The OP282 and OP482 are single and dual JFET op amps that
have been optimized for high speed at low power. This
combination makes these amplifiers excellent choices for battery
powered or low power applications requiring above average
performance. Applications benefiting from this performance
combination include telecom, geophysical exploration, portable
medical equipment and navigational instrumentation.
HIGH SIDE SIGNAL CONDITIONING
There are many applications that require the sensing of signals
near the positive rail. OP282s and OP482s have been tested and
guaranteed over a common-mode range (–11 V ≤ VCM ≤ +15 V)
that includes the positive supply.
One application where this is commonly used is in the sensing of
power supply currents. This enables it to be used in current
sensing applications such as the partial circuit shown in Figure
1. In this circuit, the voltage drop across a low value resistor,
such as the 0.1 Ω shown here, is amplified and compared to 7.5
volts. The output can then be used for current limiting.
Figure 1. Phase Inversion
Most JFET-input amplifiers will invert the phase of the input
signal if either input exceeds the input common-mode range.
For the OP282 and OP482 negative signals in excess of approxi-
mately 14 volts will cause phase inversion. The cause of this
effect is saturation of the input stage leading to the forward-
biasing of a drain-gate diode. A simple fix for this in noninverting
applications is to place a resistor in series with the noninverting
input. This limits the amount of current through the forward-
biased diode and prevents the shutting down of the output
stage. For the OP282/OP482, a value of 200 kΩ has been found
to work. However, this adds a significant amount of noise.
Figure 2. OP282 Phase Reversal
The OP282 and OP482’s wide bandwidth and high slew rates
make either an excellent choice for many filter applications.
There are many types of active filter configurations, but the four
most popular configurations are Butterworth, elliptical, Bessel,
and Chebyshev. Each type has a response that is optimized for a
given characteristic as shown in Table I.
PROGRAMMABLE STATE-VARIABLE FILTER
Selectivity Overshoot Phase
–4– REV. B