MCP6H81_12 datasheet(19/42 Pages) MICROCHIP

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DS22320B-page 19

MCP6H81/2/4

4.6

PCB Surface Leakage

In applications where low input bias current is critical,

PCB surface leakage effects need to be considered.

Surface leakage is caused by humidity, dust or other

contamination on the board. Under low-humidity condi-

tions, a typical resistance between nearby traces is

1012

. A 15V difference would cause 15 pA of current

to flow, which is greater than the MCP6H81/2/4 family’s

bias current at +25°C (10 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 4-7.

FIGURE 4-7:

Example Guard Ring Layout

for Inverting Gain.

Non-Inverting Gain and Unity-Gain Buffer:

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

input with a wire that does not touch the

PCB surface.

b.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):

a.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).

b.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

DIFFERENCE AMPLIFIER

The MCP6H81/2/4 op amps can be used in current

sensing applications. Figure 4-8 shows a resistor

(RSEN) that converts the sensor current (ISEN) to

voltage, as well as a difference amplifier that amplifies

the voltage across the resistor while rejecting common

mode noise. R1 and R2 must be well matched to obtain

acceptable

Common

Mode

Rejection

Ratio

(CMRR). Moreover, RSEN should be much smaller than

R1 and R2 in order to minimize the resistive loading of

the source.

To ensure proper operation, the op amp common mode

input voltage must be kept within the allowed range.

The reference voltage (VREF) is supplied by a

low-impedance source. In single-supply applications,

VREF is typically VDD/2.

FIGURE 4-8:

High Side Current Sensing

Using Difference Amplifier.

Guard Ring

VIN–VIN+

VSS

VOUT

RSEN

ISEN

RSEN << R1, R2

OUT

–



------





REF

VREF

VDD

MCP6H81

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