ISL28110_1012 Datasheet, PDF (15/25 Pages) Intersil Corporation – Precision Low Noise JFET Operational Amplifiers

ISL28110, ISL28210
In applications where one or both amplifier input
terminals are at risk of exposure to voltages beyond the
supply rails, current limiting resistors may be needed at
each input terminal (see Figure 41 R_IN+, R_IN-) to limit
current through the power supply ESD diodes to 20mA.
V+
V_IN-
R_IN-
-
V_IN+
R_IN
+
R_L
V-
FIGURE 41. INPUT ESD DIODE CURRENT LIMITING
JFET Input Stage Performance
The ISL28110, ISL28210 JFET input stage has the linear
gain characteristics of the MOSFET but can operate at
high frequency with much lower noise. The reversed-
biased gate PN gate junction has significantly lower gate
capacitance than the MOSFET, enabling input slew rates
that rival op amps using bipolar input stages. The added
advantage for high impedance, precision amplifiers is
the lack of a significant 1/f component of current noise
(Figures 13, 14) as there is virtually no gate current.
The input stage JFETs are bootstrapped to maintain a
constant JFET drain to source voltage which keeps the
JFET gate currents and input stage frequency response
nearly constant over the common mode input range of
the device. These enhancements provide excellent
CMRR, AC performance and very low input distortion
over a wide temperature range. The common mode
input performance for offset voltage and bias current is
shown in Figure 42. Note that the input bias current
remains low even after the maximum input stage
common mode voltage is exceeded (as indicated by the
abrupt change in input offset voltage).
10
500
8
V_S = ±15V
INPUT OFFSET VOLTAGE (V_OS) _{T = +25°C}
400
6
300
4
200
2
100
0
0
-2
-100
-4
-200
-6
INPUT BIAS (I_B)
-8
-300
-400
-10
-15
-10
-5
0
-500
5
10
15
V_CM (V)
FIGURE 42. INPUT OFFSET VOLTAGE AND BIAS
CURRENT vs COMMON MODE INPUT
VOLTAGE
Output Drive Capability
The complementary bipolar emitter follower output
stage features low output impedance (Figure 40) and is
capable of substantial current drive over the full
temperature range (Figures 27, 28) while driving the
output voltage close to the supply rails. The output
current is internally limited to approximately ±50mA at
+25°C. The amplifiers can withstand a short circuit to
either rail as long as the power dissipation limits are not
exceeded. This applies to only 1 amplifier at a time for
the dual op amp. Continuous operation under these
conditions may degrade long term reliability.
Output Phase Reversal
Output phase reversal is a change of polarity in the
amplifier transfer function when the input voltage
exceeds the supply voltage. The ISL28110 and ISL28210
are immune to output phase reversal, out to 0.5V
beyond the rail (V_ABS MAX) limit. Beyond these limits,
the device is still immune to reversal to 1V beyond the
rails but damage to the internal ESD protection diodes
can result unless these input currents are limited.
Maximizing Dynamic Signal Range
The amplifiers maximum undistorted output swing is a
figure of merit for precision, low distortion applications.
Audio amplifiers are a good example of amplifiers that
require low noise and low signal distortion over a wide
output dynamic range. When these applications operate
from batteries, raising the amplifier supply voltage to
overcome poor output voltage swing has the penalty of
increased power consumption and shorter battery life.
Amplifiers whose input and output stages can swing
closest to the power supply rails while providing low
noise and undistorted performance, will provide
maximum useful dynamic signal range and longer
battery life.
Rail-to-rail input and output (RRIO) amplifiers have the
highest dynamic signal range but their added complexity
degrades input noise and amplifier distortion. Many
contain two input pairs, one pair operating to each supply
rail. The trade-offs for these are increased input noise
and distortion caused by non-linear input bias current
and capacitance when amplifying high impedance
sources. Their rail-to-rail output stages swing to within a
few millivolts of the rail, but output impedances are high
so that their output swing decreases and distortion
increases rapidly with increasing load current. At heavy
load currents the maximum output voltage swing of RRO
op amps can be lower than a good emitter follower
output stage.
The ISL28110 and ISL28210 low noise input stage and
high performance output stage are optimized for low
THD+N into moderate loads over the full -40°C to
+125°C temperature range. Figures 19 and 20 show the
1kHz THD+N unity gain performance vs output voltage
swing at load resistances of 2kΩ and 600Ω. Figure 43
shows the unity-gain THD+N performance driving
600Ω from ±5V supplies.
15
FN6639.1
December 8, 2010