Input Offset Voltage (VOS) & Input Bias Current (IB) TIPL 1100 TI Precision Labs – Op Amps

Presented by Ian Williams Prepared by Art Kay and Ian Williams

Input Offset Voltage - VOS

2

VOUT

VOS = 50!V

Input Offset Voltage - VOS

3

VOUT

Input offset from mismatch of input

transistors

Offset Voltage Specs and Distribution

4

OPA827

Simulate Input Offset Voltage

5

+

-

+

U1 OPA350

R1

1k

Vcm 2.5

Vload 2.5

V+

VM1

VEE 5V+

VM2 -150uV

-150uV

Drift Slope – Positive and Negative

6

OPA835 OPA835

For this example VOS drift is defined as:

!Vos!T

Vos T1( ) Vos 25C( )"

T1 25C"

Drift Slope – Common Definition

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Vosi at Temp

0

20

40

60

80

100

120

140

160

-40 -20 0 20 40 60 80 100

Temp (degC)

Vosi

(uV)

! Vos! T

Vos T1( ) Vos 25C( )! Vos T1( ) Vos 25C( )!+

T1 T2!=

(-40C, 100uV)

(85C, 150uV)

(25C, 30uV)

! Vos! T

100"V 30"V! 150"V 30"V!+

85C 40! C( )!= 1.52

"VC

=°

T2

Application Example

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-

++

Vosi 0.1mV

R1 1k R2 99k

R3

1k

Vout = (1mV+ 0.1mV)*(100)= 110mV

Vin 1m

VOS introduces 10% error!

Input Offset Drift Calculations

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Temp (°C) VOS Initial + Drift +1.5uV/C

VOS Initial + Drift -1.5uV/C

-25 °C 25uV 175uV 0 °C 62.5uV 137.5uV 25 °C 100uV 100uV 50 °C 137.5uV 62.5uV 85 °C 190uV 10uV 125 °C 250uV -50uV

Vosi Vosi_room Vosi_drift T 25C!( )"+=

Example calculations:

Vosi 100!V 1.5!VC

25C( ) 25C![ ]"+= 100!V=

At 25C

Vosi 100!V 1.5!VC

125C( ) 25C![ ]"+= 250!V=

At 125C

-

++

R1 1k R2 99k

R3

1k

Vout

Vin 1m

+

Vosi 0.1mVVosi drift

1.5uV/C x (T – 25C)

Range of Offset - !V to mV

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Op Amp VOS (max)

(high grade) VOS Drift (max)

(high grade) Technology

OPA333 10 !V 0.05 !V/°C Zero Drift CMOS

OPA277 20 !V 0.15 !V/°C Precision Bipolar

OPA188 25 !V 0.085 !V/°C Auto-Zero CMOS

OPA192 25 !V 0.5 !V/°C CMOS

OPA211 50 !V 1.5 !V/°C Precision Bipolar

OPA827 150 !V 1.5 !V/°C (typ.) JFET input, Bipolar, Precision

OPA350 500 !V 4 !V/°C (typ.) CMOS

OPA835 1.85 mV 13.5 !V/°C High Speed Bipolar

LM741 3.00 mV 15 !V/°C Bipolar commodity (lower cost)

Input Bias Current - IB

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Input bias offset current: IB_OS = IB1 – IB2 = 210 nA – 150 nA = 60 nA

VOUT

150 nA

210 nA

Simple Bipolar, No IB Cancellation

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Bias current in bipolar amplifiers is from input transistor base current. It is typically larger than in FET-input amplifiers and it flows into the input terminals.

Vcc

IS1

Q1 Q2

R1 R2

Vin2Vin1

Ib1

Ib2

Bipolar with IB Cancellation

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Vcc

IS1

Q1 Q2

R1 R2

Vin2Vin1

Ib1

Ib2

Ib Cancel Circuit

!

Ib Cancel Circuit

!

The input bias currents are mirrored and summed back in to cancel the bias current. This has the effect of significantly reducing input IB. Note that when this is done, IB can flow in both directions. Also, IB_OS is no longer smaller then IB.

IB_OS = IB1 – IB2

Bias Current for CMOS

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Bias current in FET-input amplifiers is mainly from leakage into ESD protection diodes.

Vcc

IS1

Q1 Q2

R1 R2

Vin2

Vin1

Ib2

Vcc

Vcc

Ib1

IB over Temperature

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OPA350

CMOS amplifier: In this case you see a dramatic increase in bias current at 25 °C. Note the logarithmic graph, which doubles every 10 °C.

OPA277

Bipolar amplifier: In this case you see a dramatic increase in bias current at 75 °C.

IB Calculation – OPA211 at High Temp.

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-

+

R1 1k R2 99k

R3

1kVin 1m

Ib 200nA

Ib 150nA Ib 200nA

Using nodal analysis

VinR1

Vin Vout!

Rf+ Ib+ 0=

Vout Rf IbVinR1

+VinRf

+"#$

%&'

(=

Using superposition set Vin=0V

Vout Rf Ib0R1

+0Rf

+"#$

%&'

(= Ib Rf(=

In this example

Vout_Ib 200nA( ) 99k!( )(= 20mV=

Vout_vin 1mV991

1+"#$

%&'

(= 100mV=

Vout_total 20mV_100mV= 120mV=

Point for nodal analysis

Range of Bias Current – fA to nA

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Op Amp IB (max)

(high grade) IB at max

temp. Technology

OPA129 100 fA 20 pA (typ.) Difet – Ultra Low Bias Current

OPA627 5 pA 1nA Difet – Precision High Speed

OPA350 10 pA 500 pA (typ.) CMOS

OPA827 50 pA 50 nA max JFET input, Bipolar, Precision

OPA333 70 pA 150 pA (typ) Zero Drift CMOS

OPA277 1 nA 2 nA (max) Precision Bipolar

OPA211 125 nA 200 nA Precision Bipolar

OPA835 400 nA 530 nA High Speed Bipolar

LM741 80 nA 0.2 !A (max) Bipolar commodity (lower cost)

Thanks for your time! Please try the quiz.

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