|Description||1 MHz to 10 GHz, 45 dB Log Detector/Controller|
|AD8319ACPZ-R7 Datasheet PDF : 20 Pages |
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 x 3 mm add to the low cost
and high performance benefits found in the AD8319.
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, 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 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
ID × log10(VIN/VINTERCEPT)
ID is the internally set detector current.
VIN is the input signal voltage.
VINTERCEPT is the intercept voltage (that is, when VIN = V , INTERCEPT
the output voltage would be 0 V, if it were capable of going to 0 V).
Rev. 0 | Page 10 of 20
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