Figure 5

. A 50 Ω resistor is added to the output to prevent package resonance.

Although power is the traditional metric used in the analysis of cascaded systems, most active circuit blocks

fundamentally respond to voltage. The relationship between power and voltage is defined by the impedance level.

When input and output impedance levels are the same, power gain and voltage gain are identical. However, when

impedance levels change between input and output, they differ. Thus, one must be very careful to use the appropriate

gain for system chain analyses. Quantiti

es such as OIP3 are quoted in dBVrms as well as dBm reference to 200 Ω

and dBm reference to 50 Ω. The dBVrms unit is defined as decibels relative to 1 Vrms. In a 200 Ω environment, the

conversion from dBVrms to dBm

referenced to 200 Ω requires the addition of 7dB to the dBVrms value. For example,

a 2 dBVrms

level corresponds to 9 dBm relative to 200 Ω. The conversion from dBVrms to dBm referenced to 50 Ω

50 Ω. Another perhaps simpler way to express the conversion between these different units is given in Equation 3:

Equation 3:

dBm re 50 = dBm re 200 + 6 = dBVrms + 13.

Since the AD8367S consists of a passive variable attenuator followed by a fixed gain amplifier, the noise and

distortion characteristics as a function of the gain voltage are easily predicted. The input-referred noise increases in

The minimum NF of 7.5 dB occurs at maximum gain and increases 1 dB for every 1 dB reduction in gain. In receiver

applications, the minimum NF should occur at the maximum gain where the received signal presumably is weakest. At

higher signal levels, a lower gain is needed, and the increased NF becomes less important.

The input-referred distortion varies in a similar manner to the noise. Figure 6 illustrates how the third-order intercept

. The highest IIP3 of 20 dBVrms (27 dBm re 200 Ω) occurs at

minimum gain. The IIP3 then decreases 1 dB for 1 dB increase in gain. At lower gain levels, a degraded IIP3 is

acceptable. Overall, the dynamic range, represented by the difference between IIP3 and NF, remains reasonably

constant as a function of gain. The output distortion and compression are essentially independent of the gain. At low

gains, when the input level is high, input overload can occur, causing premature distortion.