MCP6V969 datasheet(24/52 Pages) MICROCHIP

24 / 52 page

MCP6V96/6U/7/9

DS20006467A-page 24

 2020 Microchip Technology Inc.

Typical thermojunctions have temperature-to-voltage

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

higher).

Microchip’s 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.

4.3.10.2

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

4.3.10.3

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 triboelectric 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 some ceramics) output small voltages. Use

more appropriate capacitor types in the signal path and

minimize mechanical stresses and vibration.

Humidity can cause electrochemical 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 4-11 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

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

enough for moderate Common-mode noise.

FIGURE 4-11:

Simple Design.

4.4.2

RESISTANCE TEMPERATURE

DETECTOR (RTD) SENSOR

The ratiometric circuit in Figure 4-12 conditions a

two-wire RTD for applications with a limited

temperature range. U1 acts as a difference amplifier

with a low-frequency pole. The sensor’s wiring

resistance (RW) is corrected in firmware. Failure (open)

of the RTD is detected by an out-of-range voltage.

FIGURE 4-12:

RTD Sensor.

VDD

100R

0.01C

ADC

VDD

0.2R

1kΩ

MCP6V96

10 nF

ADC

VDD

1.0 µF

VDD

RRTD

100Ω

1.00 kΩ

4.99 kΩ

34.8 kΩ

2.00 MΩ

10.0 kΩ

MCP6V96

10.0 kΩ

2.00 MΩ

10 nF

100 nF

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