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Data Sheet

AD8362

Rev. E | Page 17 of 32

loop to settle. Because the scaling parameters of the two

squarers are accurately matched, it follows that Equation 4

is satisfied only when

MEAN(VSIG2) = VATG2

(5)

In a formal solution, extract the square root of both sides to

provide an explicit value for the root-mean-square (rms) value.

However, it is apparent that by forcing this identity through

varying the VGA gain and extracting the mean value by the

filter provided by the capacitor(s), the system inherently

establishes the relationship

rms(VSIG) = VATG

(6)

Substituting the value of VSIG from Equation 3,

rms[GOVIN exp(−VSET/VGNS)] = VATG

(7)

As a measurement device, VIN is the unknown quantity and all

other parameters can be fixed by design. To solve Equation 7,

rms[GO

VIN/VATG] = exp(VSET/VGNS)

(8)

therefore,

VSET = VGNS log[rms(VIN)/VZ]

(9)

The quantity VZ = VATG/GO is defined as the intercept voltage

because VSET must be 0 when rms (VIN) = VZ.

When connected as a measurement device, the output of the

buffer is tied directly to VSET, which closes the AGC loop.

Making the substitution VOUT = VSET and changing the

log base to 10, as needed in a decibel conversion,

VOUT = VSLP log10[rms(VIN)/VZ]

(10)

where VSLP is the slope voltage, that is, the change in output

voltage for each decade of change in the input amplitude.

Note that VSLP = VGNS log (10) = 2.303 VGNS.

In the AD8362, VSLP is laser-trimmed to 1 V using a 100 MHz

test signal. Because a decade corresponds to 20 dB, this slope

can also be stated as 50 mV/dB. The Altering the Slope section

explains how the effective value of VSLP can be altered by the

user. The intercept, VZ, is also laser-trimmed to 224 µV (−60 dBm

relative to 50 Ω). In an ideal system, VOUT would cross zero

for an rms input of that value. In a single-supply realization of

the function, VOUT cannot run fully down to ground; here, VZ

is the extrapolated value.

VOLTAGE VS. POWER CALIBRATION

The AD8362 can be used as an accurate rms voltmeter from

arbitrarily low frequencies to microwave frequencies. For low

frequency operation, the input is usually specified either in

volts rms or in dBV (decibels relative to 1 V rms).

At high frequencies, signal levels are commonly specified in

power terms. In these circumstances, the source and termina-

tion impedances are an essential part of the overall scaling. For

this condition, the output voltage can be expressed as

VOUT = SLOPE × (PIN − PZ)

(11)

where PIN and the intercept PZ are expressed in dBm.

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 45. Output Voltage and Law Conformance Error

@ TA = −40°C, +25°C, and +85°C

Figure 45 shows the output of the circuit of Figure 47 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 for the +25°C data.

The error at −40°C, +25°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 45).

The error curves generated in this way reveal not only the devia-

tions from the ideal transfer function at a nominal temperature,

but also the additional errors caused by temperature changes.

Notice that there is a small temperature dependence in the

intercept (the vertical position of the error plots).

Figure 45 further reveals 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 attenua-

tion values. The selected signal is then applied to the 3.5 GHz,

40 dB fixed gain amplifier in the remaining stages of the VGA

of the AD8362.

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