LMC66 datasheet(20/24 Pages) NSC

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Filters (Continued)

The LMC6681/2/4, with its rail-to-rail input common-mode

voltage range and high gain (120 dB typical, R

L = 10 kΩ)is

extremely well suited for such filter applications. The

rail-to-rail input range allows for large input signals to be pro-

cessed without distortion. The high gain means that the cir-

cuit can provide filtering and gain in one stage, instead of the

typical two stage filter. This implies a reduction in cost, and

savings of space and power.

This is an illustration of the conceptual use of the LMC6681/

2/4. The selectivity of the filter can be improved by increas-

ing the order (number of poles) of the design.

Sample-and-Hold Circuits

When the “Switch” is closed during the Sample Interval,

HOLD charges up to the value of the input signal when the

“Switch” is open, C

HOLD retains this value as it is buffered by

the high input impedance of the LMC6681.

Errors in the “hold” voltage are caused by the input current of

the amplifier, the leakage current of the CD4066, and the

leakage current of the capacitor. While an input current of 80

fA minimizes the accumulation rate for error in this circuit, the

LMC6681’s CMRR of 82 dB allows excellent accuracy

throughout the amplifier’s rail-to-rail dynamic capture range.

Battery Monitoring Circuit

The LMC6681/2/4 has been optimized for performance at

3V, and also has guaranteed specs at 1.8V and 2.2V. In por-

table applications, the R

LOAD

represents the laptop/

notebook, or any other computer which the battery is power-

ing. A desired output voltage can be achieved by

manipulating the ratios of the feedback resistors. During the

charging cycle, the current flows out of the battery as shown.

While during discharge, the current is in the reverse direc-

tion. Since the current can range from a few milliamperes to

amperes, the amplifier will have to sense a signal below

ground during the discharge cycle. At 3V, the LMC6681/2/4

can accept a signal up to 300 mV below ground. The

common-mode voltage range of the LMC6681/2/4, which ex-

tends beyond both rails, is thus a very useful feature in this

application.

A typical offset voltage of 0.5 mV, and CMRR of 82 dB main-

tain accuracy in the circuit output, while the rail-to-rail output

performance allows for a maximum signal range.

DS012042-34

FIGURE 25. Sample-and-Hold Application

DS012042-37

FIGURE 26. Circuit Used to Sense Charging

DS012042-38

FIGURE 27. Circuit Used to Sense Discharging

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