AD8307-EB datasheet(15/20 Pages) AD

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AD8307

–15–

REV. A

APPLICATIONS

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 connec-

tions shown in the preceding section. A few examples of more

specialized applications are provided here.

Buffered Output

The output may be buffered, and the slope optionally increased,

using an op amp. If the single-supply capability is to be pre-

served, 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-pin version and in

dual form as the 8-pin AD8032. Figure 36 shows how the slope

may 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 k

Ω, the slope is adjustable to 25 mV/dB, allow-

ing the use of a 2.7 V supply. Setting R2 to 80.6 k

Ω, 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.

INP VPS ENB INT

INM COM OFS OUT

AD8307

INPUT

–75dBm TO

+16dBm

4.7

3dB

0.1 F

VP, 2.7V – 5.5V

20mV/dB

VR2

50k

32.4k

VR1

50k

AD8031

20k

30.1k

OUTPUT

50mV/dB

10%

COM

FOR VP = 3V, RS = 20k

VP = 5V, RS = 51k

NC = NO CONNECT

Figure 36. 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%-

90% response time is under 200 ns, and is typically 300 ns to

99% of final value. To achieve faster response times, it is neces-

sary 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 k

Ω, 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%-90%

response time is under 100 ns. Figure 37 shows how the output

current capability can be augmented to drive a 50

Ω load; R

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.

INP VPS ENB INT

INM COM OFS OUT

AD8307

INPUT

–75dBm TO

+16dBm

4.7

3dB

0.1 F

VP, 2.7–5.5V

10mV/dB

18%

VR2

50k

6.34k

VR1

AD8031

3.01k

OUTPUT

MIN

COM

2N3904

NC = NO CONNECT

(OPTIONAL)

25mV/dB

(SEE FIGURE 36)

Figure 37. Cable-Driving Log Amp

In Figure 38, the capacitor values were chosen for operation in

the audio field, providing a corner frequency of 10 Hz, an attenua-

tion 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 may easily be adapted to other frequencies, by proportional

scaling of C5–C7 (e.g., 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.

INP VPS ENB INT

INM COM OFS OUT

AD8307

INPUT

5mV TO

160V rms

COM

0.1 F

VR2

50k

SLOPE

32.4k

80.6k

100k

OUTPUT

100mV/dB

OP AMP IS AD8032

SCALE C1 – C8 AS NEEDED

(SEE TEXT). NOTE POLARITIES

IF TANTALUM CAPACITORS

ARE USED.

1 F

34k

1 F

7.32k

93k

1 F

10 F

2.5nF

10 F

VR1

INT

4dB

422

4.7

50k

75k

1 F

34k

Figure 38. Log Amp with Four-Pole Low-Pass Filter

Figure 38 also shows the use of an input attenuator which may

optionally be employed here, or in any other of these applica-

tions, 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 band-

width 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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