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FEATURES
Complete Log/Antilog Amplifier
External Components Not Required;
Internal Reference; Temperature Compensated
6 Decades Current Operation - 1nA to 1mA
% max Error - 10nA to 100
A
1% max Error - 1nA to 1mA
4 Decades Voltage Operation - 1mV to 10V,
% max Error - 1mV to 1V
1% max Error - 1mV to 10V
APPLICATIONS
Log Current or Voltage
Antilog Voltage
Data Compression or Expansion
Absorbence Measurements
Computing Powers and Log Ratios
GENERAL DESCRIPTION
Model 755 is a complete dc logarithmic amplifier consisting of an
accurate temperature compensated antilog element, and a low bias
current FET amplifier. In addition to offering 120dB of current logging
(1nA to 1mA) and 80dB of voltage logging (1mV to 10V), the 755
features exceptionally low bias currents of 10pA and 15
V/
C voltage
drift to satisfy most wide range applications. Conformance to ideal log
operation is held to 1% over its total 120dB current range (1nA to
1mA), with 0.5% conformity guaranteed over an 80dB range (10nA
to 100
A). Two models are available, model 755N and model 755P.
The N version computes the log of positive input signals and the P
version computes the log of negative input signals.
Advanced design techniques and improved component selection are
used to obtain exceptionally good performance. For example, the use
of monolithic devices greatly reduces the influence of temperature
variations. Offering both log and antilog operation, model 755's price
and performance are especially attractive as an alternative to in-house
designs of OEM applications. This log design also improves
significantly over competitive designs in price, performance, and
package size.
MAJOR IMPROVEMENTS IN I
OS
For most low level applications, the input bias current I
OS
, is especially
critical, since it is the major source of error when processing low level
currents. At 1nA of input current there is an error contribution of 1%
for every 10pA of I
OS.
Recognizing the importance of this parameter,
bias current of model 755 is maintained below 10pA.
APPLICATIONS
When connected in the current or voltage logging configuration, as
shown in Figure 1, the model 755 may be used in several key
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applications. A plot of input current versus output is also presented
to illustrate the log amplifier's transfer characteristics.
Models 755N, 755P
6-Decade, High Accuracy
Log, Antilog Amplifiers
Figure 1. Functional Block Diagram and Transfer Function
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