AD538
FUNCTIONAL DESCRIPTION
STABILITY PRECAUTIONS
As shown in Figure 1 and Figure 11, the VZ and VX inputs
connect directly to the input log ratio amplifiers of the AD538.
This subsection 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
At higher frequencies, the multistaged signal path of the AD538
can result in large phase shifts (as illustrated in Figure 11). If a
condition of high incremental gain exists along that path (for
example, VO = VY × VZ/VX = 10 V × 10 mV/10 mV = 10 V so
that ΔVO/ΔVX = 1000), then small amounts of capacitive feedback
from VO to the current inputs IZ or IX can result in instability.
Appropriate care should be exercised in board layout to prevent
capacitive feedback mechanisms under these conditions.
Ln Z – Ln X
VZ
VX
kT
q
VB =
ln
I
X
Ln X
M(Ln Z – Ln X)
M(Ln Z – Ln X) +Ln Y
LOG
e
where:
V
X
k is 1.3806 × 10−23 J/K.
q is 1.60219 × 10−19 C.
T is in Kelvins.
–
ANTILOG
0.2≤M≤5
BUFFER
Σ
Σ
e
+
+
+
M
The log ratio configuration may be used alone, if correctly
temperature compensated and scaled to the desired output
level (see the Applications Information section).
V
V
Z
X
I
I
Z
Z
Y
V
= V
Y
O
LOG
LOG
e
e
Ln Z
Ln Y
V
V
Y
Figure 11. Model Circuit
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:
USING THE VOLTAGE REFERENCES
A stable band gap voltage reference for scaling is included in the
AD538. It is laser-trimmed to provide a selectable voltage output of
+10 V buffered (Pin 4), +2 V unbuffered (Pin 5) or any voltages
between +2 V and +10.2 V buffered as shown in Figure 12. The
output impedance at Pin 5 is approximately 5 kΩ. Note that any
loading of this pin produces an error in the +10 V reference
voltage. External loads on the +2 V output should be greater
than 500 kΩ to maintain errors less than 1%.
q
kT
V =V e V
C
O
Y
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:
I
V
V
kT
q
kT
1
2
3
4
5
6
18
17
16
15
14
13
12
11
A
D
Z
Z
Z
X
ln
Q
e
;VB =VC
LOG
RATIO
25kΩ
VO = VY
V
+2V TO +10.2V
BUFFERED
which reduces to:
B
I
X
VZ
VX
REF OUT
V
X
VO = VY
25kΩ
100Ω
100Ω
+2V
SIGNAL
GND
50kΩ
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 mth power. Without external
programming, m is unity. Thus, the overall AD538 transfer
function equals:
INTERNAL
VOLTAGE
REFERENCE
11.5kΩ
PWR
GND
+V
S
AD538
–V
S
7
8
9
C
OUTPUT
25kΩ
V
O
I
Y
ANTILOG
m
VZ
VX
10
V
Y
I
LOG
VO = VY
25kΩ
Figure 12. +2 V to +10.2 V Adjustable Reference
where 0.2 < m < 5.
In situations not requiring both reference levels, the +2 V output
can be converted to a buffered output by tying Pin 4 and Pin 5
together. If both references are required simultaneously, the
+10 V output should be used directly and the +2 V output
should be externally buffered.
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.
Rev. E | Page 10 of 16
ADI [ ADI ]
