AD8307
2.50
2.25
2.00
1.75
1.50
1.25
1.00
0.75
0.50
0.25
0
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.
WITHOUT
FILTER
5V
2
1
4.7Ω
0.1µF
C1
R1
0
10µF
5kΩ
+
V
IN
–1
–2
NC
5
ERROR
(WITH FILTER)
0.5mV TO
20V SINE
AMPLITUDE
8
7
6
INP VPS ENB INT
C3
750pF
AD8307
WITH FILTER
–60
INM COM OFS OUT
1
2
3
4
V
OUT
+
C2
10µF
25mV/dB
R2
5kΩ
–100
–80
–40
–20
0
20
C4
1µF
C5
1µF
INPUT LEVEL (dBm)
NC = NO CONNECT
Figure 43. Results for 120 dB Measurement System
Figure 44. Connections for Low Frequency Operation
OPERATION AT LOW FREQUENCIES
DC-COUPLED APPLICATIONS
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 in order 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 −ꢀ4 dBm, and
the input range is now 0.5 mV to 20 V (sine amplitude).
It may occasionally be necessary to provide response to dc inputs.
Since the AD8307 is internally dc-coupled, there is no fundamental
reason why this is precluded. However, there is a practical
constraint since its inputs must be positioned about 2 V above the
COM potential for proper biasing of the first stage. If the source is a
differential signal at this level, it can be directly connected to the
input. For example, a microwave detector can be ac-coupled at its
RF input and its baseband load then automatically provided by the
floating RIN and CIN of the AD8307, at about VP/2.
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 resistance
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 post-
demodulation ripple by lowering the low-pass filter time constant.
This is provided by C5; with the value shown in Figure 44, the
output time constant is 125 ms.
Usually, the source is a single sided ground-referenced signal;
thus, it is necessary to provide a negative supply for the
AD8307. This can be achieved as shown in Figure 45. The
output is now referenced to this negative supply, and it is
necessary to provide an output interface that performs a
differential-to-single sided conversion. This is the purpose of
the AD830. The slope can be arranged to be 20 mV/dB, when
the output ideally runs from zero, for a dc input of 10 μV, to
2.2 μV, for an input of 4 V. The device is fundamentally
insensitive to the sign of the input signal, but with this biasing
scheme, the maximum negative input is constrained to about
−1.5 V. The transfer function after trimming and with R7 = 0 is
V
OUT = (0.4 V) log10 (VIN/10 μV)
R1
4.7Ω
+5V FOR 20mV/dB
+10V FOR 50mV/dB
+15V FOR 100mV/dB
+5V
C1
0.1µF
VR2
V
OUT
50kΩ
–5V
5
R2
3.3kΩ
R5*
8
7
6
5
8
7
6
VP
NC VN
INP VPS ENB INT
INT
V
IN
AD830
X1 X2 Y1 Y2
AD8307
INM COM OFS OUT
C1
1µF
R7
R8
TEMP
1
2
3
4
1
2
3
4
20mV/dB
R6
VR1
2kΩ
AD589
32.4kΩ
Q1
2N3904
C3
0.1µF
R9
250Ω
VR3
50kΩ
R3
1kΩ
–5V
–2V
NC = NO CONNECT
*51kΩ FOR 20mV/dB; 5kΩ FOR 100mV/dB
Figure 45. Connections for DC-Coupled Applications
Rev. C | Page 22 of 24
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