AD8307ARZ Analog Devices Inc, AD8307ARZ Datasheet - Page 20

• Manufacturer Part Number: AD8307ARZ
• Description: IC LOGARITHMIC AMP 92DB 8-SOIC
• Manufacturer: Analog Devices Inc
• Type: Logarithmic Amplifierr

Specifications of AD8307ARZ

• Applications: Receiver Signal Strength Indication (RSSI)
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• No. Of Amplifiers: 1
• Dynamic Range, Decades: 4.6
• Scale Factor V / Decade: 0.5
• Response Time: 500ns
• Supply Voltage Range: 2.7V To 5.5V
• Supply Current: 8mA
• Input Offset Voltage: 500µV
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD8307
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.
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; R
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.
–75dBm TO
NC = NO CONNECT
+16dBm
INPUT
Figure 38. Log Amp with Buffered Output
0.1µF
INM COM OFS OUT
INP VPS ENB INT
8
1
AD8307
7
2
32.4kΩ
NC
6
3
4.7Ω
50kΩ
VR2
5
4
50kΩ
VR1
±3dB
20mV/dB
R
V
AD8031
S
P
, 2.7V TO 5.5V
C1
FOR V
T
V
optionally provides
P
P
= 3V, R
= 5V, R
R1
20kΩ
R2
30.1kΩ
S
S
OUTPUT
50mV/dB
±10%
= 20kΩ
= 51kΩ
COM
Rev. D | Page 20 of 24
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.
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.
–75dBm TO
INPUT 5mV
TO 160V rms
NC = NO CONNECT
2.5nF
+16dBm
INPUT
C3
+
50kΩ
10µF
R1
C1
C2
10µF
INM COM OFS OUT
INP VPS ENB INT
0.1µF
Figure 40. Log Amp with Four-Pole Low-Pass Filter
8
1
+
INM COM OFS OUT
0.1µF
INP VPS ENB INT
AD8307
8
1
7
2
INT ±4dB
AD8307
VR1
2kΩ
4.7Ω
Figure 39. Cable Driving Log Amp
7
2
+
6
3
6.34kΩ
NC
6
3
NC
4.7Ω
5
4
V
422Ω
50kΩ
32.4kΩ
VR2
P
VR2
50kΩ
SLOPE
5
4
VR1
5kΩ
±3dB
10mV/dB
34kΩ
±18%
R
1µF
1µF
C5
AD8031
C6
S
+
+
OP AMP IS AD8032 SCALE
C1 TO C8 AS NEEDED.
NOTE POLARITIES IF TANTALUM
CAPACITORS ARE USED.
80.6kΩ
7.32kΩ
100kΩ
34kΩ
1µF
R1
2kΩ
C7
2N3904
R2
3.01kΩ
+
25mV/dB
1µF
C8
+
V
P
, 2.7V TO 5.5V
75kΩ
OUTPUT
100mV/dB
R
(OPTIONAL)
OUTPUT
50Ω
MINIMUM
T
93kΩ
COM
COM