MCP6001/2/4
4.6
4.6.1
Application Circuits
UNITY-GAIN BUFFER
V
IN
14.3 kΩ 53.6 kΩ
100 pF
The rail-to-rail input and output capability of the
MCP6001/2/4 op amp is ideal for unity-gain buffer
applications. The low quiescent current and wide
bandwidth makes the device suitable for a buffer
configuration in an instrumentation amplifier circuit, as
shown in Figure 4-6.
+
MCP6002
33 pF
–
V
OUT
1/2
MCP6002
V
IN1
+
–
R
2
R
1
FIGURE 4-7:
Low- Pass Filter.
4.6.3
Active Second-Order
PEAK DETECTOR
–
MCP6001
+
1/2
MCP6002
V
IN2
+
R
1
V
REF
–
R
2
V
OUT
R
1
= 20 kΩ
R
2
= 10 kΩ
The MCP6001/2/4 op amp has a high input impedance,
rail-to-rail input/output and low input bias current, which
makes this device suitable for peak detector applica-
tions. Figure 4-8 shows a peak detector circuit with
clear and sample switches. The peak-detection cycle
uses a clock (CLK), as shown in Figure 4-8.
At the rising edge of CLK, Sample Switch closes to
begin sampling. The peak voltage stored on C
1
is sam-
pled to C
2
for a sample time defined by t
SAMP
. At the
end of the sample time (falling edge of Sample Signal),
Clear Signal goes high and closes the Clear Switch.
When the Clear Switch closes, C
1
discharges through
R
1
for a time defined by t
CLEAR
. At the end of the clear
time (falling edge of Clear Signal), op amp A begins to
store the peak value of V
IN
on C
1
for a time defined by
t
DETECT
.
In order to define t
SAMP
and t
CLEAR
, it is necessary to
determine the capacitor charging and discharging
period. The capacitor charging time is limited by the
amplifier source current, while the discharging time (
τ
)
is defined using R
1
(
τ
= R
1
C
1
). t
DETECT
is the time that
the input signal is sampled on C
1
and is dependent on
the input voltage change frequency.
The op amp output current limit, and the size of the
storage capacitors (both C
1
and C
2
), could create slew-
ing limitations as the input voltage (V
IN
) increases.
Current through a capacitor is dependent on the size of
the capacitor and the rate of voltage change. From this
relationship, the rate of voltage change or the slew rate
can be determined. For example, with an op amp short-
circuit current of I
SC
= 25 mA and a load capacitor of
C
1
= 0.1 µF, then:
R
1
-
V
OUT
=
(
V
IN2
–
V
IN1
) •
----- +
V
REF
R
2
FIGURE 4-6:
Instrumentation Amplifier
with Unity-Gain Buffer Inputs.
4.6.2
ACTIVE LOW-PASS FILTER
The MCP6001/2/4 op amp’s low input bias current
makes it possible for the designer to use larger resis-
tors and smaller capacitors for active low-pass filter
applications. However, as the resistance increases, the
noise generated also increases. Parasitic capacitances
and the large value resistors could also modify the fre-
quency response. These trade-offs need to be
considered when selecting circuit elements.
Usually, the op amp bandwidth is 100X the filter cutoff
frequency (or higher) for good performance. It is possi-
ble to have the op amp bandwidth 10X higher than the
cutoff frequency, thus having a design that is more
sensitive to component tolerances.
100 kHz cutoff frequency and a gain of +1 V/V; the op
amp bandwidth is only 10X higher than the cutoff
frequency. The component values were selected using
Microchip’s FilterLab
®
software.
EQUATION 4-1:
dV
C1
-
I
SC
=
C
1
------------
dt
dV
C1
I
SC
------------ = -------
-
-
dt
C
1
25mA
-
= --------------
0.1μF
dV
C1
------------ =
250mV
⁄ μs
-
dt
©
2005 Microchip Technology Inc.
DS21733F-page 10
MICROCHIP [ MICROCHIP TECHNOLOGY ]
