VOLTAGE LOG RATIO
The principle of operation for voltage log ratio is identical to that of
current log ratio after the voltage signal has been converted to a
current. To accomplish this conversion, an external resistor is
attached from the voltage signal to the appropriate input current
terminal of the 756. Input currents are then determined by:
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where R represents the total feedback resistance of A3, and K
DES
he desired scale factor.
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Norwood, MA 02062
Phone (781) 551-5500
FAX (781) 551-5555
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GENERAL DESCRIPTION
Model 756 is a complete temperature compensated DC log ratio
module, containing two channels for processing input variables.
Channel 1 features a high quality FET amplifier with bias current of
only 10pA. Using this input, signals spanning 4 decades can be
processed with less than 1% error. By applying signals spanning up
to 3 decades to channel 2, overall performance of 1% can be
achieved for ratios covering a dynamic range of 10 million to 1 (7
decades).
Designed primarily for photometer applications, model 756 replaces
two log modules, a subtractor, and associated circuitry. The signal
sources for these applications are usually photo diodes which should
be operated in the zero-volt mode (short circuit current). When
connected as shown in Figure 2, the summing junctions provide
virtual grounds, thereby forcing the input currents to be the short
circuit current of the photo diodes.
PRINCIPLES OF OPERATION
CURRENT LOG RATIO
Current log ratio is accomplished by model 756 when two currents,
I
SIG
and I
REF
, are applied directly to the input terminals (see Figure
1). The two log amps process these signals providing voltages which
are proportional to the log of their respective inputs. These voltages
are then subtracted and applied to an output amplifier. The scale
factor, when connected as shown, is 1V/dec. However, other scale
factors may be achieved by using an external/feedback resistor for A
3
instead of the internal 15k
. The governing equation for this
optional adjustment is:
FEATURES
Complete Log Ratio Module
Provides Log Ratio of Current
Provides Log Ratio of Voltage
Dynamic Range of 7 Decades of Ratio
APPLICATIONS
Log Ratio or Antilog Ratio of Voltages Log Ratio or
Antilog Ratio of Currents
Absorbance Measurements
Complete
Log Ratio Module
Model 756
Figure 1. Functional Block Diagram of Model 756
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