MCP6L1RT datasheet(12/32 Pages) MICROCHIP

12 / 32 page

MCP6L1/1R/2/4

DS22135A-page 12

4.4

Supply Bypass

With this family of operational amplifiers, the power

supply pin (VDD 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., MCP6L4)

should be configured as shown in Figure 4-4. 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 4-4:

Unused Op Amps.

4.6

PCB Surface Leakage

In applications where low input bias current is critical,

PCB (printed circuit board) 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 1012

Ω. 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 4-5 shows an example of this type of layout.

FIGURE 4-5:

Example guard ring layout.

Inverting Amplifiers (Figure 4-5) and Transim-

pedance Gain Amplifiers (convert current to

voltage, such as photo detectors).

Connect the guard ring to the non-inverting

input pin (VIN+); this biases the guard ring

to the same reference voltage as the op

amp’s input (e.g., VDD/2 or ground).

Connect the inverting pin (VIN–) to the input

with a wire that does not touch the PCB sur-

face.

Non-inverting Gain and Unity-Gain Buffer.

Connect the guard ring to the inverting input

pin (VIN–); this biases the guard ring to the

common mode input voltage.

Connect the non-inverting pin (VIN+) to the

input with a wire that does not touch the

PCB surface.

VDD

¼ MCP6L4 (A)

¼ MCP6L4 (B)

VDD

VREF

REF

------------------

⋅

Guard Ring

VIN–VIN+

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