MCP604-I Datasheet, PDF (14/34 Pages) Microchip Technology – 2.7V to 6.0V Single Supply CMOS Op Amps

MCP601/1R/2/3/4
4.6 Unused Op Amps
An unused op amp in a quad package (MCP604)
should be configured as shown in Figure 4-6. These
circuits prevent the output from toggling and causing
crosstalk. Circuits A sets the op amp at its minimum
noise gain. The resistor divider produces any desired
reference voltage within the output voltage range of the
op amp; the op amp buffers that reference voltage.
Circuit B uses the minimum number of components
and operates as a comparator, but it may draw more
current.
¼ MCP604 (A)
V_DD
R₁
V_DD
R₂
V_REF
¼ MCP604 (B)
V_DD
V_REF
=
V_DD
⋅
-------^R----²-------
R₁ + R₂
FIGURE 4-6:
Unused Op Amps.
4.7 PCB Surface Leakage
In applications where low input bias current is critical,
printed circuit board (PCB) surface leakage effects
need to be considered. Surface leakage is caused by
humidity, dust or other contamination on the board.
Under low humidity conditions, a typical resistance
between nearby traces is 10¹²Î©. A 5V difference
would cause 5 pA of current to flow. This is greater
than the MCP601/1R/2/3/4 family’s bias current at
+25°C (1 pA, typical).
The easiest way to reduce surface leakage is to use a
guard ring around sensitive pins (or traces). The guard
ring is biased at the same voltage as the sensitive pin.
An example of this type of layout is shown in
Figure 4-7.
Guard Ring V_IN– V_IN+
FIGURE 4-7:
Example Guard Ring layout.
1. Connect the guard ring to the inverting input pin
(V_IN–) for non-inverting gain amplifiers, includ-
ing unity-gain buffers. This biases the guard ring
to the common mode input voltage.
2. Connect the guard ring to the non-inverting input
pin (V_IN+) for inverting gain amplifiers and
transimpedance amplifiers (converts current to
voltage, such as photo detectors). This biases
the guard ring to the same reference voltage as
the op amp (e.g., V_DD/2 or ground).
4.8 Typical Applications
4.8.1 ANALOG FILTERS
Figure 4-8 and Figure 4-9 show low-pass, second-
order, Butterworth filters with a cutoff frequency of
10 Hz. The filter in Figure 4-8 has a non-inverting gain
of +1 V/V, and the filter in Figure 4-9 has an inverting
gain of -1 V/V.
C₁
47 nF
R₁
R₂
382 kΩ 641 kΩ
V_IN
C₂
22 nF
G = +1 V/V
f_P = 10 Hz
+
MCP60X
–
V_OUT
FIGURE 4-8:
Second-Order, Low-Pass
Sallen-Key Filter.
R₂
618 kΩ
G = -1 V/V
f_P = 10 Hz
R₁
R₃
618 kΩ 1.00 MΩ
V_IN
C₂
47 nF
V_DD/2
C₁
8.2 nF
–
MCP60X
+
V_OUT
FIGURE 4-9:
Second-Order, Low-Pass
Multiple-Feedback Filter.
The MCP601/1R/2/3/4 family of op amps have low
input bias current, which allows the designer to select
larger resistor values and smaller capacitor values for
these filters. This helps produce a compact PCB layout.
These filters, and others, can be designed using
Microchip’s Design Aids; see Section 5.2 “FilterLab®
Software” and Section 5.3 “Mindi™ Simulatior
Tool”.
DS21314G-page 14
© 2007 Microchip Technology Inc.