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AD603ARZ-REEL Arkusz danych(PDF) 11 Page - Analog Devices |
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AD603ARZ-REEL Arkusz danych(HTML) 11 Page - Analog Devices |
11 / 20 page AD603 Rev. G | Page 11 of 20 THEORY OF OPERATION The AD603 comprises a fixed-gain amplifier, preceded by a broadband passive attenuator of 0 dB to 42.14 dB, having a gain control scaling factor of 40 dB per volt. The fixed gain is laser- trimmed in two ranges, to either 31.07 dB (×35.8) or 50 dB (×358), or may be set to any range in between using one external resistor between Pin 5 and Pin 7. Somewhat higher gain can be obtained by connecting the resistor from Pin 5 to common, but the increase in output offset voltage limits the maximum gain to about 60 dB. For any given range, the bandwidth is independent of the voltage-controlled gain. This system provides an underrange and overrange of 1.07 dB in all cases; for example, the overall gain is −11.07 dB to +31.07 dB in the maximum bandwidth mode (Pin 5 and Pin 7 strapped). This X-AMP structure has many advantages over former methods of gain control based on nonlinear elements. Most importantly, the fixed-gain amplifier can use negative feedback to increase its accuracy. Since large inputs are first attenuated, the amplifier input is always small. For example, to deliver a ±1 V output in the −1 dB/+41 dB mode (that is, using a fixed amplifier gain of 41.07 dB) its input is only 8.84 mV; thus the distortion can be very low. Equally important, the small-signal gain and phase response, and thus the pulse response, are essentially independent of gain. Figure 29 is a simplified schematic. The input attenuator is a seven-section 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 ±3%, which ensures accurate operation (gain and HP corner frequency) when used in conjunction with external resistors or capacitors. The nominal maximum signal at input VINP 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. Pin 4 (COMM) must be connected directly to the input ground; significant impedance in this connection will reduce the gain accuracy. 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 29, thus providing continuous attenuation from 0 dB to 42.14 dB. It will help in understanding the AD603 to think in terms of a mechanical means for moving this slider from left to right; in fact, its position is controlled by the voltage between Pin 1 and Pin 2. The details of the gain control interface are discussed later. The gain is at all times very exactly determined, and a linear-in- dB relationship is automatically guaranteed by the exponential nature of the attenuation in the ladder network (the X-AMP principle). In practice, the gain deviates slightly from the ideal law, by about ±0.2 dB peak (see, for example, Figure 5). NOISE PERFORMANCE An important advantage of the X-AMP is its superior noise performance. The nominal resistance seen at inner tap points is 41.7 Ω (one third of 125 Ω), which exhibits a Johnson noise spectral density (NSD) of 0.83 nV/√Hz (that is, √4kTR) at 27°C, which is a large fraction of the total input noise. The first stage of the amplifier contributes a further 1 nV/√Hz, for a total input noise of 1.3 nV/√Hz. It will be apparent that it is essential to use a low resistance in the ladder network to achieve the very low specified noise level. The signal’s source impedance forms a voltage divider with the AD603’s 100 Ω input resistance. In some applications, the resulting attenuation may be unacceptable, requiring the use of an external buffer or preamplifier to match a high impedance source to the low impedance AD603. The noise at maximum gain (that is, at the 0 dB tap) depends on whether the input is short-circuited or open-circuited: when shorted, the minimum NSD of slightly over 1 nV/√Hz is achieved; when open, the resistance of 100 Ω looking into the first tap generates 1.29 nV/√Hz, so the noise increases to a total of 1.63 nV/√Hz. (This last calculation would be important if the AD603 were preceded by, for example, a 900 Ω resistor to allow operation from inputs up to 10 V rms.) As the selected tap moves away from the input, the dependence of the noise on source impedance quickly diminishes. Apart from the small variations just discussed, the signal-to- noise (S/N) ratio at the output is essentially independent of the attenuator setting. For example, on the −11 dB/+31 dB range, the fixed gain of ×35.8 raises the output NSD to 46.5 nV/√Hz. Thus, for the maximum undistorted output of 1 V rms and a 1 MHz bandwidth, the output S/N ratio would be 86.6 dB, that is, 20 log (1 V/46.5 µV). |
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