MCP6271R Microchip Technology Inc., MCP6271R Datasheet - Page 19

• Manufacturer Part Number: MCP6271R
• Description: 170 ?a, 2 Mhz Rail-to-rail Op Amp
• Manufacturer: Microchip Technology Inc.

The Three-Amp Instrumentation Amplifier
The most versatile instrumentation amplifier configuration
uses three operational amplifiers in its implementation. This
instrumentation amplifier is easy to understand because each of
the three operational amplifiers serves a specific function. With
this circuit configuration (shown in Figure 9), two of the three
operational amplifiers (A
third amplifier, A
providing a single-ended output. The transfer function of the
circuit is equal to:
Figure 9: The three-amp instrumentation amplifier has two
fundamental stages. The first stage (gain stage gains the input
signals and present the results to the second stage (difference
amplifier).
In single-supply applications, the circuits in Figure 8 generate the
center-supply reference, V
The Two-Amp Instrumentation Amplifier
The design shown in Figure 10 uses two operational amplifiers.
This design configuration is typically called the two op-amp
instrumentation amplifier.
Figure 10: Dual- and single- supply can power the two amp
instrumentation amplifier. Single-supply circuits require a
center-supply reference, V
V
V
IN-
IN+
V
Operational Amplifiers
V
SHIFT
OUT
V
V
IN-
IN+
= (V
R
IN
1
+ – V
3
R
, subtracts the two gained input signals, thereby
G
Gain Stage
IN
–
+
-)(1 + 2R
+
+
–
A
–
R
1
1
SHIFT
A
A
2
SHIFT
and A
2
1
R
R
V
F
F
DD
.
.
F
/R
2
R
) gain the two input signals. The
G
G
) (R
R
R
2
R
2
Difference Amplifier
1
1
/R
R
V
1
+
–
)
2
SHIFT
–
+
A
R
3
2
A
R
2
1
V
OUT
V
OUT
Discrete designs use dual amplifiers for good matching
of bandwidth and performance over temperature. This
instrumentation amplifier utilizes the high impedance of the non-
inverting input of the operational amplifiers, thereby significantly
reducing source impedance mismatch problems at DC. The
transfer function of this circuit is equal to:
If the application is in a single-supply environment, this circuit will
typically require a reference that is half of the way between the
power-supply voltages. In Figure 10, V
This circuit does not allow for zero-volt, common mode, input
voltages in single-supply systems.
The center-supply reference, V
in Figure 8.
Conclusion
The task of transferring your dual-supply amplifier circuits
to single-supply environments is straightforward. During the
conversion, pay attention to input common-mode range and
output-swing specification violations. If any violations of the
single-supply amplifier’s input or output stage occur as a result of
the new power supplies, the addition of a voltage reference can
eliminate these violations, making the circuit usable.
References
Input Operation Really Mean?”, Bonnie C. Baker, AnalogZone,
March 22, 2004.
Operational-Amplifier Output Characteristics”, Bonnie C. Baker,
AnalogZone, May 3, 2004.
“Operational Amplifiers Part 1 of 6: What Does “Rail-to-Rail”
“Operational Amplifiers Part 2 of 6: Working with Single-Supply
V
OUT
= (V
Analog and Interface Guide – Volume 2
IN
+ – V
IN
-)(1 + R
1
SHIFT
/R
2
, is implemented using circuits
+ 2R
SHIFT
1
/R
serves that function.
G
) + V
SHIFT
17