AD8307

Data Sheet

Rev. F | Page 20 of 24

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.

20mV/dB

AD8031

OUTPUT

50mV/dB

±10%

AD8307

INPUT

–75dBm TO

+16dBm

INP VPS ENB INT

INM COM OFS OUT

NC

NC = NO CONNECT

87

6

5

23

4

1

4.7Ω

VR2

50kΩ

R2

30.1kΩ

COM

R1

20kΩ

RS

0.1µF

VP, 2.7V TO 5.5V

±3dB

VR1

50kΩ

32.4kΩ

C1

FOR VP = 3V, RS = 20kΩ

VP = 5V, RS = 51kΩ

Figure 38. Log Amp with Buffered Output

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.

25mV/dB

2N3904

10mV/dB

±18%

AD8031

OUTPUT

50Ω

MINIMUM

AD8307

INPUT

–75dBm TO

+16dBm

INP VPS ENB INT

INM COM OFS OUT

NC

NC = NO CONNECT

87

6

5

23

4

1

4.7Ω

VR2

50kΩ

R2

3.01kΩ

R1

2kΩ

RS

0.1µF

VP, 2.7V TO 5.5V

±3dB

VR1

5kΩ

6.34kΩ

RT

(OPTIONAL)

COM

Figure 39. Cable Driving Log Amp

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.

VR2

50kΩ

SLOPE

OUTPUT

100mV/dB

AD8307

INPUT 5mV

TO 160V rms

INP VPS ENB INT

INM COM OFS OUT

87

6

5

23

4

1

4.7Ω

VR1

2kΩ

NC

R1

50kΩ

93kΩ

80.6kΩ

32.4kΩ

34kΩ

422Ω

0.1µF

VP

C6

1µF

75kΩ

C7

1µF

C2

10µF

C3

2.5nF

+

+

C1

10µF

OP AMP IS AD8032 SCALE

C1 TO C8 AS NEEDED.

NOTE POLARITIES IF TANTALUM

CAPACITORS ARE USED.

+

+

+

+

C8

1µF

7.32kΩ

100kΩ

34kΩ

+

C5

1µF

INT ±4dB

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.