ETC 757P

6 Decade, High Accuracy
Log Ratio Amplifiers
Models 757N, 757P
6 Decade Operation – 1 nA to 1 mA
1/2% Log Conformity – 10 nA to 100 µA
Symmetrical FET Inputs
Voltage or Current Operation
Temperature Compensated
Absorbence Measurements
Log Ratios of Voltages or Currents
Data Compression
Transducer Linearization
Model 757 is a complete, temperature compensated, dc-coupled log
ratio amplifier. It is comprised of two input channels for processing
signals spanning up to 6 decades in dynamic range (1nA to 1mA). By
virtue of its symmetrical FET input stages, the 757 can accommodate
this 6 decade signal range at either channel. Log conformity is
maintained to within 1/2% over 4 decades of input (10nA to 100µA)
and to with in 1% over the full input range. Unlike other log ratio
designs, model 757 does not restrict the relative magnitude of the two
signal inputs to achieve rated performance. Either input can be
operated within the specified range regardless of the signal level at the
other channel.
The model 757 log-ratio amplifier design makes available both
input amplifier summing junctions. As a result, it can directly
interface with photo diodes operating in the short-circuit current
mode without the need of additional input circuitry.
The excellent performance of model 757 can be further improved by
means of external scale factor and output offset adjustments. A
significant feature of model 757 not found on competing devices is
that, when the offset adjustment is used to establish a fixed bias at the
output, the output offset level does not vary as a function of input
signal magnitude. On other designs, the sensitivity of output offset to
input levels results in output effects resembling log conformity errors.
Current log ratio is accomplished by model 757 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, higher scale factors
may be achieved by connecting external scale factor adjusting
Model 757 can operate with either current or voltage inputs.
Its excellent performance makes it ideally suited for log ratio
applications such as blood analysis, chromatography, chemical
analysis of liquids and absorbence measurements.
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.
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