Datasheet

© 2009 Microchip Technology Inc. DS21733J-page 15
MCP6001/1R/1U/2/4
FIGURE 4-6: Example Guard Ring Layout
for Inverting Gain.
1. Non-inverting Gain and Unity-Gain Buffer:
a. Connect the non-inverting pin (V
IN
+) to the
input with a wire that does not touch the
PCB surface.
b. Connect the guard ring to the inverting input
pin (V
IN
–). This biases the guard ring to the
common mode input voltage.
2. Inverting Gain and Transimpedance Gain
Amplifiers (convert current to voltage, such as
photo detectors):
a. Connect the guard ring to the non-inverting
input pin (V
IN
+). This biases the guard ring
to the same reference voltage as the op
amp (e.g., V
DD
/2 or ground).
b. Connect the inverting pin (V
IN
–) to the input
with a wire that does not touch the PCB
surface.
4.7 Application Circuits
4.7.1 UNITY-GAIN BUFFER
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 4-7.
FIGURE 4-7: Instrumentation Amplifier
with Unity-Gain Buffer Inputs.
4.7.2 ACTIVE LOW-PASS FILTER
The MCP6001/2/4 op amp’s low input bias current
makes it possible for the designer to use larger
resistors 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 frequency 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
possible to have the op amp bandwidth 10X higher
than the cutoff frequency, thus having a design that is
more sensitive to component tolerances.
Figure 4-8 shows a second-order Butterworth filter with
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.
FIGURE 4-8: Active Second-Order
Low-Pass Filter.
Guard Ring
V
SS
V
IN
-V
IN
+
V
IN1
R
2
MCP6002
V
IN2
R
2
MCP6002
V
REF
MCP6001
V
OUT
R
1
R
1
–
+
–
+
–
+
1/2
1/2
V
OUT
V
IN2
V
IN1
–()
R
1
R
2
------
• V
REF
+=
R
1
= 20 kΩ
R
2
= 10 kΩ
14.3 kΩ
MCP6002
V
OUT
53.6 kΩ
100 pF
V
IN
33 pF
+
–