AD538AD datasheet(6/11 Pages) AD

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AD538

–6–

REV. C

25k

LOG

RATIO

100

25k

ANTILOG

LOG

OUTPUT

100

AD538

INTERNAL

VOLTAGE

REFERENCE

+10V

–VS

+VS

+2V

PWR

GND

SIGNAL

GND

118

Figure 9. Functional Block Diagram

FUNCTIONAL DESCRIPTION

As shown in Figures 9 and 10, the VZ and VX inputs connect

directly to the AD538’s input log ratio amplifiers. This subsec-

tion provides an output voltage proportional to the natural log

of input voltage VZ , minus the natural log of input voltage VX.

The output of the log ratio subsection at B can be expressed by

the transfer function:









where k = 1.3806

× 10–23 J/K,

q = 1.60219

× 10–19 C,

T is in Kelvins.

The log ratio configuration may be used alone, if correctly tem-

perature compensated and scaled to the desired output level

(see Applications section).

Under normal operation, the log-ratio output will be directly

connected to a second functional block at input C, the antilog

subsection. This section performs the antilog according to the

transfer function:

O = VY e









As with the log-ratio circuit included in the AD538, the user

may use the antilog subsection by itself. When both subsections

are combined, the output at B is tied to C, the transfer function

of the AD538 computational unit is:

O = VY e





































 ; V

B = VC

which reduces to:









Finally, by increasing the gain, or attenuating the output of the

log ratio subsection via resistor programming, it is possible to

raise the quantity VZ/VX to the m

th power. Without external

programming, m is unity. Thus the overall AD538 transfer

function equals:

O = VY









where 0.2 < m < 5.

When the AD538 is used as an analog divider, the VY input can

be used to multiply the ratio VZ /VX by a convenient scale factor.

The actual multiplication by the VY input signal is accomplished

by adding the log of the VY input signal to the signal at C, which

is already in the log domain.

INPUT FREQUENCY – Hz

150

100

1.0

0.1

10k

100k

100

VX = 10V

VY = 5V +5V SIN t VOLTS

VZ = 0V

Figure 7. VY Feedthrough vs. Frequency

DC OUTPUT VOLTAGE – Volts

100

0.01

0.10

0.01

0.1

FOR THE FREQUENCY RANGE OF 10Hz

TO 100kHz THE TOTAL RMS OUTPUT

NOISE, eo, FOR A GIVEN BANDWIDTH

Bw, IS CALCULATED eo = en Bw

VX = 10V

VX = 0.01V

Figure 8. 1 kHz Output Noise Spectral Density vs. DC Output

Voltage

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