LT6106

9

Rev. C

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APPLICATIONS INFORMATION

Output Error 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 low end of the dynamic

range. The paragraph “Selection of External Current Sense

Resistor” provides details.

Output Error 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–

⎛

⎝

⎜

⎞

⎠

⎟

Assuming IB+ ≅ IB– = IBIAS, and RSENSE << RIN then:

EOUT(IBIAS) ≅ –ROUT • IBIAS

It is convenient to refer the error to the input:

EIN(IBIAS) ≅ –RIN • IBIAS

ForinstanceifIBIASis60nAandRINis1k,theinputreferred

error is 60µV. Note that in applications where RSENSE ≅

RIN, IB+ causes a voltage offset in RSENSE that cancels the

error due to IB– and EOUT(IBIAS) ≅ 0mV. In most applica-

tions,RSENSE<<RIN,thebiascurrenterrorcanbesimilarly

reduced if an external resistor RIN+ = (RIN – RSENSE) is

connected as shown in Figure 4. Under both conditions:

EIN(IBIAS) = ±RIN • IOS; where IOS = IB+ – IB–

If the offset current, IOS, of the LT6106 amplifier is 6nA,

the 60µV error above is reduced to 6µV.

Adding RIN+ as described will maximize the dynamic

range of the circuit. For less sensitive designs, RIN+ is

not necessary.

Output Error Due to Gain Error

The LT6106 exhibits a typical gain error of –0.25% at 1mA

output current. The primary source of gain error is due to

the finite gain to the PNP output transistor, which results in

asmallpercentageofthecurrentinRINnotappearinginthe

outputloadROUT.

Minimum Output Voltage

The curves of the Output Voltage vs Input Sense Voltage

show the behavior of the LT6106 with low input sense

voltages.WhenVSENSE=0V,theoutputvoltagewillalways

be slightly positive, the result of input offset voltages and

of a small amount of quiescent current (0.7µA to 1.2µA)

flowing through the output device. The minimum output

voltage in the Electrical Characteristics table include both

these effects.

Power Dissipation Considerations

The power dissipated by the LT6106 will cause a small

increase in the die temperature. This rise in junction tem-

perature can be calculated if the output current and the

supply current are known.

The power dissipated in the LT6106 due to the output

signal is:

POUT = (VIN– – VOUT) • IOUT

Since VIN– ≅ V+, POUT ≅ (V+ – VOUT) • IOUT

The power dissipated due to the quiescent supply current is:

PQ = IS • (V+ – V–)

The total power dissipated is the output dissipation plus

the quiescent dissipation:

PTOTAL = POUT + PQ

The junction temperature is given by:

TJ = TA + θJA • PTOTAL

At the maximum operating supply voltage of 36V and the

maximum guaranteed output current of 1mA, the total

Figure 4. Second Input R Minimizes Error Due to Input Bias Current

LT6106

ROUT

VOUT

6106 F04

RIN–

RIN+

V+

LOAD

RSENSE

V+

V–

OUT

RIN+ = RIN– – RSENSE

–IN

+IN