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ADL5330 Datenblatt(PDF) 12 Page - Analog Devices |
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ADL5330 Datenblatt(HTML) 12 Page - Analog Devices |
12 / 24 page ADL5330 Data Sheet Rev. B | Page 12 of 24 THEORY OF OPERATION The ADL5330 is a high performance, voltage controlled variable gain amplifier/attenuator for use in applications with frequencies up to 3 GHz. This device serves as an output variable gain amplifier (OVGA) for applications where a reasonably constant input level is available and the output level adjusts over a wide range. One aspect of an OVGA is the output metrics, OIP3 and OP1dB, decrease with decreasing gain. The signal path is fully differential throughout the device in order to provide the usual benefits of differential signaling, including reduced radiation, reduced parasitic feedthrough, and reduced susceptibility to common-mode interference with other circuits. Figure 31 provides a simplified schematic of the ADL5330. GAIN CONTROL OPHI OPLO INHI INLO TRANSIMPEDANCE AMPLIFIER Gm STAGE Figure 31. Simplified Schematic A controlled input impedance of 50 Ω is achieved through a combination of passive and active (feedback-derived) termination techniques in an input Gm stage. The input compression point of the Gm stage is 1 dBm to 3 dBm, depending on the input frequency. Note that the inputs of the Gm stage are internally biased to a dc level, and dc blocking capacitors are generally needed on the inputs to avoid upsetting operation of the device. The currents from the Gm stage are then injected into a balanced ladder attenuator at a deliberately diffused location along the ladder, wherein the location of the centroid of the injection region is dependent on the applied gain control voltage. The steering of the current injection into the ladder is accomplished by proprietary means to achieve linear in dB gain control and low distortion. Linear in dB gain control is accomplished by the application of a voltage in the range of 0 V dc to 1.4 V dc to the gain control pin, with maximum gain occurring at the highest voltage. The output of the ladder attenuator is passed into a fixed gain transimpedance amplifier (TZA) to provide gain and buffer the ladder terminating impedance from load variations. The TZA uses feedback to improve linearity and to provide controlled 50 Ω differential output impedance. The quiescent current of the output amplifier is adaptive; it is slaved to the gain control voltage to conserve power at times when the gain (and output power) are low. The outputs of the ADL5330 require external dc bias to the positive supply voltage. This bias is typically supplied through external inductors. The outputs are best taken differentially to avoid any common-mode noise that is present, but, if necessary, can be taken single-ended from either output. If only a single output is used, it is still necessary to provide bias to the unused output pin, and it is advisable to arrange a reasonably equivalent ac load on the unused output. Differential output can be taken via a 1:1 balun into a 50 Ω environment. In virtually all cases, it is necessary to use dc blocking in the output signal path. At high gain settings, the noise floor is set by the input stage, in which case the noise figure (NF) of the device is essentially independent of the gain setting. Below a certain gain setting, however, the input stage noise that reaches the output of the attenuator falls below the input equivalent noise of the output stage. In such a case, the output noise is dominated by the output stage itself; therefore, the overall NF of the device gets worse on a dB per dB basis, because the gain is reduced below the critical value. Figure 9 through Figure 13 provide details of this behavior. |
Ähnliche Teilenummer - ADL5330_17 |
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Ähnliche Beschreibung - ADL5330_17 |
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