AD8307

–13–

REV. A

USING THE AD8307

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 quite indis-

tinguishable from the “wanted” signal, and will have the effect

of raising the apparent noise floor (that is, lowering the useful

dynamic range). For example, while the signal of interest may

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; spuri-

ous coupling from a digital clock source on the same PC board;

local radio stations; etc.

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, however, and neither of the inputs should be ac-

coupled directly to the ground plane, but kept separate from it,

being returned instead to the low associated with the source.

This may mean isolating the low side of an input connector with

a small resistance to the ground plane.

Basic Connections

Figure 30 shows the simple connections suitable for many appli-

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

INP VPS ENB INT

INM COM OFS OUT

AD8307

INPUT

–75dBm TO

+16dBm

1.1k

4.7

NC

NC

0.1 F

AT

8mA

OUTPUT

25mV/dB

NC = NO CONNECT

Figure 30. Basic Connections

Where it is necessary to terminate the source at a low imped-

shunting effect of the basic 1.1 k

Ω input resistance (R

AD8307. For example, to terminate a 50

Ω source a 52.3 Ω 1%

tolerance resistor should be used. This may 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 fre-

directly at the log-amp inputs.

Figure 31 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 32 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. But if the termi-

nation 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.

INPUT LEVEL – dBm

3

2.5

0

–80

20

–60

–40

–20

0

2

1.5

1

0.5

500MHz

–70

–50

–30

–10

10

100MHz

10MHz

Figure 31. Log Response at 10 MHz, 100 MHz and 500 MHz

INPUT LEVEL – dBm

5

4

–5

–80

20

–60

–40

–20

0

3

2

1

–4

500MHz

–70

–50

–30

–10

10

100MHz

10MHz

–3

–2

–1

0

Figure 32. 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 trans-

formation also eliminates the need for coupling capacitors,

lowers the offset voltage generated directly at the input, and

balances the drives to INP and INM. The choice of turns ratio

will depend 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 will lower the input imped-

ance to 50

Ω while raising the input voltage, and thus lowering

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

There will be a small contribution from the input noise current,

so the total noise will be reduced by a somewhat smaller factor.

The intercept will also be lowered by the turns ratio; for a

50

Ω match, it will be reduced by 20 log