Datasheet

MCP6L01/1R/1U/2/4
DS22140B-page 12  2009-2011 Microchip Technology Inc.
4.4 Supply Bypass
With this family of operational amplifiers, the power
supply pin (V
DD
for single supply) should have a local
bypass capacitor (i.e., 0.01 µF to 0.1 µF) within 2 mm
for good high-frequency performance. It also needs a
bulk capacitor (i.e., 1 µF or larger) within 100 mm to
provide large, slow currents. This bulk capacitor can be
shared with other nearby analog parts.
4.5 Unused Op Amps
An unused op amp in a quad package (e.g., MCP6L04)
should be configured as shown in Figure 4-3. These
circuits prevent the output from toggling and causing
crosstalk. Circuit 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.
FIGURE 4-3: Unused Op Amps.
4.6 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 10
12
. A 5V difference would
cause 5 pA of current to flow; this is greater than this
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.
Figure 4-4 shows an example of this type of layout.
FIGURE 4-4: Example Guard Ring
Layout.
1. Inverting Amplifiers (Figure 4-4) and Trans-
impedance Gain Amplifiers (convert current to
voltage, such as photo detectors).
a) Connect the guard ring to the noninverting
input pin (V
IN
+); this biases the guard ring
to the same reference voltage as the op
amp’s input (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 sur-
face.
2. Noninverting Gain and Unity-Gain Buffer.
a) Connect the guard ring to the inverting input
pin (V
IN
–); this biases the guard ring to the
common mode input voltage.
b) Connect the noninverting pin (V
IN
+) to the
input with a wire that does not touch the
PCB surface.
4.7 Application Circuit
4.7.1 ACTIVE LOW-PASS FILTER
The MCP6L01/1R/1U/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. Para-
sitic capacitances and the large value resistors could
also modify the frequency response. These trade-offs
need to be considered when selecting circuit elements.
Figure 4-5 shows a second-order Bessel filter with
100 Hz cutoff frequency and a gain of +1 V/V. The
component values were selected using Microchip’s
FilterLab
®
software; the capacitor values were reduced
to a more common range.
FIGURE 4-5: Bessel Filter.
V
DD
V
DD
¼ MCP6L04 (A) ¼ MCP6L04 (B)
R
1
R
2
V
DD
V
REF
V
REF
V
DD
R
2
R
1
R
2
+
------------------=
Guard Ring V
IN
–V
IN
+
R
1
V
IN
V
OUT
R
2
11.3 k 20.5 k
MCP6L01
C
2
68 pF
C
1
100 pF