MCP6141 datasheet(13/32 Pages) MICROCHIP

13 / 32 page

 2002 Microchip Technology Inc.

21668A-page 13

MCP6141/2/3/4

3.8.3

SIGNAL COUPLING

The input pins of the MCP6141/2/3/4 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 3-8 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 3-8:

Equivalent circuit for

capacitive coupling between traces on a PC

board (with ground plane).

When noise is coupled magnetically, the 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.

3.9

Typical Applications

3.9.1

BATTERY CURRENT SENSING

The MCP6141/2/3/4 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 3-9 shows a high side battery current sensor cir-

cuit. The feedback and input resistors are sized to min-

imize power losses. The battery current (IDD) through

the 1 k

Ω resistor causes its top terminal to be more

negative than the bottom terminal. This keeps the com-

mon 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 2-26), which is a

smaller error than the offset voltage.

FIGURE 3-9:

High Side Battery Current

Sensor.

3.9.2

SUMMING AMPLIFIER

The rail-to-rail input and output, the 600 nA (typ.) qui-

escent current and the wide bandwidth make the

MCP6141/2/3/4 family of operational amplifiers fit well

in a summing amplifier circuit, as shown in Figure 3-10.

FIGURE 3-10:

Summing amplifier circuit.

VTRACE 1

RT2

CSH2

CSH1

VTRACE 2

VDD

Ω

MCP614X

100 k

Ω

VDD

IDD

+1.4 V

5.5 V

VSS

VREF

MCP614X

VOUT

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