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LOG104

10

SBOS243C

(8)

(9)

(10)

(11)

(12)

(13)

INDIVIDUAL ERROR COMPONENTS

The ideal transfer function with current input is:

VV

I

I

OUT =

(

) •

05

1

2

.log

The actual transfer function with the major components of

error is:

VV

K

II

II

Nm

V

OUT

B

B

OS O

=

(

) ±

(

)

±±

05

1

11

22

.

log

–

–

∆

The individual component of error is:

∆K = gain accuracy (0.15%, typ), as specified in

specification table.

IB1 = bias current of A1 (5pA, typ)

IB2 = bias current of A2 (5pA, typ)

N = log conformity error (0.01%, 0.06%, typ)

0.01% for n = 5, 0.06% for n = 7.5

VOSO = output offset voltage (3mV, typ)

m = number of decades over which N is specified:

Example: what is the error when

I1 = 1µA and I2 = 100nA

VmV

OUT =±

(

)

−

−

±

( )(

) ±

−−

−−

0 5

0 0015

10

5 10

10

5 10

2 0 0001 5

3 0

612

712

.

.

log

•

•

..

= 0.5055V

Since the ideal output is 0.5V, the error as a percent of

reading is

%

.

.

%. %

error

=•

=

0 5055

05

100

1 1

For the case of voltage inputs, the actual transfer function is

VV

K

V

R

I

E

R

V

R

I

E

R

Nm

V

OUT

B

OS

B

OS

OSO

=

(

) ±

(

)

±

±

±±

05

1

1

1

1

1

1

2

2

2

2

2

.log

–

–

∆

Where

E

R

and

E

R

OS

OS

1

1

2

2

are considered to be zero for large

values of resistance from external input current sources.