MCP6051-ESLASHST datasheet(17/34 Pages) MICROCHIP

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DS22182A-page 17

MCP6051/2/4

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 1012

Ω. A 5V difference would

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

MCP6051/2/4 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

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

Connect the inverting pin (VIN–) to the input

with a wire that does not touch the PCB

surface.

4.7

Application Circuits

4.7.1

GYRATOR

The MCP6051/2/4 op amps can be used in gyrator

applicaitons. The gyrator is an electric circuit which can

make a capacitive circuit behave inductively. Figure 4-

shows

example

gyrator

simulating

inductance, with an approximately equivalent circuit

below. The two ZIN have similar values in typical

applications. The primary application for a gyrator is to

reduce the size and cost of a system by removing the

need for bulky, heavy and expensive inductors. For

example, RLC bandpass filter characteristics can be

realized with capacitors, resistors and operational

amplifiers without using inductors. Moreover, gyrators

will typically have higher accuracy than real inductors,

due to the lower cost of precision capacitors than

inductors.

FIGURE 4-7:

Gyrator.

Guard Ring

VIN–VIN+

VSS

VOUT

Gyrator

ZIN

ωL

LRC

ZIN

Equivalent Circuit

MCP6051

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