AD8307AR-REEL datasheet(8/20 Pages) AD

Numer części	AD8307AR-REEL

Szczegółowy opis	Low Cost DC-500 MHz, 92 dB Logarithmic Amplifier

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AD8307

–8–

REV. A

voltage. The use of dBV (decibels with respect to 1 V rms) would

be more precise, though still incomplete, since waveform is

involved, too. Since most users think about and specify RF

signals in terms of power—even more specifically, in dBm re 50

Ω

—we will use this convention in specifying the performance of

the AD8307.

Progressive Compression

Most high speed high dynamic range log amps use a cascade of

nonlinear amplifier cells (Figure 20) to generate the logarithmic

function from a series of contiguous segments, a type of piece-

wise-linear technique. This basic topology immediately opens

up the possibility of enormous gain-bandwidth products. For

example, the AD8307 employs six cells in its main signal path,

each having a small-signal gain of 14.3 dB (

×5.2) and a –3 dB

bandwidth of about 900 MHz; the overall gain is about 20,000

(86 dB) and the overall bandwidth of the chain is some 500 MHz,

resulting in the incredible gain-bandwidth product (GBW) of

10,000 GHz, about a million times that of a typical op amp.

This very high GBW is an essential prerequisite to accurate

operation under small-signal conditions and at high frequencies.

Equation 2 reminds us, however, that the incremental gain will

decrease rapidly as VIN increases. The AD8307 continues to

exhibit an essentially logarithmic response down to inputs as

small as 50

µV at 500 MHz.

A

VX

STAGE 1

STAGE 2

STAGE N –1

STAGE N

VW

A

Figure 20. Cascade of Nonlinear Gain Cells

To develop the theory, we will first consider a slightly different

scheme to that employed in the AD8307, but which is simpler

to explain and mathematically more straightforward to analyze.

This approach is based on a nonlinear amplifier unit, which we

may call an A/1 cell, having the transfer characteristic shown in

Figure 21. The local small-signal gain

∂V

OUT/∂VIN is A, main-

tained for all inputs up to the knee voltage EK, above which the

incremental gain drops to unity. The function is symmetrical: the

same drop in gain occurs for instantaneous values of VIN less

than –EK. The large-signal gain has a value of A for inputs in the

range –EK ≤ VIN ≤ +EK, but falls asymptotically toward unity for

very large inputs. In logarithmic amplifiers based on this ampli-

fier function, both the slope voltage and the intercept voltage

must be traceable to the one reference voltage, EK. Therefore, in

this fundamental analysis, the calibration accuracy of the log amp

is dependent solely on this voltage. In practice, it is possible to

separate the basic references used to determine VY and VX and

SLOPE = A

SLOPE = 1

AEK

0

EK

INPUT

A/1

Figure 21. The A/1 Amplifier Function

in the case of the AD8307, VY is traceable to an on-chip band-

gap reference, while VX is derived from the thermal voltage kT/q

and later temperature-corrected.

Let the input of an N-cell cascade be VIN, and the final output

VOUT. For small signals, the overall gain is simply A

N. A six-

stage system in which A = 5 (14 dB) has an overall gain of

15,625 (84 dB). The importance of a very high small-signal gain

in implementing the logarithmic function has been noted; how-

ever, this parameter is of only incidental interest in the design of

log amps.

From here onward, rather than considering gain, we will analyze

the overall nonlinear behavior of the cascade in response to a

simple dc input, corresponding to the VIN of Equation 1. For

very small inputs, the output from the first cell is V1 = AVIN;

from the second, V2 = A

2 V

IN, and so on, up to VN = A

N V

IN. At

a certain value of VIN, the input to the Nth cell, VN–1, is exactly

equal to the knee voltage EK. Thus, VOUT = AEK and since there

are N–1 cells of gain A ahead of this node, we can calculate that

VIN = EK /A

N–1. This unique situation corresponds to the lin-log

transition, labeled 1 on Figure 22. Below this input, the cascade

of gain cells is acting as a simple linear amplifier, while for higher

values of VIN, it enters into a series of segments which lie on a

logarithmic approximation (dotted line).

VOUT

LOG VIN

0

RATIO

OF A

EK/AN–1

EK/AN–2

EK/AN–3

EK/AN–4

(A-1) EK

(4A-3) EK

(3A-2) EK

(2A-1) EK

AEK

Figure 22. The First Three Transitions

Continuing this analysis, we find that the next transition occurs

when the input to the (N–1) stage just reaches EK; that is, when

VIN = EK /A

N–2. The output of this stage is then exactly AE

K,

and it is easily demonstrated (from the function shown in Figure

21) that the output of the final stage is (2A–1) EK (labeled on

Figure 22). Thus, the output has changed by an amount (A–1)EK

for a change in VIN from EK /A

N–1 to E

K /A

N–2, that is, a ratio

change of A. At the next critical point, labeled , we find the

input is again A times larger and VOUT has increased to (3A–2)EK,

that is, by another linear increment of (A–1)EK. Further analysis

shows that right up to the point where the input to the first cell

is above the knee voltage, VOUT changes by (A–1)EK for a ratio

change of A in VIN. This can be expressed as a certain fraction

of a decade, which is simply log10(A). For example, when A = 5

a transition in the piecewise linear output function occurs at

regular intervals of 0.7 decade (that is, log10(A), or 14 dB divided

by 20 dB). This insight allows us to immediately write the Volts

per Decade scaling parameter, which is also the Scaling Voltage

VY, when using base-10 logarithms, as:

VY

=

Linear Change in VOUT

Decades Change in VIN

=

A

−1

()E

K

log10 A

()

(4)

Podobny numer części - AD8307AR-REEL

Producent	Numer części	Arkusz danych	Szczegółowy opis
Analog Devices	AD8307AR-REEL	508Kb / 24P	Low Cost DC-500 MHz, 92 dB Logarithmic Amplifier REV. C
Analog Devices	AD8307AR-REEL7	508Kb / 24P	Low Cost DC-500 MHz, 92 dB Logarithmic Amplifier REV. C

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Analog Devices	AD8307	508Kb / 24P	Low Cost DC-500 MHz, 92 dB Logarithmic Amplifier REV. C
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