EL8170 Datasheet, PDF (11/14 Pages) Intersil Corporation – Micropower, Single Supply, Rail-to-Rail Input-Output Instrumentation Amplifiers

EL8170, EL8173
Description of Operation and Applications
Information
Product Description
The EL8170 and EL8173 are micropower instrumentation
amplifiers (in-amps) which deliver rail-to-rail input amplification
and rail-to-rail output swing on a single +2.4V to +5.5V supply.
The EL8170 and EL8173 also deliver excellent DC and AC
specifications while consuming only 65µA typical supply
current. Because the EL8170 and EL8173 provide an
independent pair of feedback terminals to set the gain and to
adjust output level, these in-amps achieve high common-mode
rejection ratio regardless of the tolerance of the gain setting
resistors. The EL8173 is internally compensated for a minimum
closed loop gain of 10 or greater, well suited for moderate to
high gains. For higher gains, the EL8170 is internally
compensated for a minimum gain of 100.
Input Protection
All input and feedback terminals of the EL8170 and EL8173
have internal ESD protection diodes to both positive and
negative supply rails, limiting the input voltage to within one
diode drop beyond the supply rails. The inverting inputs and
FB- inputs have ESD diodes to the V-rail, and the
non-inverting inputs and FB+ terminals have ESD diodes to
the V+ rail. The EL8170 has 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. On the other hand,
the EL8173 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 input terminals of the EL8173 are not overdriven beyond
1V to avoid offset drift. An external series resistor may be
used as an external protection to limit excessive external
voltage and current from damaging the inputs.
Input Stage and Input Voltage Range
The input terminals (IN+ and IN-) of the EL8170 and EL8173
are single differential pair bipolar PNP devices aided by an
Input Range Enhancement Circuit 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
of both the EL8170 and EL8173 is rail-to-rail. These in-amps
are able to handle input voltages that are at or slightly beyond
the supply and ground making these in-amps well suited for
single +5V or +3.3V low voltage supply systems. There is no
need to move the common-mode input of the in-amps to
achieve symmetrical input voltage.
Input Bias Cancellation, Input Bias Compensation
Both EL8170 and EL8173 feature an Input Bias
Cancellation/Compensation Circuit for both the input and
feedback terminals (IN+, IN-, FB+ and FB-), achieving a low
input bias current all 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 transistor 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, Input Bias Compensation
Circuit operates from input voltages of 10mV above the
negative supply to input voltages slightly above the positive
supply. See “Average Input Bias Current vs Common-Mode
Input Voltage” in the “Typical Performance Curves” beginning
on page 4.
Output Stage and Output Voltage Range
A pair of complementary MOSFET devices drives the output
V_OUT 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, while 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 of the EL8170
and EL8173 are internally limited to 26mA.
Gain Setting
V_IN, the potential difference across IN+ and IN-, is replicated
(less the input offset voltage) across FB+ and FB-. The
objective of the EL8170 and EL8173 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 gain of the EL8170 and EL8173
is set by two external resistors, the feedback resistor R_F, and
the gain resistor R_G.
11
FN7490.6
August 31, 2009