AD8362 datasheet(25/36 Pages) AD

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AD8362

Rev. B | Page 25 of 36

lowers the signal currents in the squaring cells by a factor of 25.

As well as making the system more sensitive to small static

errors (offsets) in the postdetection circuitry, such a reduction

also reduces the peak slew rate. A suitable adjustment to the

value of CLPF is needed to maintain a given AGC loop

bandwidth. On the other hand, increasing the target voltage can

improve the accuracy and stability of the intercept for low crest

factor signals. Thus, using VTGT = 2.5 V, the peak output

currents of the squaring cell are quadrupled and the peak slew

rate is increased by the same factor. CLPF should be increased

to maintain an adequate stability margin in the AGC loop.

In many applications, it is useful to use a nonstandard value of

VTGT to shift the measurement range by a constant amount to

accommodate either a reduced or increased range of signal

inputs. The dynamic span remains >60 dB for such changes.

This technique is particularly useful when the sensitivity can be

lowered by raising VTGT, and there is little expectation of high

crest factor signals.

ADJUSTING THE INTERCEPT

Another way to take advantage of the effect of VTGT is to use it

to introduce an adjustment to the log intercept, represented by

the voltage VZ in Equation 14. Formally, this can be expressed in

terms of a modified value of VZ'.

VTGT

(14)

A lower VTGT effectively increases the sensitivity of the

measurement system, which is just another way of stating that the

intercept moves to a lower value. This raises VOUT for all input

amplitudes, as demonstrated by the plots in Figure 45. This

control of the measurement system’s intercept could therefore be

brought about by applying the output of a DAC to the VTGT pin,

if that suits the overall objectives of an application.

For many purposes, a small manual adjustment range of ±3 dB

is sufficient. This can be implemented as shown in Figure 55.

Here, the largest fraction of VTGT is still provided by the built-

in reference to minimize the sensitivity to supply voltage

variations. Now a variable component is provided by the trim

network. For a 5 V supply, this added component of VTGT is 0

when VR1 is centered. With the slider closest to ground, VTGT

is lowered by 366 mV, which corresponds to a 3 dB decrease in

intercept; in the opposite condition, it is raised by 518 mV,

which increases the intercept by 3 dB. That is, VTGT ranges

from 1.25 V/√2 to √2 × 1.25 V.

Other adjustment ranges can be readily calculated from this

example. The resistance at the VTGT pin is nominally 52 kΩ;

resistor values should be calculated with this in mind. In some

situations, this control interface might be driven from a

programmable source. In the simplest case, a logic level could

provide two intercept values, differing by say, 10 dB, thus

providing essentially two switched input ranges.

Also, it is worth remembering that these shifts in intercept are

equivalent, in most respects, to a dc offset applied to the

AD8362’s output, with the main differences being that:

•

Varying VTGT affects the crest factor capacity to some

extent

•

This technique makes better use of the available output

range than a post-VOUT adjustment would

COMM

CHPF

DECL

INHI

INLO

DECL

PWDN

COMM

ACOM

VREF

VTGT

VPOS

VOUT

VSET

ACOM

CLPF

AD8362

4.02k

Ω

4.02k

Ω

5.75k

Ω

VR1

20k

Ω

Figure 55. Adjustments of the Intercept by ±3 dB

ALTERING THE SLOPE

None of the changes in operating conditions discussed so far

affect the logarithmic slope, VSLP, in Equation 9. However, this

can readily be altered by controlling the fraction of VOUT that

is fed back to the setpoint interface at the VSET pin. When the

full signal from VOUT is applied to VSET, the slope assumes its

nominal value of 50 mV/dB. It can be increased by including an

attenuator between these pins, as shown in Figure 56. Moder-

ately low resistance values should be used to minimize scaling

errors due to the 70 kΩ input resistance at the VSET pin. Keep

in mind that 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

(

50 −

(15)

where SD is the desired slope, expressed in mV/dB, and

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. This choice of scaling is useful

when the output is applied to a digital voltmeter because the

displayed number reads as a decibel quantity directly, with only

a decimal point shift.

COMM

CHPF

DECL

INHI

INLO

DECL

PWDN

COMM

ACOM

VREF

VTGT

VPOS

VOUT

VSET

ACOM

CLPF

AD8362

VOUT

Figure 56. External Network to Raise Slope

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