MCP65R46T datasheet(25/36 Pages) MICROCHIP

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DS20002269C-page 25

MCP65R41/6

4.6

PCB Surface Leakage

In applications where the low input bias current is

critical, the Printed Circuit Board (PCB) surface

leakage effects need to be considered. Surface

leakage is caused by humidity, dust or other type of

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 MCP65R41/6 family’s

bias current at +25°C (1 pA, typical).

The easiest way to reduce the surface leakage is to use

a guard ring around the 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 of a Guard Ring

Layout for Inverting Circuit.

Use the following steps for an inverting configuration

(Figures 4-6):

Connect the guard ring to the noninverting 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.

Use the following steps for a noninverting configuration

(Figure 4-4):

Connect the noninverting pin (VIN+) to the input

pad without touching the guard ring.

Connect the guard ring to the inverting input pin

(VIN-).

4.7

Typical Applications

4.7.1

PRECISE COMPARATOR

Some applications require a higher DC precision. A

simple way to address this need is using an amplifier

(such as the MCP6041 – a 600 nA low power and

14 kHz bandwidth op amp) to gain-up the input signal

before it reaches the comparator. Figure 4-10 shows

an example of this approach, which also level shifts to

VPU using the Open-Drain option, the MCP65R46.

FIGURE 4-10:

Precise Inverting

Comparator.

4.7.2

BISTABLE MULTI-VIBRATOR

A simple bistable multi-vibrator design is shown in

Figure 4-11. VREF needs to be between ground and the

maximum comparator internal VREF of 2.4V to achieve

oscillation. The output duty cycle changes with VREF.

FIGURE 4-11:

Bistable Multi-Vibrator.

Guard Ring

VSS

IN-

IN+

VREF

VDD

VOUT

VIN

VREF

VPU

RPU

MCP65R46

MCP6041

VREF

VDD

VREF

VOUT

MCP65R41

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