AD8361

Rev. C | Page 11 of 24

CIRCUIT DESCRIPTION

The AD8361 is an rms-responding (mean power) detector that

provides an approach to the exact measurement of RF power

that is basically independent of waveform. It achieves this

function through the use of 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, which presents a nominal (LF) resistance of

225 Ω between the RFIN and COMM pins (connected to the

ground plane). Because the input pin is at a bias voltage of about

0.8 V above ground, a coupling capacitor is required. By making

this an external component, the measurement range may be

extended to arbitrarily low frequencies.

squared. This is applied to an internal load resistor, across which

a capacitor is connected. These form a low-pass filter, which

responding, the associated input impedance calibrates this port

in terms of equivalent power. Therefore, 1 mW corresponds to a

voltage input of 447 mV rms. The Applications section shows

how to match this input to 50 Ω.

The voltage across the low-pass filter, whose frequency may 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 AD8361. When the voltage at the input of the second

stable state, and the output then represents the rms value of the

input. The feedback ratio is nominally 0.133, making the rms-dc

conversion gain ×7.5, that is

rms

V

V

IN

OUT

×

= 5

.

7

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;

thus, the overall calibration may 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 fairly 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 AD8361 have a

Class-AB aspect; the peak input is not limited by their 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 fairly 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 less than 100 mV below the

supply. An example of the output induced limit: given a gain of

7.5 and assuming a maximum output of 2.9 V with a 3 V supply,

the maximum input is (2.9 V rms)/7.5 or 390 mV rms.

Filtering

An important aspect of rms-dc conversion is the need for

averaging (the function is root-MEAN-square). For complex RF

waveforms, such as those that occur in CDMA, the filtering

provided by the on-chip, low-pass filter, although satisfactory

for CW signals above 100 MHz, is inadequate when the signal

has modulation components that extend down into the

kilohertz region. 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.

Offset

An offset voltage can be added to the output (when using the

MSOP version) to allow the use of ADCs whose range does not

extend down to ground. However, accuracy at the low end

degrades because of the inherent error in this added voltage.

This requires that the IREF (internal reference) pin be tied to

VPOS and SREF (supply reference) to ground.

In the IREF mode, the intercept is generated by an internal

reference cell and is a fixed 350 mV, independent of the supply

voltage. To enable this intercept, IREF should be open-circuited,

and SREF should be grounded.

In the SREF mode, the voltage is provided by the supply. To

implement this mode, tie IREF to VPOS and SREF to VPOS.

The offset is then proportional to the supply voltage and is

400 mV for a 3 V supply and 667 mV for a 5 V supply.