LTC6103

7

6103f

APPLICATIONS INFORMATION

In this dual current sense device, amplifiers A and B are

voltage and component values can be chosen independently

for each amplifier.

Selection of External Current Sense Resistor

on the function of a current sensing system and must be

chosen with care.

First, the power dissipation in the resistor should be

considered. The system load current will cause both heat

tor should be as small as possible while still providing

the input dynamic range required by the measurement.

Note that input dynamic range is the difference between

the maximum input signal and the minimum accurately

reproduced signal, and is limited primarily by input DC

offset of the internal amplifier of the LTC6103.

As an example, an application may require that the

maximum sense voltage be 100mV. If this application is

larger than 50m

Ω.

R

V

I

mV

A

m

SENSE

SENSE

PEAK

==

=

100

2

50

Ω

mum sense resistor value will be set by the resolution or

dynamic range required. The minimum signal that can be

accurately represented by this sense amp is limited by the

input offset. As an example, the LTC6103 has a typical

input offset of 85µV. If the minimum current is 20mA, a

sense resistor of 4.25m

the same value as the input offset. A larger sense resistor

will reduce the error due to offset by increasing the sense

voltage for a given load current.

Choosing a 50m

and provide a system that has 100mV across the sense

resistor at peak load (2A), while input offset causes an

error equivalent to only 1.7mA of load current.

Peak dissipation is 200mW. If instead a 5m

Ω sense resis-

tor is employed, then the effective current error is 17mA,

while the peak sense voltage is reduced to 10mV at 2A,

dissipating only 20mW.

The low offset and corresponding large dynamic range of

the LTC6103 make it more flexible than other solutions

in this respect. The 85µV typical offset gives 60dB of

dynamic range for a sense voltage that is limited to 85mV

max, and over 75dB of dynamic range for a maximum

input of 500mV.

THEORY OF OPERATION

An internal sense amplifier loop forces –IN to have the

The high impedance inputs of the sense amplifier will not

conduct this input current, so the current will flow through

an internal MOSFET to the OUT pin. In most application

The output current can be transformed into a voltage by