
ADL6010 Data Sheet
Rev. E | Page 16 of 23
THEORY OF OPERATION
The ADL6010 uses eight Schottky diodes in a novel two path
detector topology. One path responds during the positive half
cycles of the input, and the second responds during the negative
half cycles of the input, thus achieving full wave rectification. This
arrangement presents a constant input impedance throughout
the full RF cycle, thereby preventing the reflection of even-
order distortion components back toward the source, which is a
well-known limitation of the widely used traditional single
Schottky diode detectors.
Eight diodes are arranged on the chip in such a way as to minimize
the effect of chip stresses and temperature variations. They are
biased by small keep alive currents chosen in a trade-off between
the inherently low sensitivity of a diode detector and the need to
preserve envelope bandwidth. Thus, the corner frequency of the
front-end low-pass filtering is a weak function of the input
level. At low input levels, the −3 dB corner frequency is at
approximately 0.5 GHz. The overall envelope bandwidth is
limited mainly by the subsequent linearizing and output
circuitry.
At small input levels, all Schottky diode detectors exhibit an
extremely weak response which approximates a square law
characteristic (having zero slope at the origin). For large inputs,
the response approaches a linear transfer function. In the
ADL6010, this nonlinearity and variations in the response are
corrected using proprietary circuitry having an equally shaped
but inverse amplitude function, resulting in an overall envelope
response that is linear across the whole span of input levels.
The composite signal is buffered and presented at the output
pin (VOUT). The transfer function relating the instantaneous
RF voltage amplitude to the quasi-dc output is a scalar constant
of a little over ×2. This scalar constant is mainly determined by
ratios of resistors, which are independent of temperature and
process variations. Errors associated with the minuscule voltages
generated by the Schottky front-end under low level conditions,
and other errors in the nonlinear signal processing circuitry, are
minimized by laser trimming, permitting accurate measurement
of RF input voltages down to the millivolts level. An aspect of
the linear in volts response is that the minimum VOUT is limited by
the ability of the output stage to reach down to absolute zero (the
potential on the COMM pin) when using a single positive supply.
DC voltages at the input are blocked by an on-chip capacitor.
The two ground pins (RFCM) on either side of RFIN (Pin 5)
form part of an RF coplanar waveguide (CPW) launch into
the detector. The RFCM pins must be connected to the signal
ground. Give careful attention to the design of the PCB in this
area.
The envelope voltage gain of the ADL6010 is nominally
×2.2 V/VPEAK from 1 GHz to 35 GHz. This factor becomes
3.2 V/V when the input signal is specified as the rms voltage of
a CW carrier. For example, a steady −30 dBm input generates a
dc output voltage of 22.5 mV, at which level the output buffer is
able to track the envelope. In fact, the sensitivity at ambient
temperatures typically extends below −30 dBm. However, over
the specified temperature range, the measurement error tends
to increase at the bottom of the specified range.
For large inputs, the voltage headroom in the signal processing
stages limits the measurement range. Using a 5 V supply, the
maximum signal is approximately 3.6 V p-p, corresponding to a
power of 15 dBm, referenced to 50 Ω. Therefore, the ADL6010
achieves a 45 dB dynamic range of high accuracy measurement.
Note that, above 43.5 GHz, accuracy is limited by the package,
PCB, and instrumentation. The RF input interface provides a
broadband (flat) 50 Ω termination without the need for
external components. Although the input return loss inevitably
degrades at very high frequencies, the slope of the transfer
function holds near 2.2 V/VPEAK up to 35 GHz, owing to the
voltage responding behavior of the ADL6010.