MCP6401 datasheet(15/38 Pages) MICROCHIP

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DS22229B-page 15

MCP6401/1R/1U/2/4

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:

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

PRECISION HALF-WAVE

RECTIFIER

The precision half-wave rectifier, which is also known

as a super diode, is a configuration obtained with an

operational amplifier in order to have a circuit behaving

like an ideal diode and rectifier. It effectively cancels the

forward voltage drop of the diode so that very low level

signals can still be rectified with minimal error. This can

be useful for high-precision signal processing. The

MCP6401/1R/1U/2/4

amps

have

high

input

impedance, low input bias current and rail-to-rail input/

output, which makes this device suitable for precision

rectifier applications.

Figure 4-8 shows a precision half-wave rectifier and its

transfer characteristic. The rectifier’s input impedance

is determined by the input resistor R1. To avoid loading

effect, it must be driven from a low impedance source.

When VIN is greater than zero, D1 is OFF, D2 is ON, and

VOUT is zero. When VIN is less than zero, D1 is ON, D2

is OFF, and VOUT is the VIN with an amplification of -R2/

R1.

The rectifier circuit shown in Figure 4-8 has the benefit

that the op amp never goes in saturation, so the only

thing

affecting

its

frequency

response

the

amplification and the gain bandwidth product.

FIGURE 4-8:

Precision Half-Wave

Rectifier.

Guard Ring

VIN–VIN+

VSS

VOUT

VIN

VOUT

VIN

-R2/R1

Transfer Characteristic

Precision Half-Wave Rectifier

MCP6401

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