参数资料
型号: AD8307AR-REEL
厂商: Analog Devices Inc
文件页数: 15/24页
文件大小: 0K
描述: IC LOGARITHMIC AMP 8-SOIC T/R
标准包装: 2,500
类型: 对数放大器
应用: 接收器信号强度指示(RSSI)
安装类型: 表面贴装
封装/外壳: 8-SOIC(0.154",3.90mm 宽)
供应商设备封装: 8-SO
包装: 带卷 (TR)
AD8307
Rev. D | Page 22 of 24
2.50
2.25
0
2.00
1.25
1.00
0.50
WITH FILTER
ERROR
(WITH FILTER)
WITHOUT
FILTER
0.25
0.75
1.50
1.75
–2
–1
0
1
2
–100
–80
–60
–40
–20
20
0
01
08
2
-04
3
V
OU
T
(V
)
E
RRO
R
(
d
B)
INPUT LEVEL (dBm)
Figure 43. Results for 120 dB Measurement System
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, provid-
ing 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.
C2
10F
C1
10F
01
08
2-
04
5
AD8307
NC
INP VPS ENB INT
INM COM OFS OUT
NC = NO CONNECT
87
6
5
23
4
1
5V
R1
5k
R2
5k
0.1F
C5
1F
VOUT
25mV/dB
VIN
0.5mV TO
20V SINE
AMPLITUDE
C4
1F
+
C3
750pF
4.7
Figure 44. Connections for Low Frequency Operation
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