EL8171 查看數據表（PDF） - Intersil
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EL8171 Datasheet PDF : 11 Pages
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EL8171, EL8172
Description of Operation and Application
Information
Product Description
The EL8171 and EL8172 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
5V supply. The EL8171 and EL8172 also deliver excellent
DC and AC specifications while consuming only 60µA typical
supply current. Because EL8171 and EL8172 provide an
independent pair of feedback terminals to set the gain and to
adjust the output level, these in-amps achieve high common-
mode rejection ratio regardless of the tolerance of the gain
setting resistors. The EL8171 is internally compensated for a
minimum closed loop gain of 10 or greater, well suited for
moderate to high gains. For higher gains, the EL8172 is
internally compensated for a minimum gain of 100. An
ENABLE pin is used to reduce power consumption, typically
2.9µA, while the instrumentation amplifier is disabled.
Input Protection
All input and feedback terminals of the EL8171 and EL8172
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. If overdriving the inputs
is necessary, the external input current must never exceed
5mA. External series resistor may be used as a 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 EL8171 and EL8172
are single differential pair P-MOSFET 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 EL8171 and EL8172 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 then to move the common-mode input of
the in-amps to achieve symmetrical input voltage.
Output Stage and Output Voltage Range
A pair of complementary MOSFET devices drives the output
VOUT to within a few mV 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 EL8171
and EL8172 are internally limited to 29mA.
Gain Setting
VIN, the potential difference across IN+ and IN-, is replicated
(less the input offset voltage) across FB+ and FB-. The
obsession of the EL8171 and EL8172 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 EL8171 and EL8172
is set by two external resistors, the feedback resistor RF, and
the gain resistor RG.
2.9V to 5V
EN_BAR
VIN/2
VCM
VIN/2
71
2 IN+
VS+ EN
+
3 IN-
-
8 FB+ EL8171/2
6
+
5 FB- -
VS-
VOUT
4
RG
RF
FIGURE 28. CIRCUIT 1 - GAIN IS BY EXTERNAL RESISTORS
RF AND RG
VOUT
=
1
+
-R-----F--
V
I
N
RG
In Figure 28, the FB+ pin and one end of resistor RG are
connected to GND. With this configuration, the above gain
equation is only true for a positive swing in VIN; negative
input swings will be ignored and the output will be at ground.
Reference Connection
Unlike a three-opamp instrumentation amplifier, a finite
series resistance seen at the REF terminal does not degrade
the EL8171 and EL8172's high CMRR performance
eliminating the need for an additional external buffer
amplifier. Circuit 2 (Figure 29) uses the FB+ pin to provide a
high impedance REF terminal.
VIN/2
VIN/2
VCM
2.9V to 5V
R1
REF
R2
RG
2.9V to 5V
71
2 IN+
VS+ EN
+
3 IN-
-
8 FB+ EL8171/2
6
+
5 FB-
- VS-
4
RF
EN_BAR
VOUT
FIGURE 29. CIRCUIT 2 - GAIN SETTING AND REFERENCE
CONNECTION
VOUT
=
1
+
R-R----G-F--
(VIN)
+
1
+
R-R----G-F--
(VREF)
9
FN6293.1
March 9, 2006
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