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LOG101

10

SBOS242B

(10)

FIGURE 14. High Side Shunt for Avalanche Photodiode (APD) Measures 3-Decades of APD Current.

A

1

–5V

SO-8

5

LOG101

Q

1

2

Q

2

I

OUT

I

OUT = 0.1 • ISHUNT

6

1.2k

Ω

Receiver

Irx = 1

µA to 1mA

I to V

Converter

APD

10Gbits/sec

1k

Ω

C

C

A

2

OPA703

V

OUT = 2.5V to 0V

I

SHUNT

100

µA

500

Ω

5k

Ω

5k

Ω

+15V to +60V

+5V

25k

Ω

INA168

SOT23-5

REF3025

2.5V

+5V

1

1

8

4

3

The individual component of error is:

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

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

VOSO = output offset voltage (3mV, typ)

n = number of decades over which N is specified:

Example: what is the error when

I1 = 1µA and I2 = 100nA

VmV

OUT =±

(

)

−

−

±

( )(

) ±

−−

−−

1 0 0015

10

5 10

10

5 10

2 0 0001 5

3 0

612

712

.log

•

•

..

= 1.005055V

(11)

(12)

(13)

Since the ideal output is 1.000V, the error as a percent of

reading is

%

.

%. %

error

=•

=

0 005055

1

100

0 5

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

VV

K

V

R

I

E

R

V

R

I

E

R

Nn

V

OUT

B

OS

B

OS

OSO

=

( ) ±

(

)

±

±

±±

11

2

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.