AD641APZ データシートの表示（PDF） - Analog Devices

部品番号

コンポーネント説明

メーカー

AD641APZ

250 MHz Demodulating Logarithmic Amplifier

Analog Devices 

AD641

FUNDAMENTALS OF LOGARITHMIC CONVERSION

The conversion of a signal to its equivalent logarithmic value

involves a nonlinear operation, the consequences of which can be

very confusing if not fully understood. It is important to realize

from the outset that many of the familiar concepts of linear

circuits are of little relevance in this context. For example, the

incremental gain of an ideal logarithmic converter approaches

infinity as the input approaches zero. Further, an offset at the

output of a linear amplifier is simply equivalent to an offset at

the input, while in a logarithmic converter it is equivalent to a

change of amplitude at the input—a very different relationship.

We assume a dc signal in the following discussion to simplify the

concepts; ac behavior and the effect of input waveform on cali-

bration are discussed later. A logarithmic converter having a

voltage input VIN and output VOUT must satisfy a transfer func-

tion of the form

VOUT = VY LOG (VIN/VX)

Equation (1)

where VY and VX are fixed voltages which determine the scaling

of the converter. The input is divided by a voltage because the

argument of a logarithm has to be a simple ratio. The logarithm

must be multiplied by a voltage to develop a voltage output.

These operations are not, of course, carried out by explicit com-

putational elements, but are inherent in the behavior of the

converter. For stable operation, VX and VY must be based on

sound design criteria and rendered stable over wide temperature

and supply voltage extremes. This aspect of RF logarithmic

amplifier design has traditionally received little attention.

When VIN = VX, the logarithm is zero. VX is, therefore, called

the Intercept Voltage, because a graph of VOUT versus LOG

(VIN)—ideally a straight line—crosses the horizontal axis at this

point (see Figure 20). For the AD641, VX is calibrated to ex-

actly 1 mV. The slope of the line is directly proportional to VY.

Base 10 logarithms are used in this context to simplify the rela-

tionship to decibel values. For VIN = 10 VX, the logarithm has a

value of 1, so the output voltage is VY. At VIN = 100 VX, the

output is 2 VY, and so on. VY can therefore be viewed either as

the Slope Voltage or as the Volts per Decade Factor.

The AD641 conforms to Equation (1) except that its two out-

puts are in the form of currents, rather than voltages:

IOUT = IY LOG (VIN/VX)

Equation (2)

VYLOG (VIN/VX)

2VY

IDEAL

ACTUAL

SLOPE = VY

YY

+

0

ACTUAL

–

VIN = VX

IDEAL

VIN = 10VX

VIN = 100VX

INPUT ON

LOG SCALE

Figure 20. Basic DC Transfer Function of the AD641

IY, the Slope Current, is 1 mA. The current output can readily

be converted to a voltage with a slope of 1 V/decade, for ex-

ample, using one of the 1 kΩ resistors provided for this purpose,

in conjunction with an op amp, as shown in Figure 21.

1mA PER DECADE

R2

R1

48.7⍀

C1

330pF

15 14 13 12 11

LOG LOG +VS SIG

OUT COM

+OUT

AD641

SIG

–VS ITC BL2 –OUT

6 7 8 9 10

AD846

OUTPUT VOLTAGE

1V PER DECADE

FOR R2 = 1k⍀

100mV PER dB

FOR R2 = 2k⍀

Figure 21. Using an External Op Amp to Convert the

AD641 Output Current to a Buffered Voltage Output

Intercept Stabilization

Internally, the intercept voltage is a fraction of the thermal volt-

age kT/q, that is, VX = VXOT/TO, where VXO is the value of VX

at a reference temperature TO. So the uncorrected transfer

function has the form:

IOUT = IY LOG (VIN TO/VXOT)

Equation (3)

Now, if the amplitude of the signal input VIN could somehow be

rendered PTAT, the intercept would be stable with tempera-

ture, since the temperature dependence in both the numerator

and denominator of the logarithmic argument would cancel.

This is what is actually achieved by interposing the on-chip

attenuator, which has the necessary temperature dependence to

cause the input to the first stage to vary in proportion to abso-

lute temperature. The end limits of the dynamic range are now

totally independent of temperature. Consequently, this is the pre-

ferred method of intercept stabilization for applications where

the input signal is sufficiently large.

When the attenuator is not used, the PTAT variation in VX will

result in the intercept being temperature dependent. Near 300K

(+27°C) it will vary by 20 LOG (301/300) dB/°C, about 0.03 dB/

°C. Unless corrected, the whole output function would drift up

or down by this amount with changes in temperature. In the

AD641 a temperature compensating current IYLOG(T/TO) is

added to the output. This effectively maintains a constant inter-

cept VXO. This correction is active in the default state (Pin 8

open circuited). When using the attenuator, Pin 8 should be

grounded, which disables the compensation current. The drift

term needs to be compensated only once; when the outputs of

two AD641s are summed, Pin 8 should be grounded on at least

one of the two devices (both if the attenuator is used).

Conversion Range

Practical logarithmic converters have an upper and lower limit

on the input, beyond which errors increase rapidly. The upper

limit occurs when the first stage in the chain is driven into limit-

ing. Above this, no further increase in the output can occur and

the transfer function flattens off. The lower limit arises because

a finite number of stages provide finite gain, and therefore at

low signal levels the system becomes a simple linear amplifier.

–8–

REV. D

Share Link: