AD538 Analog Devices, AD538 Datasheet - Page 10

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

AD538
TRANSDUCER LINEARIZATION
Many electronic transducers used in scientific, commercial or
industrial equipment monitor the physical properties of a device
and/or its environment. Sensing (and perhaps compensating for)
changes in pressure, temperature, moisture or other physical
phenomenon can be an expensive undertaking, particularly
where high accuracy and very low nonlinearity are important. In
conventional analog systems accuracy may be easily increased
by offset and scale factor trims, however, nonlinearity is usually
the absolute limitation of the sensing device.
With the ability to easily program a complex analog function,
the AD538 can effectively compensate for the nonlinearities of
an inexpensive transducer. The AD538 can be connected be-
tween the transducer preamplifier output and the next stage of
monitoring or transmitting circuitry. The recommended proce-
dure for linearizing a particular transducer is first to find the
closest function which best approximates the nonlinearity of the
device and then, to select the appropriate exponent resistor
value(s).
ARC-TANGENT APPROXIMATION
The circuit of Figure 17 is typical of those AD538 applications
where the quantity V
In an approximate arc-tangent function, the AD538 will accu-
rately compute the angle that is defined by X and Y displace-
ments represented by input voltages V
to within one degree (for input voltages between 100 V and
10 volts), the AD538 arc-tangent circuit is more precise than
conventional analog circuits and is faster than most digital tech-
niques. For a direct arc-tangent computation that requires fewer
external components, refer to the AD639 data sheet. The circuit
shown is set up for the transfer function:
where:
The (V
adding together the output, V , and an externally applied refer-
ence voltage, V
capacitor connected around the AD547’s 100 k feedback
resistor frequency compensates the loop (formed by the ampli-
fier between V and V
REF
– V ) function is implemented in this circuit by
REF
V
, via an external AD547 op amp. The 1 F
Z
Y
/V
).
V
X
REF
is raised to powers greater than one.
Tan 1
V
X
Z
X
V
V
and V
X
Z
1 21
.
Z
. With accuracy
–10–
The V
one-quadrant divider circuit, except that the resulting quotient
is raised to the 1.21 power. Resistor R
the power or m factor.
For the highest arc-tangent accuracy the external resistors R1
and R2 should be ratio matched; however, the offset trim
scheme shown in other circuits is not required since nonlinearity
effects are the predominant source of error. Also note that insta-
bility will occur as the output approaches 90 because, by defini-
tion, the arc-tangent function is infinite and therefore, the AD538’s
gain will be extremely high.
+15V
–15V
FULL-SCALE
V
ADJUST
10k
B
/V
V
1 F
1 F
Z
A
quantity is calculated in the same manner as in the
Figure 17. The Arc-Tangent Function
118k
+10V
+V
–V
+2V
V
V
I
B
S
S
O
Z
Z
I
1 F
1
2
3
4
5
6
7
8
9
V = [V
25k
25k
100k
OUTPUT
R1
REF
*
100
REFERENCE
100k
INTERNAL
VOLTAGE
–V ]
ANTILOG
RATIO
( )
LOG
AD547JH
V
V
0.1 F
Z
X
+15V
–15V
1.21
LOG
AD538
A
100
*
(nominally 931 ) sets
100k
RATIO MATCH 1% METAL
FILM RESISTORS FOR BEST
ACCURACY
= TAN
R2
*
25k
25k
–1
( )
Z
X
18
17
16
15
14
13
12
10
11
A
D
I
V
SIGNAL
GND
PWR
GND
C
I
V
X
Y
X
Y
R
931 , 1%
A
REV. C
IN4148
V
X