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MCP6V26/7/8

The largest value of R

that should be used depends

on noise gain (see G

in Section 4.3.6, Capacitive

Loads), C

and the open-loop gain’s phase shift. An

approximate limit for R

is:

EQUATION 4-2:

Some applications may modify these values to reduce

either output loading or gain peaking (step response

overshoot).

At high gains, R

and C

need to be small in order to

prevent positive feedback and oscillations.

4.3.9 REDUCING UNDESIRED NOISE

AND SIGNALS

Reduce undesired noise and signals with:

• Low bandwidth signal filters:

- Minimizes random analog noise

- Reduces interfering signals

• Good PCB layout techniques:

- Minimizes crosstalk

- Minimizes parasitic capacitances and

inductances that interact with fast switching

edges

• Good power supply design:

- Provides isolation from other parts

- Filters interference on supply line(s)

4.3.10 SUPPLY BYPASSING AND

FILTERING

With this family of op amps, the power supply pin (V

for single supply) should have a local bypass capacitor

(i.e., 0.01 µF to 0.1 µF) within 2 mm of the pin for good

high-frequency performance.

These parts also need a bulk capacitor (i.e., 1 µF or

larger) within 100 mm 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 resistor into the

supply connection can be helpful. This resistor needs

to be small enough to prevent a large drop in V

for

the op amp, which would cause a reduced output range

and possible load-induced power supply noise. It also

needs to be large enough to dissipate little power when

is turned on and off quickly. Figure 4-11 shows a

circuit with resistors in the supply connections. It gives

good rejection out to 1 MHz for switched mode power

supplies. Smaller resistors and capacitors are a better

choice for designs where the power supply is not as

noisy.

FIGURE 4-11: Additional Supply Filtering.

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 has a strong impact on the

precision achieved. A poor PCB design can easily be

more than 100 times worse than the MCP6V26/7/8 op

amps minimum and maximum 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 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).

Microchip’s AN1258 (“Op Amp Precision Design: PCB

Layout Techniques”) contains in depth information on

PCB layout techniques that minimize thermo-junction

effects. It also discusses other effects, such as

crosstalk, impedances, mechanical stresses and

humidity.

2 k

12 pF

---------------

×≤

S_ANA

50Ω 50Ω

100 µF

0.1 µF

1/4W 1/10W

to other analog parts

MCP6V2X