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LOG101

9

SBOS242B

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.

USING A LARGER REFERENCE VOLTAGE

REDUCES OFFSET ERRORS

Using a larger reference voltage to create the reference

current minimizes errors due to the LOG101’s input offset

voltage. Maintaining an increasing output voltage as a func-

tion of increasing photodiode current is also important in

many optical sensing applications. All zeros from the

A/D converter output represent zero or low-scale photodiode

maximum photodiode current is accomplished using this

requirement. The LOG101 configured with the reference

A

1

A

2

LOG101

Q

1

Q

2

OPA703

R

2

R

3

R

2

6

R

3

C

C

V

OUT

V

REF

I

REF

I

I

V

REF

R

1

V

• (1V)LOG

(

)

R

2

R

3

I

REF

I

PHOTO

I

1

I

2

8

1

I

PHOTO

A/D

Converter

R

1

V

3

level shifter with inverting gain and is used to scale the

photodiode current directly into the A/D converter input

voltage range.

The wide dynamic range of the LOG101 is also useful for

measuring avalanche photodiode current (APD) (see Figure 14).

LOG CONFORMITY

For the LOG101, log conformity is calculated the same as

applications, log conformity is the most important specifica-

tion. This is 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.

INDIVIDUAL ERROR COMPONENTS

The ideal transfer function with current input is:

VV

I

I

1

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

=

(

)

±±

11

2

11

22

∆ log

–

–

(6)

(7)

(8)

(9)