MCP609-ISLASHP datasheet(17/42 Pages) MICROCHIP

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DS11177F-page 17

MCP606/7/8/9

4.6

Unused Op Amps

An unused op amp in a quad package (MCP609)

should be configured as shown in Figure 4-6. These

circuits prevent the output from toggling and causing

crosstalk. Circuits A sets the op amp at its minimum

noise gain. The resistor divider produces any desired

reference voltage within the output voltage range of the

op amp; the op amp buffers that reference voltage.

Circuit B uses the minimum number of components

and operates as a comparator, but it may draw more

current.

FIGURE 4-6:

Unused Op Amps.

4.7

PCB Surface Leakage

In applications where low input bias current is critical,

Printed Circuit Board (PCB) surface-leakage effects

need to be considered. Surface leakage is caused by

humidity, dust or other contamination on the board.

Under low humidity conditions, a typical resistance

between nearby traces is 1012

Ω. A 5V difference would

cause 5 pA of current to flow, which is greater than the

MCP606/7/8/9 family’s bias current at +25°C (1 pA,

typical).

The easiest way to reduce surface leakage is to use a

guard ring around sensitive pins (or traces). The guard

ring is biased at the same voltage as the sensitive pin.

An example of this type of layout is shown in Figure 4-7.

FIGURE 4-7:

Example Guard Ring Layout

for Inverting Gain.

Non-inverting Gain and Unity-gain Buffer:

Connect the non-inverting pin (VIN+) to the

input with a wire that does not touch the

PCB surface.

Connect the guard ring to the inverting input

pin (VIN–). This biases the guard ring to the

common mode input voltage.

Inverting

Gain

and

Transimpedance

Gain

(convert current to voltage, such as photo

detectors) amplifiers:

Connect the guard ring to the non-inverting

input pin (VIN+). This biases the guard ring

to the same reference voltage as the op

amp (e.g., VDD/2 or ground).

Connect the inverting pin (VIN–) to the input

with a wire that does not touch the PCB

surface.

4.8

Application Circuits

4.8.1

LOW-SIDE BATTERY CURRENT

SENSOR

The MCP606/7/8/9 op amps can be used to sense the

load current on the low-side of a battery using the

circuit in Figure 4-8. In this circuit, the current from the

power supply (minus the current required to power the

MCP606) flows through a sense resistor (RSEN), which

converts it to voltage. This is gained by the the amplifier

and resistors, RG and RF.Since the non-inverting input

of the amplifier is at the load’s negative supply (VLM),

the gain from RSEN to VOUT is RF/RG.

FIGURE 4-8:

Low Side Battery Current

Sensor.

Since the input bias current and input offset voltage of

the MCP606 are low, and the input is capable of

swinging below ground, there is very little error

generated by the amplifier. The quiescent current is

very low, which helps conserve battery power. The

rail-to-rail output makes it possible to read very low

currents.

VDD

VREF

REF

-------------------

⋅

¼ MCP609 (A)

¼ MCP609 (B)

Guard Ring

VSS

VIN-VIN+

OUT

SEN

⁄

()

To Load

2.5V

50 k

To Load

VOUT

RSEN

(VLM)

(VLP)

6.0V

MCP606

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