MCP6V06T datasheet(26/44 Pages) MICROCHIP

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MCP6V06/7/8

DS22093B-page 26

4.3.9

PCB DESIGN FOR DC PRECISION

In order to achieve DC precision on the order of ±1 µV,

many physical errors need to be minimized. The design

of the Printed Circuit Board (PCB), the wiring, and the

thermal environment has a strong impact on the

precision achieved. A poor PCB design can easily be

more than 100 times worse than the MCP6V06/7/8 op

amps minimum and maximum specifications.

4.3.9.1

Thermo-junctions

Any time two dissimilar metals are joined together, a

temperature dependent voltage appears across the

junction (the Seebeck or thermo-junction effect). This

effect is used in thermocouples to measure tempera-

ture. The following are examples of thermo-junctions

on a PCB:

• Components (resistors, op amps, …) soldered to

a copper pad

• Wires mechanically attached to the PCB

• Jumpers

•Solder joints

•PCB vias

Typical thermo-junctions have temperature to voltage

conversion coefficients of 10 to 100 µV/°C (sometimes

higher).

There are three basic approaches to minimizing

thermo-junction effects:

• Minimize thermal gradients

• Cancel thermo-junction voltages

• Minimize difference in thermal potential between

metals

4.3.9.2

Non-inverting and Inverting Amplifier

Layout for Thermo-junctions

Figure 4-11 shows the recommended non-inverting

and inverting gain amplifier circuits on one schematic.

Usually, to minimize the input bias current related off-

set, R1 is chosen to be R2||R3.

The guard traces (with ground vias at the ends) help

minimize the thermal gradients. The resistor layout

cancels the resistor thermal voltages, assuming the

temperature gradient is constant near the resistors:

EQUATION 4-2:

FIGURE 4-11:

PCB Layout and Schematic

for Single Non-inverting and Inverting Amplifiers.

Note:

Changing the orientation of the resistors

will usually cause a significant decrease in

the cancellation of the thermal voltages.

VOUT ≈ VPGP,VM =GND

≈ -VMGM,VP =GND

Where:

GM =R3/R2, inverting gain magnitude

GP =1 + GM, non-inverting gain

magnitude

VOS is neglected

VOUT

MCP6V06

VOUT

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