MCP6041 datasheet(13/32 Pages) MICROCHIP

13 / 32 page

 2002 Microchip Technology Inc.

DS21669B-page 13

MCP6041/2/3/4

3.7.2

COMPONENT PLACEMENT

Separate digital from analog and low speed from high

speed. This helps prevent crosstalk.

Keep sensitive traces short and straight. Separate

them from interfering components and traces. This is

especially important for high frequency (low rise time)

signals.

Use a 0.1 µF supply bypass capacitor within 0.1”

(2.5 mm) of the VDD pin. It must connect directly to the

ground plane.

3.7.3

SIGNAL COUPLING

The input pins of the MCP6041/2/3/4 family of op amps

are high impedance, which allows noise injection. This

noise can be capacitively or magnetically coupled. In

either case, using a ground plane helps reduce noise

injection.

When noise is coupled capacitively, the ground plane

provides shunt capacitance to ground for high fre-

quency signals. Figure 3-6 shows the equivalent cir-

cuit. The coupled current, IM, produces a lower voltage

(VTRACE 2) on the victim trace when the trace to ground

plane capacitance (CSH2) is large and the terminating

resistor (RT2) is small. Increasing the distance between

traces, and using wider traces, also helps.

FIGURE 3-6:

Equivalent circuit for

capacitive coupling between traces on a PC

board (with ground plane).

When noise is coupled magnetically, ground plane

reduces the mutual inductance between traces. This

occurs because the ground return current at high fre-

quencies will follow a path directly beneath the signal

trace. Increasing the separation between traces makes

a significant difference. Changing the direction of one

of the traces can also reduce magnetic coupling.

If these techniques are not enough, it may help to place

guard traces next to the victim trace. They should be on

both sides of the victim trace and be as close as possi-

ble. Connect the guard traces to ground plane at both

ends, and in the middle, for long traces.

3.8

Typical Applications

3.8.1

BATTERY CURRENT SENSING

The MCP6041/2/3/4 op amps’ Common Mode Input

Range, which goes 300 mV beyond both supply rails,

supports their use in high side and low side battery

current sensing applications. The very low quiescent

current (0.6 µA, typ) help prolong battery life while the

rail-to-rail output allows you to detect low currents.

Figure 3-7 shows a high side battery current sensor cir-

cuit. The 10

Ω resistors are sized to minimize power

losses. The battery current (IDD) through the 10 Ω

resistor causes its top terminal to be more negative

than the bottom terminal. This keeps the common

mode input voltage of the op amp

≤ VDD, which is within

its allowed range. The output of the op amp can reach

VDD - 0.1 mV (see Figure 2-32), which is a smaller

error than the offset voltage.

FIGURE 3-7:

High Side Battery Current

Sensor.

3.8.2

INSTRUMENTATION AMPLIFIER

The MCP6041/2/3/4 op amp is well suited for condition-

ing sensor signals in battery-powered applications.

Figure 3-8 shows a two op amp instrumentation

amplifier, using the MCP6042, that works well for appli-

cations requiring rejection of common mode noise at

higher gains. The reference voltage (VREF) is supplied

by a low impedance source. In single supply

applications, VREF is typically VDD/2.

FIGURE 3-8:

Two Op Amp

Instrumentation Amplifier.

VTRACE 1

RT2

CSH2

CSH1

VTRACE 2

VDD

Ω

MCP604X

100 k

Ω

VDD

IDD

+2.5 V

5.5 V

VSS

VREF

½ MCP6042

VOUT

½ MCP6042

VOUT

V1 V2

–

() 1

------

2R1

---------



 V

REF

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