adl5504 Analog Devices, Inc., adl5504 Datasheet - Page 13

• Manufacturer Part Number: adl5504
• Description: 450 Mhz To 6000 Mhz Trupwr Detector
• Manufacturer: Analog Devices, Inc.

CIRCUIT DESCRIPTION
The ADL5504 employs two-stage detection. The critical aspect
of this technical approach is the concept of first stripping the
carrier to reveal the envelope and then performing the required
analog computation of rms.
RMS CIRCUIT DESCRIPTION AND FILTERING
The rms processing is executed using a proprietary translinear
technique. This method is a mathematically accurate rms
computing approach and allows achieving unprecedented rms
accuracies for complex modulation signals irrespective of the
crest factor of the input signal. An integrating filter capacitor
performs the square-domain averaging. The VRMS output can
be expressed as
Note that A is a scaling parameter that is determined by the
on-chip resistor ratio, and there are no other scaling parameters
involved in this computation, which means that the rms output
is inherently free from any sources of error due to temperature,
supply, and process variations.
FILTERING
An important aspect of rms-dc conversion is the need for
averaging (the function is root-mean-square). The on-chip
averaging in the square domain has a corner frequency of
approximately 40 kHz and is sufficient for common modulation
signals, such as CDMA-, CDMA2000-, WCDMA-, and QPSK-/
QAM-based OFDM (for example, LTE, WLAN, and WiMAX).
VRMS
=
A
×
T2
T1
∫
T2
V
IN
2
−
×
T1
dt
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For improved accuracy with more complex RF waveforms
(with modulation components extending down into the
kilohertz region), more filtering is necessary to supplement
the on-chip, low-pass filter. For this reason, the FLTR pin is
provided; a capacitor attached between this pin and VPOS
can extend the averaging time to very low frequencies (see
the Selecting the Square-Domain Filter and Output Low-Pass
Filter section). Any external capacitor acts on a 1 kΩ resistor to
yield a new corner frequency for the rms filter (see Figure 1).
Adequate filtering ensures the accuracy of the rms measurement;
however, some ripple or ac residual can still be present on the
dc output. To reduce this ripple, an external shunt capacitor can
be used at the output to form a low-pass filter with the on-chip,
100 Ω resistance (see the Selecting the Square-Domain Filter
and Output Low-Pass Filter section).
OUTPUT BUFFER
A buffer takes the internal rms signal and amplifies it accor-
dingly before it is output on the VRMS pin. The output stage
of the rms buffer is a common source PMOS with a resistive
load to provide a rail-to-rail output. The buffer has a 100 Ω
on-chip series resistance on the output, allowing for easy low-
pass filtering.
ADL5504