AD8307_06 Datasheet, PDF (20/24 Pages) Analog Devices – Low Cost DC-500 MHz, 92 dB Logarithmic Amplifier

AD8307
APPLICATIONS INFORMATION
The AD8307 is a highly versatile and easily applied log amp
requiring very few external components. Most applications of this
product can be accommodated using the simple connections
shown in the preceding section.
BUFFERED OUTPUT
The output can be buffered, and the slope optionally increased,
using an op amp. If the single-supply capability is to be preserved, a
suitable component is the AD8031. Like the AD8307, it is
capable of operating from a 2.7 V supply and features a rail-to-
rail output capability; it is available in a 5-lead version and in
dual form as the 8-lead AD8032. Figure 38 shows how the slope
can be increased to 50 mV/dB (1 V per decade), requiring a
5 V supply (90 dB times 50 mV is a 4.5 V swing). VR1 provides
a ±10% slope adjustment; VR2 provides a ±3 dB intercept
range. With R2 = 4.99 kΩ, the slope is adjustable to 25 mV/dB,
allowing the use of a 2.7 V supply. Setting R2 to 80.6 kΩ, it is
raised to 100 mV/dB, providing direct reading in decibels on a
digital voltmeter. Since a 90 dB range now corresponds to a 9 V
swing, a supply of at least this amount is needed for the op amp.
0.1µF
4.7Ω
VR2
50kΩ
V_P, 2.7V TO 5.5V
R_S
INPUT
–75dBm TO
+16dBm
±3dB
8 765
FOR V_P = 3V, R_S = 20kΩ
V_P = 5V, R_S = 51kΩ
INP VPS ENB INT
AD8307
AD8031
OUTPUT
50mV/dB
±10%
INM COM OFS OUT
1 234
NC
20mV/dB
R2
30.1kΩ
NC = NO CONNECT
32.4kΩ VR1
C1
50kΩ
R1
20kΩ
Figure 38. Log Amp with Buffered Output
COM
C1 is optional; it lowers the corner frequency of the low-pass
output filter. A value of 0.1 μF should be used for applications in
which the output is measured on a voltmeter or other low speed
device. On the other hand, when C1 is omitted, the 10% to 90%
response time is under 200 ns and is typically 300 ns to 99% of
final value. To achieve faster response times, it is necessary to
lower the load resistance at the output of the AD8307, then
restore the scale using a higher gain in the op amp. Using
8.33 kΩ, the basic slope is 10 mV/dB; this can be restored to
25 mV/dB using a buffer gain of 2.5. The overall 10% to 90%
response time is under 100 ns. Figure 39 shows how the output
current capability can be augmented to drive a 50 Ω load; RT
optionally provides reverse termination, which halves the slope
to 12.5 mV/dB.
FOUR POLE FILTER
In low frequency applications, for example, audio down to
20 Hz, it is useful to employ the buffer amplifier as a multipole
low-pass filter in order to achieve low output ripple while
maintaining a rapid response time to changes in signal level.
0.1µF
4.7Ω
VR2
50kΩ RS
V_P, 2.7V TO 5.5V
INPUT
–75dBm TO
+16dBm
±3dB
8 765
INP VPS ENB INT
AD8307
AD8031
INM COM OFS OUT
1 2 3 4 10mV/dB
NC
±18%
2N3904
25mV/dB
R2
3.01kΩ
NC = NO CONNECT
6.34kΩ VR1
R1
5kΩ
2kΩ
Figure 39. Cable Driving Log Amp
R_T
(OPTIONAL)
OUTPUT
50Ω
MINIMUM
COM
In Figure 40, the capacitor values are chosen for operation in
the audio field, providing a corner frequency of 10 Hz, an
attenuation of 80 dB/decade above this frequency, and a 1%
settling time of 150 ms (0.1% in 175 ms). The residual ripple is
4 mV (±0.02 dB) when the input to the AD8307 is at 20 Hz.
This filter can easily be adapted to other frequencies by
proportional scaling of C5 to C7 (for example, for 100 kHz use
100 pF). Placed ahead of a digital multimeter, the convenient
slope scaling of 100 mV/dB requires only a repositioning of the
decimal point to read directly in decibels. The supply voltage for
the filter must be large enough to support the dynamic range; a
minimum of 9 V is needed for most applications; 12 V is
recommended.
INPUT 5mV
TO 160V rms ^0.1µF _4.7Ω
R1
VR1
50kΩ
2kΩ
V_P
422Ω
C1
10µF ₊
INT ±4dB
NC
8 765
OP AMP IS AD8032 SCALE
C1 TO C8 AS NEEDED.
NOTE POLARITIES IF TANTALUM
CAPACITORS ARE USED.
C5
1µF
C8
7.32kΩ ^1µF
OUTPUT
100mV/dB
+
+
C3
2.5nF
INP VPS ENB INT
AD8307
INM COM OFS OUT
1 234
+
34kΩ
100kΩ
34kΩ
93kΩ
+ _C2
10µF
VR2
50kΩ
SLOPE _{C6 +}
32.4kΩ 1µF
C7 +
1µF
80.6kΩ
75kΩ
COM
Figure 40. Log Amp with Four Pole Low Pass Filter
Figure 40 also shows the use of an input attenuator that can
optionally be employed here, or in any other of these
applications, to produce a useful wide range ac voltmeter with
direct decibel scaling. The basic range of −73 dBm to +17 dBm
(that is, 50 μV rms to 1.6 V rms, for sine excitations) is shifted
for illustrative purposes to 5 mV to 160 V rms (at which point
the power in R1 is 512 mW). Because the basic input resistance
of the AD8307 is not precise, VR1 is used to center the signal
range at its input, doubling as a ±4 dB intercept adjustment. The
low frequency response extends to 15 Hz; a higher corner
frequency can be selected as needed by scaling C1 and C2. The
shunt capacitor C3 is used to lower the high frequency
bandwidth to about 100 kHz, and thus lower the susceptibility
to spurious signals. Other values should be chosen as needed
for the coupling and filter capacitors.
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