AD8307AN datasheet(20/24 Pages) AD

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AD8307

Rev. C | 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

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.

20mV/dB

AD8031

OUTPUT

50mV/dB

±10%

AD8307

INPUT

–75dBm TO

+16dBm

INP VPS ENB INT

INM COM OFS OUT

NC = NO CONNECT

4.7Ω

VR2

50kΩ

30.1kΩ

COM

20kΩ

0.1µF

VP, 2.7V TO 5.5V

±3dB

VR1

50kΩ

32.4kΩ

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

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.

25mV/dB

2N3904

10mV/dB

±18%

AD8031

OUTPUT

50Ω

MINIMUM

AD8307

INPUT

–75dBm TO

+16dBm

INP VPS ENB INT

INM COM OFS OUT

NC = NO CONNECT

4.7Ω

VR2

50kΩ

3.01kΩ

2kΩ

0.1µF

VP, 2.7V TO 5.5V

±3dB

VR1

5kΩ

6.34kΩ

(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

4.7Ω

VR1

2kΩ

50kΩ

93kΩ

80.6kΩ

32.4kΩ

34kΩ

422Ω

0.1µF

1µF

75kΩ

1µF

10µF

2.5nF

10µF

OP AMP IS AD8032 SCALE

C1 TO C8 AS NEEDED.

NOTE POLARITIES IF TANTALUM

CAPACITORS ARE USED.

1µF

7.32kΩ

100kΩ

34kΩ

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 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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