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发表于 2018-2-12 13:31:30
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OPERATION AT LOW FREQUENCIES
The AD8307 provides excellent logarithmic conformance at
signal frequencies that can be arbitrarily low, depending only on
the values used for the input coupling capacitors.
It can also be desirable to add a low-pass input filter to desensitize the log
amp to HF signals.
Figure 44 shows a simple arrangement, providing coupling with an attenuation of 20 dB; the intercept is shifted up by this attenuation, from −84 dBm to −64 dBm, and the
input range is now 0.5 mV to 20 V (sine amplitude).
A high-pass 3 dB corner frequency of nominally 3 Hz is set by
the 10 μF coupling capacitors, C1 and C2, which are preferably
tantalum electrolytics (note the polarity) and a low-pass 3 dB
corner frequency of 200 kHz (set by C3 and the effective resis-
tance at the input of 1 kΩ). The −1% amplitude error points
occur at 20 Hz and 30 kHz. These are readily altered to suit other
applications by simple scaling. When C3 is zero, the low-pass
corner is at 200 MHz. Note that the lower end of the dynamic
range is improved by this capacitor, which essentially provides
an HF short circuit at the input. This significantly lowers the
wideband noise; the noise reduction is about 2 dB compared to
when the AD8307 is driven from a 50 Ω source. Ensure that the
output is free of postdemodulation ripple by lowering the low-pass
filter time constant, which is provided by C5. With the value shown
in Figure 44, the output time constant is 125 ms.
See Figure 40 for a more elaborate filter. To improve the law
conformance at very low signal levels and at low frequencies, add
C4 to the offset compensation loop.
低频操作
AD8307在。时具有出色的对数一致性
信号频率可以任意低，只取决于
用于输入耦合电容的值。
 也可能需要添加一个低通输入滤波器来使日志不敏感
放大器到HF信号。
 图44显示了一个简单的安排，提供20 dB衰减的耦合;截距由这个衰减向上移动，从-84 dBm增加到-64 dBm，
输入范围现在为0.5 mV至20 V（正弦幅值）。
标称3 Hz的高通3 dB转角频率由设定
10μF的耦合电容C1和C2
钽电解（注意极性）和低通3 dB
转角频率为200 kHz（由C3和有效电阻 -
在1kΩ的输入端）。 -1％幅度误差点
发生在20 Hz和30 kHz。这些很容易改变，以适应其他
应用程序通过简单缩放当C3为零时，低通
角落在200 MHz。请注意，动态的低端
通过这个电容器来改善范围，这基本上提供了
输入端发生短路。这显着降低了
宽带噪声;相比之下，降噪大约是2dB
当AD8307由50Ω电源驱动时。确保
通过降低低通，输出没有后解调纹波
滤波时间常数，由C5提供。显示值
在图44中，输出时间常数是125 ms。
请参见图40以获取更详细的过滤器。改善法律
在非常低的信号电平和低频率下的一致性
C4到偏移补偿环路。 |
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