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LOG104

9

SBOS243B

TOTAL ERROR

The total error is the deviation (expressed in mV) of the

Thus,

It represents the sum of all the individual components of error

normally associated with the log amp when operated in the

current input mode. The worst-case error for any given ratio

considered separately. Temperature can affect total error.

ERRORS RTO AND RTI

As with any transfer function, errors generated by the func-

tion itself may be Referred-to-Output (RTO) or Referred-to-

Input (RTI). In this respect, log amps have a unique property:

Given some error voltage at the log amp’s output, that error

corresponds to a constant percent of the input regardless of

the actual input level.

(6)

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

A

1

–5V

SO-8

5

LOG104

Q

1

2

Q

2

I

OUT

I

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

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

MEASURING AVALANCHE PHOTODIODE CURRENT

The wide dynamic range of the LOG104 is useful for measuring

avalanche photodiode current (APD), as shown in Figure 13.

LOG CONFORMITY

For the LOG104, log conformity is calculated the same as

applications, log conformity is the most important specifica-

tion. This is true because bias current errors are negligible

(5pA compared to input currents of 100pA and above) and

the scale factor and offset errors may be trimmed to zero or

removed by system calibration. This leaves log conformity as

the major source of error.

Log conformity is defined as the peak deviation from the best

expressed as a percent of ideal full-scale output. Thus, the

nonlinearity error expressed in volts over m decades is:

where N is the log conformity error, in percent.

(7)