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AD8307

Rev. C | 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); local radio stations; and so on.

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

provide a low impedance connection to the common pin,

COM, for the decoupling capacitor(s) used at VPS, and as the

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

is an 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, say, 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 applications,

fHP should be as large as possible in order 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

extends from −85 dBV to +6 dBV.

C2 = CC

C1 = CC

OUTPUT

25mV/dB

AD8307

INPUT

–75dBm TO

+16dBm

INP VPS ENB INT

INM COM OFS OUT

NC = NO CONNECT

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 versus 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

2.0

1.5

1.0

0.5

500MHz

100MHz

10MHz

–80

–70

–60

–50

–40

–30

–20

–10

INPUT LEVEL (dBm)

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

–4

–5

500MHz

100MHz

10MHz

–3

–2

–1

–80

–70

–60

–50

–40

–30

–20

–10

INPUT LEVEL (dBm)

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

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 Pin INP and Pin INM. 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

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