AD8319

Rev. A | Page 10 of 20

THEORY OF OPERATION

The AD8319 is a 5-stage demodulating logarithmic amplifier,

specifically designed for use in RF measurement and power

control applications at frequencies up to 10 GHz. A block

diagram is shown in Figure 21. Sharing much of its design with

the AD8318 logarithmic detector/controller, the AD8319

maintains tight intercept variability vs. temperature over a

40 dB range. Additional enhancements over the AD8318, such

as reduced RF burst response time of 8 ns to 10 ns, 22 mA

supply current, and board space requirements of only 2 mm ×

3 mm add to the low cost and high performance benefits found

in the AD8319.

GAIN

BIAS

SLOPE

DET

DET

DET

DET

INHI

INLO

IV

VOUT

VSET

CLPF

TADJ

VPSO

COMM

IV

Figure 21. Block Diagram

A fully differential design, using a proprietary, high speed

SiGe process, extends high frequency performance. Input INHI

receives the signal with a low frequency impedance of nominally

500 Ω in parallel with 0.7 pF. The maximum input with ±1 dB

log conformance error is typically 0 dBm (re: 50 Ω). The noise

spectral density referred to the input is 1.15 nV/√Hz, which is

equivalent to a voltage of 118 μV rms in a 10.5 GHz bandwidth

or a noise power of −66 dBm (re: 50 Ω). This noise spectral

density sets the lower limit of the dynamic range. However, the

low end accuracy of the AD8319 is enhanced by specially

shaping the demodulating transfer characteristic to partially

compensate for errors due to internal noise. The common pin,

COMM, provides a quality low impedance connection to the

printed circuit board (PCB) ground. The package paddle, which

is internally connected to the COMM pin, should also be

grounded to the PCB to reduce thermal impedance from the

die to the PCB.

The logarithmic function is approximated in a piecewise

fashion by five cascaded gain stages. (For a more comprehensive

explanation of the logarithm approximation, refer to the AD8307

data sheet.) The cells have a nominal voltage gain of 9 dB each

and a 3 dB bandwidth of 10.5 GHz. Using precision biasing, the

gain is stabilized over temperature and supply variations. The

overall dc gain is high due to the cascaded nature of the gain

stages. An offset compensation loop is included to correct for

offsets within the cascaded cells. At the output of each of the

gain stages, a square-law detector cell is used to rectify the signal.

The RF signal voltages are converted to a fluctuating differential

current having an average value that increases with signal level.

Along with the five gain stages and detector cells, an additional

detector is included at the input of the AD8319, providing a

40 dB dynamic range in total. After the detector currents are

summed and filtered, the following function is formed at the

summing node

(1)

where:

the output voltage would be 0 V, if it were capable of going to 0 V).