MCP6544 datasheet(21/46 Pages) MICROCHIP

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DS21696G-page 21

MCP6541/1R/1U/2/3/4

4.8

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

. A 5V difference would

cause 5 pA of current to flow. This is greater than the

MCP6541/1R/1U/2/3/4 family’s bias current at 25°C

(1 pA, typ.).

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 4-9.

FIGURE 4-9:

Example Guard Ring Layout

for Inverting Circuit.

Inverting Configuration (Figures 4-6 and 4-9):

Connect the guard ring to the non-inverting

input pin (VIN+). This biases the guard ring

to the same reference voltage as the

comparator (e.g., VDD/2 or ground).

Connect the inverting pin (VIN–) to the input

pad without touching the guard ring.

Non-inverting Configuration (Figure 4-4):

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

input pad without touching the guard ring.

Connect the guard ring to the inverting input

pin (VIN–).

4.9

Unused Comparators

An unused amplifier in a quad package (MCP6544)

should be configured as shown in Figure 4-10. This

circuit prevents the output from toggling and causing

crosstalk. It uses the minimum number of components

and draws minimal current (see Figure 2-15 and

Figure 2-18).

FIGURE 4-10:

Unused Comparators.

Guard Ring

VIN-VIN+

¼ MCP6544

VDD

–

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