LTC6103CMS8 datasheet(10/16 Pages) LINER

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LTC6103

6103f

Error Sources

The current sense system uses an ampliﬁer and resistors

to apply gain and level shift the result. The output is then

dependent on the characteristics of the ampliﬁer, such as

bias current and input offset, as well as resistor matching.

Ideally, the circuit output is:

OUT

SENSE

OUT

SENSE

•

In this case, the only error is due to resistor mismatch,

which provides an error in gain only. However, offset

voltage, bias current and ﬁnite gain in the ampliﬁer cause

additional errors.

Output Error, EOUT, Due to the Ampliﬁer DC Offset

Voltage, VOS

OUT VOS

OUT

()

•

The DC offset voltage of the ampliﬁer adds directly to the

value of the sense voltage, VSENSE. This is the dominant

error of the system and it limits the available dynamic

range. The paragraph, Selection of External Current Sense

Resistor provides details.

Output Error, EOUT, Due to Bias Currents

The bias current IB(+) ﬂows into the positive input of the

internal op amp. IB(–) ﬂows into the negative input.

EOUT(IBIAS) = ROUT(IB(+) • (RSENSE/RIN) – IB(–))

Since IB(+) ≈ IB(–) = IBIAS, if RSENSE << RIN then:

EOUT(IBIAS) ≈ –ROUT • IBIAS

For instance, if IBIAS is 100nA and ROUT is 1k, then the

output error is 0.1mV.

Output Error, EOUT, Due to PCB Trace Resistance

The LTC6103 uses the +IN pin for both the positive ampliﬁer

input and the positive supply input for the ampliﬁer. The

supply current can cause an output error if trace resistance

between RSENSE and +IN is signiﬁcant (Figure 4).

EOUT(RT_+IN) = (IS • RT/RIN) • ROUT

Trace resistance to the –IN pin will increase the effective

RIN value, causing a gain error. In addition, internal device

resistance will add approximately 0.3

Ω to RIN.

Minimizing the trace resistance is important and care

should be taken in the PCB layout. Make the trace short

and wide. Kelvin connection to the shunt resistor pad

should be used.

APPLICATIONS INFORMATION

Figure 4. Error Due to PCB Trace Resistance

–

+IN

OUT

1/2

LTC6103

–IN

RIN

RSENSE

V+

V–

ROUT

6103 F04

LOAD

ILOAD

Output Error, EOUT, Due to the Finite DC Open-Loop Gain,

AOL, of the LTC6103 Ampliﬁer

This error is inconsequential as the AOL of the LTC6103

is very large.

Design Example:

If ISENSE range = (1A to 1mA) and:

OUT

SENSE

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