### ETC 756N

```Complete
Log Ratio Module
Model 756
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
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.
where R represents the total feedback resistance of A3, and KDES
he desired scale factor.
Figure 1. Functional Block Diagram of Model 756
PRINCIPLES OF OPERATION
VOLTAGE LOG RATIO
CURRENT 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:
Current log ratio is accomplished by model 756 when two currents,
ISIG and IREF, 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 A3
instead of the internal 15kΩ. The governing equation for this
optional adjustment is:
Information furnished by Intronics is believed to be accurate and reliable. However, no
responsibility is assumed by Intronics for its use; nor for any infringements of patents or
other rights of third parties which may result from its use. No license is granted by
implication or otherwise under any patent or patent rights of Intronics.
1400 Providence Highway, Building #2
Norwood, MA 02062
Phone (781) 551-5500
FAX (781) 551-5555
www.intronicspower.com
```