LTC1921 datasheet(12/22 Pages) LINER

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LTC6101/LTC6101HV

6101fh

APPLICATIONS INFORMATION

Selection of External Output Resistor, ROUT

The output resistor, ROUT, determines how the output cur-

rent is converted to voltage. VOUT is simply IOUT • ROUT.

In choosing an output resistor, the max output voltage

must ﬁrst be considered. If the circuit that is driven by

the output does not limit the output voltage, then ROUT

must be chosen such that the max output voltage does

not exceed the LTC6101 max output voltage rating. If the

following circuit is a buffer or ADC with limited input range,

then ROUT must be chosen so that IOUT(MAX) • ROUT is less

than the allowed maximum input range of this circuit.

In addition, the output impedance is determined by ROUT. If

the circuit to be driven has high enough input impedance,

then almost any useful output impedance will be accept-

able. However, if the driven circuit has relatively low input

impedance, or draws spikes of current, such as an ADC

might do, then a lower ROUT value may be required in order

to preserve the accuracy of the output. As an example, if

the input impedance of the driven circuit is 100 times ROUT,

then the accuracy of VOUT will be reduced by 1% since:

OUT =IOUT •

OUT •RIN(DRIVEN)

OUT + RIN(DRIVEN)

OUT •ROUT •

100

101

=0.99 •I

OUT •ROUT

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

gain and input offset, as well as resistor matching.

Ideally, the circuit output is:

OUT = VSENSE •

OUT

SENSE = RSENSE •ISENSE

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

EOUT(VOS) = VOS • (ROUT/RIN)

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 the Bias Currents,

IB(+) and IB(–)

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 1kΩ, the output

error is 0.1mV.

Note that in applications where RSENSE ≈ RIN, IB(+) causes

a voltage offset in RSENSE that cancels the error due to

IB(–) and EOUT(IBIAS) ≈ 0. In applications where RSENSE <

RIN, the bias current error can be similarly reduced if an

external resistor RIN(+) = (RIN – RSENSE) is connected as

shown in Figure 4 below. Under both conditions:

EOUT(IBIAS) = ± ROUT • IOS; IOS = IB(+) – IB(–)

LTC6101

ROUT

VOUT

6101 F04

RIN

–

V+

LOAD

RSENSE

RIN

+ = RIN– – RSENSE

V+

V–

OUT

–IN

+IN

Figure 4. Second Input R Minimizes

Error Due to Input Bias Current

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