ADL5902ACPZ-R2 datasheet(15/28 Pages) AD

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ADL5902

Rev. 0 | Page 15 of 28

THEORY OF OPERATION

The ADL5902 is a 50 MHz to 9 GHz true rms responding

detector with a 65 dB measurement range at 2.14 GHz and a

greater than 56 dB measurement range at frequencies up to

6 GHz. It incorporates a modified AD8362 architecture that

increases the frequency range and improves measurement

accuracy at high frequencies. Transfer function peak-to-peak

ripple has been reduced to <±0.1 dB over the entire dynamic

range. Temperature stability of the rms output measurements

provides <±0.3 dB error, typically, over the specified temperature

range of −40°C to 125°C through proprietary techniques. The

device accurately measures waveforms that have a high peak-to-

rms ratio (crest factor).

The ADL5902 consists of a high performance AGC loop. As

shown in Figure 36, the AGC loop comprises a wide bandwidth

variable gain amplifier (VGA), square law detectors, an amplitude

target circuit, and an output driver. For a more detailed description

of the functional blocks, see the AD8362 data sheet.

The nomenclature used in this data sheet to distinguish

between a pin name and the signal on that pin is as follows:

•

The pin name is all uppercase, for example, VPOS,

COMM, and VOUT.

•

The signal name or a value associated with that pin is the

pin mnemonic with a partial subscript, for example, CLPF

and VOUT.

SQUARE LAW DETECTOR AND AMPLITUDE TARGET

The VGA gain has the form

GSET = GO e

)

(

GNS

SET

−

(1)

where:

GO is the basic fixed gain.

VGNS is a scaling voltage that defines the gain slope (the decibel

change per voltage). The gain decreases with increasing VSET.

The VGA output is

VSIG = GSET × RFIN = GO × RFIN e

)

(

GNS

SET

−

(2)

where RFIN is the ac voltage applied to the input terminals of the

ADL5902.

The output of the VGA, VSIG, is applied to a wideband square

law detector. The detector provides the true rms response of the

RF input signal, independent of waveform. The detector output,

ISQR, is a fluctuating current with positive mean value. The

difference between ISQR and an internally generated current,

ITGT, is integrated by the parallel combination of CF and the

external capacitor attached to the CLPF pin at the summing

node. CF is an on-chip 26 pF filter capacitor, and CLPF, the

external capacitance connected to the CLPF pin, can be used to

arbitrarily increase the averaging time while trading off with the

response time. When the AGC loop is at equilibrium

Mean(ISQR) = ITGT

(3)

This equilibrium occurs only when

Mean(VSIG2) = VTGT2

(4)

where VTGT is the voltage presented at the VTGT pin. This pin

can conveniently be connected to the VREF pin through a voltage

divider to establish a target rms voltage, VATG, of ~40 mV rms when

VTGT = 0.8 V.

Because the square law detectors are electrically identical and

well matched, process and temperature dependent variations

are effectively cancelled.

TADJ/PWDN

BAND GAP

REFERENCE

COMM

VOUT

TEMP (1.4V)

VREF (2.3V)

ISQR

ITGT

GSET

CLPF

(EXTERNAL)

(INTERNAL)

VSIG

VGA

SUMMING

NODE

VSET

(INTERNAL)

VPOS

INHI

INLO

TEMPERATURE COMPENSATION

AND BIAS

TEMPERATURE

SENSOR

VTGT

CLPF

VATG =

VTGT

Figure 36. Simplified Architecture Details

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