AD8362 datasheet(23/32 Pages) AD

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Data Sheet

AD8362

Rev. E | Page 23 of 32

Moderately low resistance values should be used to minimize

scaling errors due to the 70 kΩ input resistance at the VSET

pin. This resistor string also loads the output, and it eventually

reduces the load-driving capabilities if very low values are used.

To calculate the resistor values, use

R1 = R2' (SD/50 − 1)

(15)

where:

SD is the desired slope, expressed in mV/dB.

R2' is the value of R2 in parallel with 70 kΩ.

For example, using R1 = 1.65 kΩ and R2 = 1.69 kΩ (R2' =

1.649 kΩ), the nominal slope is increased to 100 mV/dB.

Note, however, that doubling the slope in this manner reduces

the maximum input signal to approximately −10 dBm because

of the limited swing of VOUT (4.9 V with a 5 V power supply).

TEMPERATURE COMPENSATION AND REDUCTION

OF TRANSFER FUNCTION RIPPLE

The transfer function ripple and intercept drift of the AD8362

can be reduced using two techniques detailed in Figure 57.

CLPF is reduced from its nominal value. For broadband-

modulated input signals, this results in increased noise at

the output that is fed back to the VSET pin.

The noise contained in this signal causes the gain of the VGA

to fluctuate around a central point, moving the wiper of the

Gaussian Interpolator back and forth on the R-2R ladder.

Because the gain-control voltage is constantly moving across

at least one of taps of the Gaussian Interpolator, the relationship

between the rms signal strength of the VGA output and the

VGA control voltage becomes independent of the VGA gain

control ripple (see Figure 56). The signal being applied to the

squaring cell is now lightly AM modulated. However, this does

not change the peak-to-average ratio of the signal.

–1

–2

ERROR (dB –40°C)

ERROR (dB +25°C)

ERROR (dB +85°C)

VOUT (+25°C)

VOUT (–40°C)

VOUT (+85°C)

PIN (dBm)

–60

–40

–50

–30

–20

–10

4.0

3.5

2.5

3.0

2.0

0.5

1.5

1.0

Figure 56. Transfer Function and Linearity with Combined Ripple Reduction

and Temperature Compensation Circuits, Frequency = 2.14 GHz,

Single-Carrier W-CDMA, Test Model 1-64

Because of the reduced filter capacitor, the rms voltage appearing

at the output of the error amplifier now contains significant

peak-to-peak noise. While it is critical to feed this signal back

to the VGA gain control input with the noise intact, the rms

voltage going to the external measurement node can be filtered

using a simple filter to yield a largely noise-free rms voltage.

The circuit shown in Figure 57 also incorporates a temperature

sensor that compensates temperature drift of the intercept.

Because the temperature drift varies with frequency, the amount

of compensation required must also be varied using R1 and R2.

These compensation techniques are discussed in more detail in

AN-653 Application Note, Improving Temperature, Stability, and

Linearity of High Dynamic Range RMS RF Power Detectors.

0.1µF

FREQUENCY (MHz)

R1 (kΩ)

R2 (kΩ)

900

1.02

25.5

1900

82.5

2200

19.1

VPOS

VSET

VOUT

AD8031

VREF

VTGT

CLPF

440pF

ACOM

1ADDITIONAL PINS

OMITTED FOR CLARITY.

VOUT_COMP

1µF

0.1µF

1nF

TMP36F

0.1µF

VTEMP

COMM

AD83621

1kΩ

Figure 57. Temperature Compensation and Reduction of Transfer Function Ripple

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