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VCA810 データシートの表示(PDF) - Texas Instruments

VCA810 High Gain Adjust Range, Wideband and Variable Gain Amplifier Texas-Instruments
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VCA810 Datasheet PDF : 40 Pages
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Feature Description (continued)
The maximum input voltage range is the largest at full attenuation (40 dB) and decreases as the gain increases.
Similarly, the maximum useful output voltage range increases as the input decreases. We can distinguish three
overloading issues as a result of the operating mode: high attenuation, mid-range gain-attenuation, and high
From –40 dB to –10 dB, gain overdriving the input stage is the only method to overdrive the VCA810. Preventing
this type of overdrive is achieved by limiting the input voltage range.
From –10 dB to 40 dB, overdriving can be prevented by limiting the output voltage range. There are two limiting
mechanisms operating in this situation. From –10 dB to 10 dB, an internal stage is the limiting factor; from 10 dB
to 40 dB, the output stage is the limiting factor.
Output overdriving occurs when either the maximum output voltage swing or output current is exceeded. The
VCA810 high output current of ±60 mA ensures that virtually all output overdrives will be limited by voltage swing
rather than by current limiting. Table 1 summarizes these overdrive conditions.
40 dB < G < 10 dB
10 dB < G < 10 dB
5 dB < G < 40 dB
Table 1. Output Signal Compression
Input Stage Overdrive
Internal Stage Overdrive
Output Stage Overdrive
Input Voltage Range
Output Voltage Range
Output Voltage Range
8.3.2 Overdrive Recovery
As shown in Figure 11, the onset of overdrive occurs whenever the actual output begins to deviate from the ideal
expected output. If possible, the user should operate the VCA810 within the linear regions shown in order to
minimize signal distortion and overdrive delay time. However, instances of amplifier overdrive are quite common
in automatic gain control (AGC) circuits, which involve the application of variable gain to input signals of varying
levels. The VCA810 design incorporates circuitry that allows it to recover from most overdrive conditions in 200
ns or less. Overdrive recovery time is defined as the time required for the output to return from overdrive to linear
operation, following the removal of either an input or gain-control overdrive signal. See Typical Characteristics for
the overdrive plots for maximum gain and maximum attenuation.
8.3.3 Output Offset Error
Several elements contribute to the output offset voltage error; among them are the input offset voltage, the
output offset voltage, the input bias current and the input offset current. To simplify the following analysis, the
output offset voltage error is dependent only on the output-offset voltage of the VCA810 and the input offset
voltage. The output offset error can then be expressed as Equation 3:
( GdB
VOS = VOSO + 10 20 · VIOS
• VOS = Output offset error
• VOSO = Output offset voltage
• GdB = VCA810 gain in dB
• VIOS = Input offset voltage
This is shown in Figure 29.
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