AD8317

Rev. 0 | Page 11 of 20

THEORY OF OPERATION

The AD8317 is a 6-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 AD8317 maintains tight inter-

cept variability vs. temperature over a 50 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 x 3 mm, add to the low cost

and high performance benefits found in the AD8317.

GAIN

BIAS

SLOPE

DET

DET

DET

DET

INHI

INLO

IV

VOUT

VSET

CLPF

TADJ

VPOS

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 AD8317 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 six cascaded gain stages. (For a more comprehensive

explanation of the logarithm approximation, please refer to the

AD8307 data sheet, available at www.analog.com.) 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 six gain stages and detector cells, an additional

detector is included at the input of the AD8317, providing a 50 dB

dynamic range in total. After the detector currents are summed

and filtered, the following function is formed at the summing node:

where:

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