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ADL5501

Rev. B | Page 17 of

CIRCUIT DESCRIPTION

The ADL5501 is an rms-responding (mean power) detector that

provides an approach to the exact measurement of RF power that

is independent of waveform. It achieves this function by using

a proprietary technique in which the outputs of two identical

squaring cells are balanced by the action of a high gain error

amplifier.

The signal to be measured is applied to the input of the first

squaring cell through the input matching network. The input

is matched to offer a broadband 50 Ω input impedance from

50 MHz to 6 GHz. The input matching network has a high-pass

corner frequency of approximately 70 MHz.

The ADL5501 responds to the voltage, VIN, at its input by squaring

this voltage to generate a current proportional to VIN2. This current

is applied to an internal load resistor in parallel with a capacitor,

followed by a low-pass filter, which extracts the mean of VIN2.

Although essentially voltage responding, the associated input

impedance calibrates this port in terms of equivalent power.

Therefore, 1 mW corresponds to a voltage input of 224 mV rms

referenced to 50 Ω. Because both the squaring cell input impedance

and the input matching network are frequency dependent, the

conversion gain is a function of signal frequency.

The voltage across the low-pass filter, whose frequency can

be arbitrarily low, is applied to one input of an error-sensing

amplifier. A second identical voltage-squaring cell is used to

close a negative feedback loop around this error amplifier. This

second cell is driven by a fraction of the quasi-dc output voltage

of the ADL5501. When the voltage at the input of the second

squaring cell is equal to the rms value of VIN, the loop is in a stable

state, and the output then represents the rms value of the input.

By completing the feedback path through a second squaring

cell, identical to the one receiving the signal to be measured,

several benefits arise. First, scaling effects in these cells cancel;

therefore, the overall calibration can be accurate, even though

the open-loop response of the squaring cells taken separately

need not be. Note that in implementing rms-dc conversion, no

reference voltage enters into the closed-loop scaling. Second,

the tracking in the responses of the dual cells remains very close

over temperature, leading to excellent stability of calibration.

The squaring cells have very wide bandwidth with an intrinsic

response from dc to microwave. However, the dynamic range of

such a system is small, due in part to the much larger dynamic

range at the output of the squaring cells. There are practical

limitations to the accuracy of sensing very small error signals at

the bottom end of the dynamic range, arising from small random

offsets that limit the attainable accuracy at small inputs.

On the other hand, the squaring cells in the ADL5501 have a

Class AB aspect; the peak input is not limited by its quiescent

bias condition but is determined mainly by the eventual loss of

square-law conformance. Consequently, the top end of their

response range occurs at a large input level (approximately

700 mV rms), while preserving a reasonably accurate square-law

response. The maximum usable range is, in practice, limited by

the output swing. The rail-to-rail output stage can swing from a

few millivolts above ground to within 100 mV below the supply.

An example of the output induced limit, given a conversion gain

of 6.3 V/V rms at 900 MHz and assuming a maximum output of

2.9 V with a 3 V supply, has a maximum input of 2.9 V rms/6.3 or

460 mV rms.

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 100 kHz and is sufficient for common modu-

lation signals, such as CDMA-, CDMA2000-, WCDMA-, and

QPSK-/QAM-based OFDM (for example, WLAN and WiMAX).

For 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.

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).

ADL5501AKSZ-R71 数据表(PDF) 17 Page - Analog Devices

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