AD8361

–11–

REV. A

INPUT – mV

700

200

0

500

100

200

300

400

500

400

300

600

100

0

600

700

800

Figure 37. Idealized Output Step Size as Function of Input

Voltage

Plots of output voltage vs. input voltage result in a straight line. It

may sometimes be more useful to plot the error on a logarith-

mic scale, as shown in Figure 38. The deviation of the plot for

the ideal straight line characteristic is caused by output clipping

at the high end and by signal offsets at the low end. It should

however be noted that offsets at the low end can be either posi-

tive or negative, so that this plot could also trend upwards at the

low end. Figures 5, 6, 8, and 9 show a

±3 sigma distribution of

device error for a large population of devices.

INPUT – V rms

2.0

–0.5

0.01

0.5

0.0

1.5

1.0

–1.0

–1.5

–2.0

1.0

1.9GHz

2.5GHz

900MHz

100MHz

100MHz

0.02

(–21dBm)

0.1

(–7dBm)

0.4

(+5dBm)

Figure 38. Representative Unit, Error in dB vs. Input Level,

It is also apparent in Figure 38 that the error plot tends to

shift to the right with increasing frequency. Because the input

impedance decreases with frequency, the voltage actually applied

to the input will also tend to decrease (assuming a constant source

impedance over frequency). The dynamic range is almost con-

stant over frequency, but with a small decrease in conversion gain

at high frequency.

Input Coupling and Matching

The input impedance of the AD8361 decreases with increasing

frequency in both its resistive and capacitive components (Figure

13). The resistive component varies from 225

Ω at 100 MHz

down to about 95

Ω at 2.5 GHz.

A number of options exist for input matching. For operation at

multiple frequencies, a 75

Ω shunt to ground, as shown in Figure

39a, will provide the best overall match. For use at a single fre-

quency, a resistive or a reactive match can be used. By plotting the

input impedance on a Smith Chart, the best value for a

resistive match can be calculated. The VSWR can be held below

1.5 at frequencies up to 1 GHz, even as the input impedance

varies from part to part. (Both input impedance and input

capacitance can vary by up to

±20% around their nominal values.)

At very high frequencies (i.e., 1.8 GHz to 2.5 GHz), a shunt

resistor will not be sufficient to reduce the VSWR below 1.5.

Where VSWR is critical, remove shunt component and insert

an inductor in series with the coupling capacitor as shown in

Figure 39b.

Table II gives recommended shunt resistor values for various

frequencies and series inductor values for high frequencies. The

no intentional part in the matching.

AD8361

RFIN

RFIN

a. Broadband Resistor Match

AD8361

RFIN

RFIN

b. Series Inductor Match

AD8361

RFIN

RFIN

c. Narrowband Reactive Match

AD8361

RFIN

RFIN

d. Attenuating the Input Signal

Figure 39. Input Coupling/Matching Options

Table II. Recommended Component Values for Resistive or

Inductive Input Matching (Figures 39a and 39b)

Frequency

Matching Component

100 MHz

63.4

Ω Shunt

800 MHz

75

Ω Shunt

900 MHz

75

Ω Shunt

1800 MHz

150

Ω Shunt or 4.7 nH Series

1900 MHz

150

Ω Shunt or 4.7 nH Series

2500 MHz

150

Ω Shunt or 2.7 nH Series