EL8170 Datasheet, PDF (12/14 Pages) Intersil Corporation – Micropower, Single Supply, Rail-to-Rail Input-Output Instrumentation Amplifiers

VIN/2
VIN/2
VCM
RG
+2.4V TO +5.5V
71
3 IN+
V+
+
2 IN-
- _EL8170,
6
8 FB+ EL8173
+
5 FB- _-
V-
4
RF
EL8170, EL8173
VOUT
VIN/2
VIN/2
+2.4V TO +5.5V
VCM
R1
REF
R2 _RG
+2.4V TO +5.5V
71
3 IN+
V+
+
2 IN-
- _{EL8170, 6}
8
FB+
+
EL8173
5 FB- _-
V-
4
RF
VOUT
FIGURE 37. GAIN IS SET BY TWO EXTERNAL RESISTORS,
R_F AND R_G
V_OUT
=
⎛
⎜
⎝
1
+
R^-R----_G^-F--_{⎠⎟⎞}
V_IN
(EQ. 1)
In Figure 37, the FB+ pin and one end of resistor R_G are
connected to GND. With this configuration, Equation 1 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 op amp instrumentation amplifier, a finite
series resistance seen at the REF terminal does not degrade
the EL8170 and EL8173's high CMRR performance,
eliminating the need for an additional external buffer
amplifier. The circuit shown in Figure 38 uses the FB+ pin as
a REF terminal to center or to adjust the output. Because the
FB+ pin is a high impedance input, an economical resistor
divider can be used to set the voltage at the REF terminal.
The reference voltage error due to the input bias current is
minimized by keeping the values of the voltage divider
resistors, R₁ and R₂, as low as possible. Any voltage applied
to the REF terminal will shift V_OUT by VREF times the closed
loop gain, which is set by resistors R_F and R_G according to
Equation 2. Note that any noise or unwanted signals on the
reference supply will be amplified at the output according to
Equation 2.
V_OUT
=
⎛
⎜1
⎝
+
R^-R----_G^-F--_{⎠⎟⎞}
(V_IN)
+
⎛
⎜1
⎝
+
R^-R----_G^-F--_{⎠⎟⎞}
(V_REF)
(EQ. 2)
FIGURE 38. GAIN SETTING AND REFERENCE CONNECTION
The FB+ pin can also be connected to the other end of
resistor, R_G (see Figure 39). Keeping the basic concept that
the EL8170 and EL8173 in-amps maintain constant
differential voltage across the input terminals and feedback
terminals (IN+ - IN- = FB+ - FB-), the transfer function of
Figure 39 can be derived (Equation 3). Note that the V_REF
gain term is eliminated, and susceptibility to external noise is
reduced.
+2.4V TO +5.5V
VIN/2
VIN/2
VCM
71
3 IN+
V+
+
2 IN-
- _EL8170,
6
8 FB+ EL8173
+
5 FB- _-
V-
4
VOUT
VREF
RG
RF
FIGURE 39. REFERENCE CONNECTION WITH AN AVAILABLE
V_REF
V_OUT
=
⎛
⎜
⎝
1
+
R^-R----_G^-F--_{⎠⎟⎞}
(V_IN
)
+
(
V_REF
)
(EQ. 3)
External Resistor Mismatches
Because of the independent pair of feedback terminals
provided by the EL8170 and EL8173, the CMRR is not
degraded by any resistor mismatches. Hence, unlike a three
op amp and especially a two op amp in-amp, the EL8170
and EL8173 reduce the cost of external components by
allowing the use of 1% or more tolerance resistors without
sacrificing CMRR performance. The EL8170 and EL8173
CMRR is maintained regardless of the tolerance of the
resistors used.
12
FN7490.6
August 31, 2009