MCP6V5XTESLASHMS datasheet(26/50 Pages) MICROCHIP

26 / 50 page

MCP6V51/2/4

DS20006136B-page 26

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Some applications may modify these values to reduce

either output loading or gain peaking (step response

overshoot).

At high gains, RN needs to be small in order to prevent

positive feedback and oscillations. Large CN values

can also help.

4.3.9

REDUCING UNDESIRED NOISE

AND SIGNALS

Reduce undesired noise and signals with:

• Low Bandwidth Signal Filters:

- Minimize random analog noise

- Reduce interfering signals

• Good PCB Layout Techniques:

- Minimize crosstalk

- Minimize parasitic capacitances and

inductances that interact with fast switching

edges

• Good Power Supply Design:

- Isolation from other parts

- Filtering of interference on supply line(s)

4.3.10

SUPPLY BYPASSING AND

FILTERING

With this operational amplifier, the power supply pins

(only VDD for single supply) should have a low-ESR

ceramic bypass capacitor (i.e., 0.01 µF to 0.1 µF) within

2 mm of the pins for good high-frequency decoupling.

It is recommended to place a bulk capacitor (i.e., 1 µF

or larger) within 100 mm of the device to provide large,

slow currents. This bulk capacitor can be shared with

other low-noise analog parts.

In some cases, high-frequency power supply noise

(e.g., Switched-Mode Power Supplies) may cause

undue intermodulation distortion with a DC offset shift;

this noise needs to be filtered. Adding a small resistor

or ferrite bead into the supply connection can be

helpful.

4.3.11

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 have a strong impact on the

precision achieved. A poor PCB design can easily be

more than 100 times worse than the MCP6V51/2/4

op amps’ specifications.

4.3.11.1

PCB Layout

Any time two dissimilar metals are joined together, a

temperature-dependent voltage appears across the

junction (the Seebeck or thermojunction effect). This

effect is used in thermocouples to measure tempera-

ture. The following are examples of thermojunctions on

a PCB:

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

a copper pad

• Wires mechanically attached to the PCB

• Jumpers

• Solder joints

• PCB vias

Typical thermojunctions have temperature-to-voltage

conversion coefficients of 1 to 100 µV/°C (sometimes

higher).

Microchip’s Application Note – AN1258, “Op Amp

Precision Design: PCB Layout Techniques” (DS01258)

contains in-depth information on PCB layout techniques

that minimize thermojunction effects. It also discusses

other effects, such as crosstalk, impedances,

mechanical stresses and humidity.

4.3.11.2

Crosstalk

DC crosstalk causes offsets that appear as a larger

input offset voltage. Common causes include:

• Common-Mode noise (remote sensors)

• Ground loops (current return paths)

• Power supply coupling

Interference from the mains (usually 50 Hz or 60 Hz)

and other AC sources can also affect the DC

performance. Nonlinear distortion can convert these

signals to multiple tones, including a DC shift in voltage.

When the signal is sampled by an ADC, these AC

signals can also be aliased to DC, causing an apparent

shift in offset.

To reduce interference:

• Keep traces and wires as short as possible

• Use shielding

• Use ground plane (at least a star ground)

• Place the input signal source near the DUT

• Use good PCB layout techniques

• Use a separate power supply filter (bypass

capacitors) for these zero-drift op amps

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