ISL28134IBZ Datasheet, PDF (14/25 Pages) Intersil Corporation – 5V Ultra Low Noise, Zero Drift Rail-to-Rail Precision Op Amp

ISL28134
Applications Information
Functional Description
The ISL28134 is a single 5V rail-to-rail input/output amplifier
that operates on a single or dual supply. The ISL28134 uses a
proprietary chopper-stabilized technique that combines a 3.5MHz
main amplifier with a very high open loop gain (174dB) chopper
amplifier to achieve very low offset voltage and drift (0.2µV,
0.5nV/°C) while having a low supply current (675µA). The very low
1/f noise corner <0.1Hz and low input noise voltage (8nV/√Hz @
100Hz) of the amplifier makes it ideal for low frequency precision
applications requiring very high gain and low noise.
This multi-path amplifier architecture contains a time continuous
main amplifier whose input DC offset is corrected by a
parallel-connected, high gain chopper stabilized DC correction
amplifier operating at 100kHz. From DC to ~10kHz, both
amplifiers are active with the DC offset correction active with
most of the low frequency gain provided by the chopper
amplifier. A 10kHz crossover filter cuts off the low frequency
chopper amplifier path leaving the main amplifier active out to
the -3dB frequency (3.5MHz GBWP).
The key benefits of this architecture for precision applications are
rail-to-rail inputs/outputs, high open loop gain, low DC offset and
temperature drift, low 1/f noise corner and low input noise
voltage. The noise is virtually flat across the frequency range
from a few mHz out to 100kHz, except for the narrow noise peak
at the amplifier crossover frequency (10kHz).
Power Supply Considerations
The ISL28134 features a wide supply voltage operating range. The
ISL28134 operates on single (+2.25V to +6.0V) or dual (±1.125 to
±3.0V) supplies. Power supply voltages greater than the +6.5V
absolute maximum (specified in the “Absolute Maximum Ratings”
on page 4) can permanently damage the device. Performance of
the device is optimized for supply voltages greater than 2.5V. This
makes the ISL28134 ideal for portable 3V battery applications that
require the precision performance. It is highly recommended that a
0.01µF or larger high frequency decoupling capacitor is placed
across the power supply pins of the IC to maintain high performance
of the amplifier.
Rail-to-rail Input and Output (RRIO)
Unlike some amplifiers whose inputs may not be taken to the power
supply rails or whose outputs may not drive to the supply rails, the
ISL28134 features rail-to-rail inputs and outputs. This allows the
amplifier inputs to have a wide common mode range (100mV
beyond supply rails) while maintaining high CMRR (135dB) and
maximizes the signal to noise ratio of the amplifier by having the
V_OH and V_OL levels be at the V+ and V- rails, respectively.
Low Input Voltage Noise Performance
In precision applications, the input noise of the front end
amplifier is a critical parameter. Combined with a high DC gain to
amplify the small input signal, the input noise voltage will result
in an output error in the amplifier. A 1µV_P-P input noise voltage
with an amplifier gain of 10,000V/V will result in an output offset
in the range of 10mV, which can be an unacceptable error
source. With only 250nV_P-P at the input, along with a flat noise
response down to 0.1Hz, the ISL28134 can amplify small input
signals with minimal output error.
The ISL28134 has the lowest input noise voltage compared to
other competitor Zero Drift amplifiers with similar supply
currents (See Table 1). The overall input referred voltage noise of
an amplifier can be expressed as a sum of the input noise
voltage, input noise current of the amplifier and the Johnson
noise of the gain-setting resistors used. The product of the input
noise current and external feedback resistors along with the
Johnson noise increases the total output voltage noise as the
value of the resistance goes up. For optimizing noise
performance, choose lower value feedback resistors to minimize
the effect of input noise current. Although the ISL28134 features
a very low 200fA/√Hz input noise current, at source impedances
>100kΩ, the input referred noise voltage will be dominated by
the input current noise. Keep source input impedances under
10kΩ for optimum performance.
TABLE 1.
Part
Voltage Noise @ 100Hz
Competitor A
22nV/√Hz
Competitor B
16nV/√Hz
Competitor C
90nV/√Hz
ISL28134
8nV/√Hz
0.1Hz to 10Hz Peak to Peak
Voltage Noise
600nV_P-P
260nV_P-P
1500nV_P-P
250nV_P-P
High Source Impedance Applications
The input stage of Chopper Stabilized amplifiers do not behave
like conventional amplifier input stages. The ISL28134 uses
switches at the chopper amplifier input that continually ‘chops’
the input signal at 100kHz to reduce input offset voltage down to
1µV. The dynamic behavior of these switches induces a charge
injection current to the input terminals of the amplifier. The
charge injection current has a DC path to ground through the
resistances seen at the input terminals of the amplifier. Higher
input impedance cause an apparent shift in the input bias
current of the amplifier. Input impedances larger than 10kΩ
begin to have significant increases in the bias currents. To
minimize the effect of impedance on input bias currents, an
input resistance of <10kΩ is recommended.
Because the chopper amplifier has charge injection currents at
each terminal, the input impedance should be balanced across
each input (see Figure 43). The input impedance of the amplifier
should be matched between the IN+ and IN- terminals to
minimize total input offset current. Input offset currents show up
as an additional output offset voltage, as shown in Equation 1:
V_OSTOT = V_OS - R_F*I_OS
(EQ. 1)
If the offset voltage of the amplifier is negative, the input offset
currents will add to the total output offset. For a 10,000V/V gain
amplifier using 1MΩ feedback resistor, a 500pA total input offset
current will have an additional output offset voltage of 0.5mV. By
keeping the input impedance low and balanced across the
amplifier inputs, the input offset current is kept below 100pA,
resulting in an offset voltage 0.1mV or less.
14
FN6957.3
December 22, 2011