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AD8362

Rev. B | Page 22 of 36

OPERATION IN MEASUREMENT MODES

Figure 50 shows the general connections for operating the

AD8362 as an RF power detector, more correctly viewed

as an accurate measurement system. The full performance

potential of this part, particularly at very high frequencies

(above 500 MHz), is realized only when the input is presented

to the AD8362 in differential (balanced) form. In this example, a

flux-coupled transformer is used at the input. Having a 1:4

impedance ratio (1:2 turns ratio), the 200 Ω differential input

resistance of the AD8362 becomes 50 Ω at the input to the

transformer, whose outputs can be connected directly to INHI

and INLO. If a center-tapped transformer is used, connect the

tap to the DECL pins, which are biased to the same potential as

the inputs (~3.6 V). Over the 0.9 GHz to 2.2 GHz range, a

transmission line transformer (balun) may be used, as explained

later. (The evaluation board is supplied with a M/A-COM

ETC1.6-4-2-3, 0.5 GHz to 2.5 GHz, 4:1 balun.)

COMM

CHPF

DECL

INHI

INLO

DECL

PWDN

COMM

ACOM

VREF

VTGT

VPOS

VOUT

VSET

ACOM

CLPF

1nF

VOUT RAIL-TO-RAIL

CONTROL OUTPUT

AD8362

SIGNAL INPUT

Z = 50

Ω

1nF

300pF

+5V nom, @ 24mA

0.1

µF

3.3

Ω

200

Ω

1nF

1:4 Z-RATIO

(1:2 TURNS RATIO)

Figure 50. Connections for RF Power Measurement

The output in this mode of use is a continuous, decibel-scaled

voltage ranging from about 0.5 V to 3.5 V.

(

)

VOUT

(11)

The equivalent input power, PIN, is expressed in dBm (decibels

above 1 mW) in a particular system impedance, which in this

case is 50 Ω. The intercept, PZ, is that input power for which

the back-extrapolated output crosses zero. Expressed as a

voltage, it is 0.447 mV rms (−67 dBV, laser-calibrated at

100 MHz), corresponding to a PZ of −60 dBm in 200 Ω.

However, the 1:2 turns ratio of the transformer halves the

required input voltage, which moves the intercept down by

6 dB to 0.224 mV rms (−73 dBV) at the transformer’s input.

Impedance mismatches and attenuation in the coupling

elements significantly affect the intercept position. This error

is stable over temperature and time, and thus can be removed

during calibration in a specific system. The logarithmic slope of

50 mV/dB varies only slightly with frequency; corrected values

for several common frequencies are provided in the

Specifications section.

LAW CONFORMANCE ERROR

In practice, the response deviates slightly from the ideal straight

line suggested by Equation 11. This deviation is called the law

conformance error. In defining the performance of high

accuracy measurement devices, it is customary to provide plots

of this error. In general terms, it is computed by extracting the

best straight line to the measured data using linear regression

over a substantial region of the dynamic range and under

clearly specified conditions.

INPUT AMPLITUDE (dBm)

0.2

0.8

1.1

1.4

1.7

2.0

2.3

2.9

3.5

0.5

3.8

–3.0

–1.5

–1.0

–0.5

0.5

1.0

1.5

2.0

2.5

–2.0

–2.5

3.0

2.6

3.2

–60 –55 –50 –45 –40 –35 –30 –25 –20 –15

–5

10 15

–10

–40°C

+25°C

+85°C

+25°C

+85°C

–40°C

Figure 51. Output Voltage and Law Conformance Error,

at TA = −40°C, +25°C, and +85°C

Figure 51 shows the output of the circuit of Figure 50 over the

full input range. The agreement with the ideal function (law

conformance) is also shown. This was determined by linear

regression on the data points over the central portion of the

transfer function (35 mV to 250 mV rms) for the 25°C data.

The error at +25°C, −40°C, and +85°C was then calculated by

subtracting the ideal output voltage at each input signal level

from the actual output and dividing this quantity by the mean

slope of the regression equation to provide a measurement of

the error in decibels (scaled on the right-hand axis of Figure 51).

The error curves generated in this way reveal not only the

deviations from the ideal transfer function at a nominal

temperature but also all of the additional errors caused by

temperature changes. Notice there is a small temperature

dependence in the intercept (the vertical position of the error

plots); this variation is within 0.5 dB at high powers.

Figure 51 further reveals that there is a periodic ripple in the

conformance curves. This is due to the interpolation technique

used to select the signals from the attenuator, not only at

discrete tap points, but anywhere in between, thus providing

continuous attenuation values. The selected signal is then

applied to the 3.5 GHz, 40 dB fixed gain amplifier in the

remaining stages of the AD8362’s VGA.

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