ISL28233 INTERSIL [Intersil Corporation], ISL28233 Datasheet - Page 11

• Manufacturer Part Number: ISL28233
• Description: Single, Dual, and Quad Micropower, Zero-Drift, RRIO Operational Amplifiers
• Manufacturer: INTERSIL [Intersil Corporation]

Applications Information
Functional Description
The ISL28133 uses a proprietary chopper-stabilized
architecture shown in the “Block Diagram” on page 2.
The ISL28133 combines a 400kHz main amplifier with a
very high open loop gain (174dB) chopper stabilized
amplifier to achieve very low offset voltage and drift
(2µV, 0.02µV/°C typical) while consuming only 18µA of
supply current per channel.
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 ~5kHz, both amplifiers are active with DC
offset correction and most of the low frequency gain is
provided by the chopper amplifier. A 5kHz crossover filter
cuts off the low frequency amplifier path leaving the
main amplifier active out to the 400kHz gain-bandwidth
product of the device.
The key benefits of this architecture for precision
applications are very high open loop gain, very low DC
offset, and low 1/f noise. 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 (5kHz).
Rail-to-rail Input and Output (RRIO)
The RRIO CMOS amplifier uses parallel input PMOS and
NMOS that enable the inputs to swing 100mV beyond
either supply rail. The inverting and non-inverting inputs
do not have back-to-back input clamp diodes and are
capable of maintaining high input impedance at high
differential input voltages. This is effective in eliminating
output distortion caused by high slew-rate input signals.
The output stage uses common source connected PMOS
and NMOS devices to achieve rail-to-rail output drive
capability with 17mA current limit and the capability to
swing to within 20mV of either rail while driving a 10kΩ
load.
IN+ and IN- Protection
All input terminals have internal ESD protection diodes
to both positive and negative supply rails, limiting the
input voltage to within one diode beyond the supply
rails. For applications where either input is expected to
exceed the rails by 0.5V, an external series resistor
must be used to ensure the input currents never exceed
20mA (see Figure 33).
V
FIGURE 33. INPUT CURRENT LIMITING
IN
R
IN
+
-
11
R
L
V
OUT
ISL28133
Layout Guidelines for High Impedance
Inputs
To achieve the maximum performance of the high input
impedance and low offset voltage of the ISL28133
amplifiers, care should be taken in the circuit board
layout. The PC board surface must remain clean and
free of moisture to avoid leakage currents between
adjacent traces. Surface coating of the circuit board will
reduce surface moisture and provide a humidity barrier,
reducing parasitic resistance on the board.
High Gain, Precision DC-Coupled Amplifier
The circuit in Figure 34 implements a single-stage,
10kV/V DC-coupled amplifier with an input DC sensitivity
of under 100nV that is only possible using a low VOS
amplifier with high open loop gain. This circuit is practical
down to 1.8V due to it's rail-to-rail input and output
capability. Standard high gain DC amplifiers operating
from low voltage supplies are not practical at these high
gains using typical low offset precision op amps because
the input offset voltage and temperature coefficient
consume most of the available output voltage swing. For
example, a typical precision amplifier in a gain of 10kV/V
with a ±100µV VOS and a temperature coefficient of
0.5µV/°C would produce a DC error at the output of >1V
with an additional 5mV°C of temperature dependent
error. At 3V, this DC error consumes > 30% of the total
supply voltage, making it impractical to measure
sub-microvolt low frequency signals.
The ±8µV max V
produces a temperature stable maximum DC output
error of only ±80mV with a maximum temperature drift
of 0.75mV/
noise corner frequency and some feedback filtering
enables DC voltages and voltage fluctuations well below
100nV to be easily detected with a simple single stage
amplifier.
FIGURE 34. HIGH GAIN, PRECISION DC-COUPLED
V
IN
1M
100
°
C. The additional benefit of a very low 1/f
Ω
Ω
OS
and 0.075µV/
100
+
-
Ω
0.018µF
1M
+2.5V
-2.5V
C
Ω
F
°
C of the ISL28133
A
R
CL
L
= 10kV/V
August 12, 2010
V
OUT
FN6560.3