ADL5505ACBZ-P2 AD [Analog Devices], ADL5505ACBZ-P2 Datasheet - Page 13

• Manufacturer Part Number: ADL5505ACBZ-P2
• Description: 450 MHz to 6000Mhz TruPwr Detector
• Manufacturer: AD [Analog Devices]

CIRCUIT DESCRIPTION
The ADL5505 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 for achieving unprecedented
rms accuracies for complex modulation signals irrespective of
the crest factor of the input signal. An integrating filter capaci-
tor performs the square-domain averaging. The VRMS output
can be expressed as
where A is a scaling parameter that is determined by the on-chip
resistor ratio.
There are no other scaling parameters involved in this computa-
tion, which means that the rms output is inherently free from
any sources of error due to temperature, supply, and process
variations.
VRMS
=
A
×
T2
T1
∫
T2
V
IN
2
−
×
T1
dt
Rev. 0 | Page 13 of 20
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 180 kHz. and is sufficient for common modu-
lation signals, such as CDMA-, CDMA2000-, W-CDMA-, and
QPSK-/QAM-based OFDM (for example, LTE, WLAN, and
WiMAX).
Adequate filtering ensures the accuracy of the rms measure-
ment; 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 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.
ADL5505