Data Sheet

AD8307

Rev. F | Page 17 of 24

THEORY OF OPERATION

The AD8307 has very high gain and a bandwidth from dc to over

1 GHz, at which frequency the gain of the main path is still over

60 dB. Consequently, it is susceptible to all signals within this very

broad frequency range that find their way to the input terminals. It

is important to remember that these are indistinguishable from

the wanted signal, and has the effect of raising the apparent

noise floor (that is, lowering the useful dynamic range). For

example, while the signal of interest can be an IF of 50 MHz,

any of the following could easily be larger than the IF signal at

the lower extremities of its dynamic range: 60 Hz hum (picked

up due to poor grounding techniques), spurious coupling (from

a digital clock source on the same PC board), and local radio

stations, for example.

Careful shielding is essential. A ground plane should be used to

provide a low impedance connection to the common pin, COM,

for the decoupling capacitors used at VPS, and as the output

ground. It is inadvisable to assume that the ground plane is

equipotential. Neither of the inputs should be ac-coupled directly

to the ground plane, but should be kept separate from it, being

returned instead to the low associated with the source. This can

mean isolating the low side of an input connector with a small

resistance to the ground plane.

BASIC CONNECTIONS

Figure 32 shows the simple connections suitable for many

applications. The inputs are ac coupled by C1 and C2, which

should have the same value, for example, CC. The coupling time

constant is RIN CC/2, thus forming a high-pass corner with a

3 dB attenuation at fHP = 1/(pRINCC ). In high frequency applica-

tions, fHP should be as large as possible to minimize the coupling

of unwanted low frequency signals. Conversely, in low frequency

applications, a simple RC network forming a low-pass filter

should be added at the input for the same reason. For the case

where the generator is not terminated, the signal range should

be expressed in terms of the voltage response and should extend

from −85 dBV to +6 dBV.

C2 = CC

C1 = CC

OUTPUT

25mV/dB

AD8307

RT

INPUT

–75dBm TO

+16dBm

NC

INP VPS ENB INT

INM COM OFS OUT

NC

NC = NO CONNECT

87

6

5

23

4

1

1.1kΩ

4.7Ω

0.1µF

RIN ≈

VP, 2.7V TO 5.5V

AT ~8mA

Figure 32. Basic Connections

Where it is necessary to terminate the source at a low impedance,

the resistor RT should be added, with allowance for the shunting

effect of the basic 1.1 kΩ input resistance (RIN) of the AD8307.

For example, to terminate a 50 Ω source, a 52.3 Ω 1% tolerance

resistor should be used. This can be placed on the input side or

the log amp side of the coupling capacitors; in the former case,

smaller capacitors can be used for a given frequency range; in

the latter case, the effective RIN is lowered directly at the log

amp inputs.

Figure 33 shows the output vs. the input level, in dBm, when

driven from a terminated 50 Ω generator, for sine inputs at

10 MHz, 100 MHz, and 500 MHz; Figure 34 shows the typical

logarithmic conformance under the same conditions. Note that

10 dBm corresponds to a sine amplitude of 1 V, equivalent to an

rms power of 10 mW in a 50 Ω termination. However, if the

termination resistor is omitted, the input power is negligible.

The use of dBm to define input level therefore needs to be

considered carefully in connection with the AD8307.

3.0

2.5

0

2.0

1.5

1.0

0.5

500MHz

100MHz

10MHz

–80

–70

–60

–50

–40

–30

–20

–10

0

10

20

INPUT LEVEL (dBm)

Figure 33. Log Response at 10 MHz, 100 MHz, and 500 MHz

5

4

3

2

1

–4

–5

500MHz

100MHz

10MHz

–3

–2

–1

0

–80

–70

–60

–50

–40

–30

–20

–10

0

10

20

INPUT LEVEL (dBm)

Figure 34. Logarithmic Law Conformance at 10 MHz, 100 MHz, and 500 MHz