximum signal at input A1HI is 1 V rms (±1.4 V peak) when using the recommended ±5 V supplies; although, operation to ±2 V peak is permissible with some increase in HF distortion and feedthrough. Each attenuator is provided with a separate signal LO connection for use in rejecting common mode, the voltage between input and output grounds. Circuitry is included to provide rejection of up to ±100 mV. The signal applied at the input of the ladder network is attenuated by 6.02 dB by each section; thus, the attenuation to each of the taps is progressively 0 dB, 6.02 dB, 12.04 dB, 18.06 dB, 24.08 dB, 30.1 dB, 36.12 dB, and 42.14 dB. A unique circuit technique is employed to interpolate between these tap points, indicated by the slider in Figure 21, providing continuous attenuation from 0 dB to 42.14 dB. To understand the AD600, it helps to think in terms of a mechanical means for moving this slider from left to right; in fact, it is voltage controlled. The details of the control interface are discussed later. Note that the gain is exactly determined at all times, and a linear decibel relationship is guaranteed automatically between the gain and the control parameter that determines the position of the slider. In practice, the gain deviates from the ideal law by about ±0.2 dB peak (see Figure 28). Note that the signal inputs are not fully differential. A1LO, A1CM (for CH1), A2LO, and A2CM (for CH2) provide separate access to the input and output grounds. This recognizes that even when using a ground plane, small differences arise in the voltages at these nodes. It is important that A1LO and A2LO be connected directly to the input ground(s). Significant impedance in these connections reduces the gain accuracy. A1CM and A2CM should be connected to the load ground(s). NOISE PERFORMANCE An important reason for using this approach is the superior noise performance that can be achieved. The nominal resistance seen at the inner tap points of the attenuator is 41.7 Ω (one third of 125 Ω), which, at 27°C, exhibits a Johnson noise spectral density (NSD) of 0.84 nV/√Hz (that is, √4kTR), a large fraction of the total input noise. The first stage of the amplifier contributes another 1.12 nV/√Hz, for a total input noise of 1.4 nV/√Hz. The noise at the 0 dB tap depends on whether the input is short-circuited or open-circuited. When shorted, the minimum NSD of 1.12 nV/√Hz is achieved. When open, the resistance of 100 Ω at the first tap generates 1.29 nV/√Hz, so the noise increases to 1.71 nV/√Hz. This last calculation would be important if the AD600 were preceded, for example, by a 900 Ω resistor to allow operation from inputs up to ±10 V rms. However, in most cases, the low impedance of the source limits the maximum noise resistance. |
同様の部品番号 - AD602AQ |
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同様の説明 - AD602AQ |
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