MCP6271R Microchip Technology Inc., MCP6271R Datasheet - Page 17

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

Non-Inverting Configurations With Gain Built-in
The non-inverting configuration is more forgiving than the buffer
circuit. With the non-inverting amplifier configuration (Figure 2.),
the gain of the circuit is greater than +1V/V. This circuit will send
the voltage applied to V
circuit.
This circuit is not hard to convert between a dual- and single-
supply environment. The input stage limitations do not come
into play as much as they did with the buffer circuit. Most single-
supply amplifiers have an input, common-mode-range to ground.
This amplifier circuit will gain any signal on the input, V
higher value. It is probable that the output stage will limit the
signal on the positive supply rail before the input stage can reach
any internal limits.
Photo-Sensing Problems
The transimpedance amplifier that is used for photo-sensing
applications is a little more difficult to convert from dual- to
single-supply environments. A primary issue is to correctly bias
the photo sensor across the input terminals. Figure 3 illustrates
the proper connection.
Figure 3: Grounding the anode of the photo sensor and attaching
cathode to R
a positive voltage at V
reversed across the amplifier-input terminals, the sensor current
will be reversed causing the output of the amplifier to remain on
the ground rail.
Figure 2: The non-inverting, gained amplifier circuit is also easy to
transfer from a dual-supply environment to a single-supply system.
Since the signal is gained by the equation (1+ R
output, the input stage is not exercised as much as it is with the
buffer. The output stage is more likely to over-range because of the
gain in the system.
Source
V
Light
IN
Operational Amplifiers
F
allows the sensor current to flow through R
I
SC
OUT
V
R
OUT
1
IN
. If the polarity of the photo sensor is
V
–
+
MCP601
and gain it using the resistors in the
OUT
= V
R
F
= + I
IN
(1 + R
V
+
–
DD
SC
R
R
V
F
2
2
OUT
/R
1
)
2
/R
–
+
1
) to the
V
MCP3201
REF
ADC
IN
F
, to a
creating
V
OUT
V
DD
In this configuration (Figure 3), the current from the photo diode
during excitation from a light source will change the output of
the amplifier in a positive direction. This arrangement appears to
work well except for the fact that the amplifier’s output is unable
to swing all the way down to ground. Consequently, smaller
magnitudes of luminance from the light source will be not be
registered by the A/D converter.
Figure 4 provides a solution to this problem. The output of the
amplifier is raised by 300 mV (from ground) to ensure that the
amplifier is operating in its linear region. A level-shift network,
consisting of R
Figure 4: R
divider. The output of this divider is 300 mV. A buffer, A
the impedance of R
photo sensor. This circuit configuration creates a 300 mV level
shift of the signal to the output of the transimpedance amplifier
(A
inputs.
The conditions of the open-loop gain specification of the
MCP6022 are “V
circuit creates a 300 mV level shift of the signal so that the
amplifier, A
is critical. A
sensing circuit in a timely manner. Consequently, A
as fast or faster than A
requirements. A precision voltage reference (replacing V
top of R
The Inverting Configuration Will Surprise You
The inverting-amplifier configuration (Figure 5) will only work in
a single-supply circuit if you have a voltage reference. You can
imagine that by connecting the voltage reference node, V
to ground, the circuit would only work if the input signal were
negative. Figure 6 shows one example of a voltage-reference
circuit.
1
R
300 mV
R
). The A/D converter also has this level shift applied to its
1
2
1
1/2 of
MCP6022
+
–
will add stability and reduce noise.
1
1
2
, remains in its linear region. The selection of A
Analog and Interface Guide – Volume 2
and R
A
must be able to supply current to the photo-
2
1
, R
OUT
2
2
1
Source
, in conjunction with V
and A
Light
= V
and R
1
SS
. Dual amplifiers work well with these
V
2
2
+300 mV and V
, achieves this goal.
OUT
while connecting to the anode of the
= + I
I
SC
1/2 of
MCP6022
+
SC
–
R
R
F
F
V
A
DD
DD
1
DD
, form a voltage
– 300 mV.” This
V
OUT
2
must be
–
+
2
, isolates
V
MCP3201
DD
REF
SHIFT
ADC
A
) at the
3
2
,
V
15
DD