MCP6031ESLASHST datasheet(16/30 Pages) MICROCHIP

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MCP6031/2/3/4

DS22041A-page 16

4.6

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 analog parts.

4.7

Unused Op Amps

An unused op amp in a quad package (MCP6034)

should be configured as shown in Figure 4-5. These

circuits prevent the output from toggling and causing

crosstalk. Circuit A can use any reference voltage

between the supplies, provides a buffered DC

voltage,and minimizes the supply current draw of the

unused op amp. Circuit B uses fewer components and

operates as a comparator; it may draw more current.

FIGURE 4-5:

Unused Op Amps.

4.8

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 1012

Ω. A 5V difference would

cause 5 pA of current to flow; which is greater than the

MCP6031/2/3/4 family’s bias current at +25°C

(±1.0 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 4-6.

FIGURE 4-6:

Example Guard Ring Layout

for Inverting Gain.

Non-inverting Gain and Unity-Gain Buffer:

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

input with a wire that does not touch the

PCB surface.

Connect the guard ring to the inverting input

pin (VIN–). This biases the guard ring to the

common mode input voltage.

Inverting Gain and Transimpedance Gain Ampli-

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

Connect the inverting pin (VIN–) to the input

with a wire that does not touch the PCB

surface.

VDD

¼ MCP6034 (A)

¼ MCP6034 (B)

VDD

Guard Ring

VIN–VIN+

VSS

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