Ultra Low Bias Current Varactor
Bridge Operational Amplifiers
Models 310, 311
FEATURES
-14
Guaranteed Low Bias: 10 A max
Low Voltage Drift: 10µV/°C max (310K, 311K)
Versatility: Noninverting, model 31IJ/K
Inverting, model 310J/K
14
High Input Impedance: 10
Ω(311J,
311K)
APPLICATIONS
Femto-ammeter
Electrometer
Long-Time Integrator
Small-Rate Differentiator
Flame Current Detector
Phototube Amplifier
Log Compressor
pH Meter
GENERAL DESCRIPTION
The models 310 and 311 are operational amplifiers with extremely low
input bias currents and high input impedances. As such, they are
applicable to a large number of
electrometer
amplifier applications
which have traditionally been fulfilled (not without difficulty) by
vacuum electrometer tubes. These specialized requirements are
characterized by extremely high source impedances or where
infinitesimal currents must be measured or amplified. Because of
varactor bridge inputs, the all solid state 310 and 311 amplifiers are
ideally suited to this type of requirement. Voltage drifts are small, noise
is minimized, and the cost is low. In principles the operation of varactor
bridge amplifiers is similar to that of vibrating reed electrometers
(parametric), but also includes the inherent advantages of solid state
design.
VOLTAGE SOURCE OR CURRENT SOURCE?
Model 310 -
The model 310 is designed such that the high quality
signal input is the
inverting
input terminal, and is most appropriate for
measurements of
current
signals. This type of signal source is common
to gas chromatograph flame detectors, photomultiplier tubes, radiation
detectors, etc. The inverting model 310 is also useful for logarithmic
compression over an extremely wide dynamic range and in the
construction of very-long-time-constant integrators or differentiators.
Input signals from picoamps to milliamps may be accommodated with
femtoamp current resolution.
Model 311
- The311 is similar to the 310 but the high quality input is
the positive or
non-inverting
input. It is primarily intended for
measurement of voltages from very high source impedances. Such
sources are found in the glass electrodes of pH cells and other
scientific measurement apparatus. Another such source is charge
stored on the plates of a capacitor, as is found in long-time track-and-
hold applications.
Table 1. Comparison of Electrometer Types
TYPE
VARACTOR
BRIDGE
VIBRATING
CAPACITOR
MOSFET
ELECTROMETER
TUBES
I-Stability
V-Stability
Bandwidth
Overload
Protection
CMR
Microphonics
Warm-up
Size
Price
good
excellent
narrow
easy
excellent
fair
fast
small
low
excellent
excellent
narrow
easy
excellent
fair
fast
large
high
good
fair
wide
difficult
fair
good
fast
small
medium
good
poor
wide
poor
poor
slow
large
medium
The chart above shows the relative advantages of various electrometer
types. The best performance possible is given by the vibrating reed
electrometer. it should be remembered, however, that this is a large,
expensive laboratory instrument not generally suitable for instrument
construction.
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
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