MCP6V26 datasheet(27/50 Pages) MICROCHIP

27 / 50 page

DS25007B-page 27

MCP6V26/7/8

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 perfor-

mance. Non-linear 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 (e.g., encapsulant)

- Use ground plane (at least a star ground)

- Place the input signal source near to the DUT

- Use good PCB layout techniques

- Use a separate power supply filter (bypass

capacitors) for these auto-zeroed op amps

4.3.11.3

Miscellaneous Effects

Keep the resistances seen by the input pins as small

and as near to equal as possible, to minimize bias

current-related offsets.

Make the (trace) capacitances seen by the input pins

small and equal. This is helpful in minimizing switching

glitch-induced offset voltages.

Bending a coax cable with a radius that is too small

causes a small voltage drop to appear on the center

conductor (the tribo-electric effect). Make sure the

bending radius is large enough to keep the conductors

and insulation in full contact.

Mechanical stresses can make some capacitor types

(such as ceramic) to output small voltages. Use more

appropriate capacitor types in the signal path and

minimize mechanical stresses and vibration.

Humidity can cause electro-chemical potential voltages

to appear in a circuit. Proper PCB cleaning helps, as

does the use of encapsulants.

4.4

Typical Applications

4.4.1

WHEATSTONE BRIDGE

Many sensors are configured as Wheatstone bridges.

Strain gauges and pressure sensors are two common

examples. These signals can be small and the

common mode noise large. Amplifier designs with high

differential gain are desirable.

Figure 4-12 shows how to interface to a Wheatstone

bridge with a minimum of components. Because the

circuit is not symmetric, the ADC input is single ended,

there is a minimum of filtering, and the CMRR is good

enough for moderate common mode noise.

FIGURE 4-12:

Simple Design.

Figure 4-13 shows a higher performance circuit for

Wheatstone bridges. This circuit is symmetric and has

high CMRR. Using a differential input to the ADC helps

with the CMRR.

FIGURE 4-13:

High Performance Design.

VDD

100R

0.01C

ADC

VDD

0.2R

MCP6V26

20 k

1µF

200

20 k

1µF

ADC

VDD

200

1µF

VDD

10 nF

200

U1A

½ MCP6V27

U1B

½ MCP6V27

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