Datasheet

MCP6V11/1U
DS25124A-page 24 © 2012 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”) 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. 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
- 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 zero-drift 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 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 RTD SENSOR
The ratiometric circuit in Figure 4-12 conditions a two-
wire RTD, for applications with a limited temperature
range. U
1
acts a difference amplifier, with a low
frequency pole. The sensor’s wiring resistance (R
W
) is
corrected in firmware. Failure (open) of the RTD is
detected by an out-of-range voltage.
FIGURE 4-12: RTD Sensor.
V
DD
RR
RR
100R
0.01C
ADC
V
DD
0.2R
0.2R
1kΩ
U
1
MCP6V11
R
F
10 nF
ADC
V
DD
R
N
1.0 µF
V
DD
R
W
R
T
R
B
R
RTD
R
G
100Ω
1.00 kΩ
4.99 kΩ
34.8 kΩ
2.00 MΩ10.0 kΩ
U
1
MCP6V11
R
W
10.0 kΩ
R
F
2.00 MΩ
10 nF
100 nF