AD8307AR-REEL7 Datasheet AD8307ARZ, AD8307ARZ-RL7, AD8307ANZ | P19-P21

AD8307 Data Sheet

Rev. E | Page 18 of 24

INPUT MATCHING

Where higher sensitivity is required, an input matching network

is valuable. Using a transformer to achieve the impedance

transformation also eliminates the need for coupling capacitors,

which lowers the offset voltage generated directly at the input,

and balances the drives to the INP pin and the INM pin. The

choice of turns ratio depends somewhat on the frequency. At

frequencies below 50 MHz, the reactance of the input capacitance

is much higher than the real part of the input impedance. In this

frequency range, a turns ratio of about 1:4.8 lowers the input

impedance to 50  while raising the input voltage, thus lowering

the effect of the short-circuit noise voltage by the same factor.

There is a small contribution from the input noise current, so

the total noise is reduced by a lesser factor. The intercept is also

lowered by the turns ratio; for a 50  match, it is reduced by

20 log

(4.8) or 13.6 dB.

NARROW-BAND MATCHING

Transformer coupling is useful in broadband applications. How-

ever, a magnetically coupled transformer may not be convenient in

some situations. At high frequencies, it is often preferable to use

a narrow-band matching network, as shown in Figure 35.

Using a narrow-band matching network has several advantages.

The same voltage gain is achieved, providing increased sensitivity,

but a measure of selectivity is also introduced. The component

count is low: two capacitors and an inexpensive chip inductor.

Further, by making these capacitors unequal, the amplitudes at

the INP pin and the INM pin can be equalized when driving from

a single-sided source, that is, the network also serves as a balun.

Figure 36 shows the response for a center frequency of 100 MHz.

Note the very high attenuation at low frequencies. The high fre-

quency attenuation is due to the input capacitance of the log amp.

OUTPUT

25mV/dB

01082-035

AD8307

50Ω INPUT

–88dBm TO

+3dBm

INP VPS ENB INT

INM COM OFS OUT

NC = NO CONNECT

= 50Ω

8765

234

4.7Ω

0.1µ

, 2.7V TO 5.5

AT ~8mA

Figure 35. High Frequency Input Matching Network

–1

INPUT

GAIN

01082-036

DECIBELS

FREQUENCY (MHz)

60 150140130120110100908070

Figure 36. Response of 100 MHz Matching Network

Table 5 provides solutions for a variety of center frequencies (f

)

and matching impedances (Z

) of nominally 50  and 100 .

The unequal capacitor values were chosen to provide a well-

balanced differential drive and to allow better centering of the

frequency response peak when using standard value components,

which generally results in a Z

that is not exact. The full AD8307

HF input impedance and the inductor losses are included in the

modeling.

Table 5. Narrow-Band Matching Values

(MHz) Z

(Ω) C1 (pF) C2 (pF) L

(nH) Voltage Gain (dB)

10 45 160 150 3300 13.3

20 44 82 75 1600 13.4

50 46 30 27 680 13.4

100 50 15 13 330 13.4

150 57 10 8.2 220 13.2

200 57 7.5 6.8 150 12.8

250 50 6.2 5.6 100 12.3

500 54 3.9 3.3 39 10.9

10 103 100 91 5600 10.4

20 102 51 43 2700 10.4

50 99 22 18 1000 10.6

100 98 11 9.1 430 10.5

150 101 7.5 6.2 260 10.3

200 95 5.6 4.7 180 10.3

250 92 4.3 3.9 130 9.9

500 114 2.2 2.0 47 6.8

Data Sheet AD8307

Rev. E | Page 19 of 24

SLOPE AND INTERCEPT ADJUSTMENTS

Where higher calibration accuracy is needed, the adjustments

shown in Figure 37 can be used, either singly or in combination.

The log slope is lowered to 20 mV/dB by shunting the nominally

12.5 k on-chip load resistor (see Figure 31) with 50 k, adjusted

by VR1. The calibration range is ±10% (18 mV/dB to 22 mV/dB),

including full allowance for the variability in the value of the

internal load. The adjustment can be made by alternately applying

two input levels, provided by an accurate signal generator, spaced

over the central portion of the log amp’s dynamic range, for

example, −60 dBm and 0 dBm. An AM modulated signal at the

center of the dynamic range can also be used. For a modulation

depth, M, expressed as a fraction, the decibel range between the

peaks and troughs over one cycle of the modulation period is

given by







log20

(7)

For example, using an rms signal level of −40 dBm with a 70%

modulation depth (M = 0.7), the decibel range is 15 dB, as the

signal varies from −47.5 dBm to −32.5 dBm.

The log intercept is adjustable over a ±3 dB range, which is

sufficient to absorb the worst-case intercept error in the AD8307,

plus some system level errors. For greater range, set R

to zero.

VR2 is adjusted while applying an accurately known CW signal

near the lower end of the dynamic range to minimize the effect

of any residual uncertainty in the slope. For example, to position

the intercept to −80 dBm, a test level of −65 dBm can be applied

and VR2 adjusted to produce a dc output of 15 dB above zero at

25 mV/dB, which is 0.3 V.

01082-037

C2 = C

C1 = C

20mV/dB

±10%

AD8307

INPUT

–75dBm TO

+16dBm

INP VPS ENB INT

INM COM OFS OUT

NC = NO CONNECT

8765

234

4.7Ω

VR2

50kΩ

0.1µ

, 2.7V TO 5.5V

AT ~8mA

±3dB

VR1

50kΩ

32.4kΩ

FOR V

= 3V, R

= 20kΩ

= 5V, R

= 51kΩ

Figure 37. Slope and Intercept Adjustments

AD8307 Data Sheet

Rev. E | 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 k, the slope is adjustable to 25 mV/dB, allowing

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

01082-038

OUTPUT

50mV/dB

±10%

AD8307

INPUT

–

75dBm TO

+16dBm

INP VPS ENB INT

INM COM OFS OUT

NC = NO CONNECT

8765

234

4.7Ω

VR2

50kΩ

30.1kΩ

COM

20kΩ

0.1µ

, 2.7V TO 5.5V

±3dB

VR1

50kΩ

32.4kΩ

FOR V

= 3V, R

= 20kΩ

= 5V, R

= 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 F 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 k,

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  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 to achieve low output ripple while maintaining a

rapid response time to changes in signal level.

25mV/dB

2N3904

10mV/dB

±18%

AD8031

01082-039

OUTPUT

50Ω

MINIMUM

AD8307

INPUT

–75dBm TO

+16dBm

INP VPS ENB INT

INM COM OFS OUT

NC = NO CONNECT

8765

234

4.7Ω

VR2

50kΩ

3.01kΩ

2kΩ

0.1µF

, 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

01082-040

OUTPUT

100mV/dB

AD8307

INPUT 5m

O 160V rms

INP VPS ENB INT

INM COM OFS OUT

8765

234

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 to produce a useful wide range ac voltmeter

with direct decibel scaling. The basic range of −73 dBm to

+17 dBm (that is, 50 V 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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