AD8307ANZ1 View Datasheet(PDF) - Analog Devices
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AD8307ANZ1 Datasheet PDF : 25 Pages
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AD8307
Data Sheet
APPLICATIONS INFORMATION
The AD8307 is a highly versatile and easily applied log amp
requiring very few external components. Most applications of this
device can be accommodated using the simple connections shown
in the preceding section.
BUFFERED OUTPUT
The output can be buffered and the slope optionally increased by
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. Because 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
VP, 2.7V TO 5.5V
RS
INPUT
–75dBm TO
+16dBm
±3dB
8
7
6
5
FOR
VVPP
=
=
3V,
5V,
RRSS
=
=
20kΩ
51kΩ
INP VPS ENB INT
AD8307
AD8031
OUTPUT
50mV/dB
±10%
INM COM OFS OUT
1
2
3
NC
4 20mV/dB
R2
30.1kΩ
NC = NO CONNECT
32.4kΩ VR1
50kΩ
C1
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
the 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 to achieve low output ripple while maintaining a
rapid response time to changes in signal level.
0.1µF
4.7Ω
VR2
50kΩ RS
VP, 2.7V TO 5.5V
INPUT
–75dBm TO
+16dBm
±3dB
8
7
6
5
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
5kΩ
R1
2kΩ
Figure 39. Cable Driving Log Amp
RT
(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
VP
422Ω
C1
10µF +
INT ±4dB
NC
8
7
6
5
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
2
3
4
+
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 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.
Rev. E | Page 20 of 24
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