ISL28470 Intersil Corporation, ISL28470 Datasheet - Page 11

• Manufacturer Part Number: ISL28470
• Description: (ISL28270 - ISL28470) Rail-to-Rail Input and Output (RRIO) Instrumentation Amplifier
• Manufacturer: Intersil Corporation

Application Information
Product Description
The ISL28270 and ISL28273 are dual channel micropower
instrumentation amplifiers (in-amps) and the ISL28470 is a
Quad-channel which delivers rail-to-rail input amplification
and rail-to-rail output swing. The in-amps also deliver
excellent DC and AC specifications while consuming only
about 60µA per channel. Because the independent pair of
feedback terminals set the gain and adjust the output 0 level,
the ISL28270, ISL28273 and ISL28470 achieve high CMRR
regardless of the tolerance of the gain setting resistors. The
ISL28270 and ISL28470 are internally compensated for a
minimum gain of 100. The ISL28273 is internally
compensated for a minimum gain of 10.
EN pins are available to independently enable or disable a
channel. When all channels are off, current consumption is
down to typically 4µA.
Input Protection
All input terminals and feedback 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. Input signals originating from low impedance
sources should have current limiting resistors in series with
the IN+ and IN- pins to prevent damaging currents during
power supply sequencing and other transient conditions.
The ISL28270 and ISL28470 have additional back-to-back
diodes across the input terminals and also across the
feedback terminals. If overdriving the inputs is necessary,
the external input current must never exceed 5mA. External
series resistors may be used as an external protection to
limit excessive external voltage and current from damaging
the inputs. On the other hand, the ISL28273 has no clamps
to limit the differential voltage on the input terminals allowing
higher differential input voltages at lower gain applications. It
is recommended, however, that the terminals of the
ISL28273 are not overdriven beyond 1V to avoid offset drift.
Input Stage and Input Voltage Range
The input terminals (IN+ and IN-) of the in-amps are a single
differential pair of bipolar PNP devices aided by an Input Range
Enhancement Circuit (IREC), to increase the headroom of
operation of the common-mode input voltage. The feedback
terminals (FB+ and FB-) also have a similar topology. As a
result, the input common-mode voltage range is rail-to-rail
regardless of the feedback terminal settings and regardless of
the gain settings. They are able to handle input voltages that
are at or slightly beyond the supply and close to ground making
these in-amps well suited for single 5V down to 2.4V supply
systems. There is no need to bias the common-mode input to
achieve symmetrical input voltage. It is recommended,
however, that the common-mode input be biased at least 10mV
above the negative supply rail to achieve top performance. See
“Input Bias Cancellation/Compensation” on page 11.
11
ISL28270, ISL28273, ISL28470
The IREC enables rail-to-rail input amplification without the
problems usually associated with the dual differential stage
topology. The IREC ensures that there are no drastic
changes in offset voltage over the entire range of the input.
See Input Offset Voltage vs Common-Mode Input Voltage in
performance charts. IREC also cures the abrupt change and
even reverse polarity of the input bias current over the whole
range of input.
Input Bias Cancellation/Compensation
All three parts have an Input Bias Cancellation/Compensation
Circuit for both the input and feedback terminals (IN+, IN-, FB+
and FB-), achieving a low input bias current throughout the
input common-mode range and the operating temperature
range. While the PNP bipolar input stages are biased with an
adequate amount of biasing current for speed and increased
noise performance, the Input Bias Cancellation/Compensation
Circuit sinks most of the base current of the input transistors
leaving a small portion as input bias current, typically 500pA. In
addition, the Input Bias Cancellation/Compensation Circuit
maintains a smooth and flat behavior of input bias current over
the common mode range and over the operating temperature
range. The Input Bias Cancellation/Compensation Circuit
operates from input voltages of 10mV above the negative
supply to input voltages slightly above the positive supply.
Output Stage and Output Voltage Range
A Class AB common-source output stage drives the output.
The pair of complementary MOSFET devices drive the
output VOUT to within a few millivolts of the supply rails. At a
100kΩ load, the PMOS sources current and pulls the output
up to 4mV below the positive supply. The NMOS sinks
current and pulls the output down to 4mV above the negative
supply, or ground in the case of a single supply operation.
The current sinking and sourcing capability are internally
limited to 29mA. When disabled, the outputs are in a high
impedance state.
Gain Setting
VIN (the potential difference across IN+ and IN-), is
replicated (less the input offset voltage) across FB+ and FB-.
The function of the in-amp is to maintain the differential
voltage across FB- and FB+ equal to IN+ and IN-; (FB- -
FB+) = (IN+ - IN-). Consequently, the transfer function can
be derived. The in-amp gain is set by two external resistors,
the feedback resistor RF, and the gain resistor RG.
April 13, 2007
FN6260.2