AD538 Analog Devices, AD538 Datasheet - Page 6

• Manufacturer Part Number: AD538
• Description: Real-Time Analog Computational Unit ACU
• Manufacturer: Analog Devices

AD538
FUNCTIONAL DESCRIPTION
As shown in Figures 9 and 10, the V
directly to the AD538’s input log ratio amplifiers. This subsec-
tion provides an output voltage proportional to the natural log
of input voltage V
The output of the log ratio subsection at B can be expressed by
the transfer function:
where k = 1.3806
The log ratio configuration may be used alone, if correctly tem-
perature compensated and scaled to the desired output level
(see Applications section).
q = 1.60219 10
T is in Kelvins.
+10V
+2V
+V
–V
V
150
100
V
1.0
0.1
I
B
10
Figure 7. V
O
S
S
Z
Z
I
100
1
2
3
4
5
6
7
8
9
Figure 9. Functional Block Diagram
25k
OUTPUT
25k
Z
, minus the natural log of input voltage V
REFERENCE
INTERNAL
VOLTAGE
10
1k
V
Y
–23
B
Feedthrough vs. Frequency
–19
INPUT FREQUENCY – Hz
100
ANTILOG
J/K,
C,
RATIO
LOG
kT
q
V
V
V
10k
ln
X
Y
Z
AD538
LOG
= 10V
= 5V +5V SIN
= 0V
100
Z
V
V
and V
Z
X
100k
25k
25k
X
t VOLTS
inputs connect
18
17
16
15
14
13
12
11
10
A
D
I
V
SIGNAL
GND
PWR
GND
C
I
V
X
Y
X
Y
1M
X
.
–6–
Figure 8. 1 kHz Output Noise Spectral Density vs. DC Output
Voltage
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:
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:
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 V
programming, m is unity. Thus the overall AD538 transfer
function equals:
where 0.2 < m < 5.
When the AD538 is used as an analog divider, the V
be used to multiply the ratio V
The actual multiplication by the V
by adding the log of the V
is already in the log domain.
0.10
0.01
100
10
1
0.01
V
FOR THE FREQUENCY RANGE OF 10Hz
TO 100kHz THE TOTAL RMS OUTPUT
NOISE, e
Bw, IS CALCULATED e
O
Z
V
/V
o
Y
, FOR A GIVEN BANDWIDTH
e
X
DC OUTPUT VOLTAGE – Volts
V
V
V
to the m
V
O
O
kT
O
Y
X
q
0.1
= 0.01V
input signal to the signal at C, which
V
V
kT
V
Z
q
Y
Y
o
Y
/ V
= e
e
th
V
ln
X
Y
X
V
n
V
V
power. Without external
V
V
= 10V
C
V
V
input signal is accomplished
by a convenient scale factor.
Bw
Z
X
X
kT
Z
Z
X
q
m
; V
1
B
V
C
Y
input can
10
REV. C