Individually calculating the LCE for every possible input condition is not practical. The LCE expressed as an

error envelope is more useful to circuit designers. This calculation conveys the maximum LCE expected across

a given input range as a percentage of the expected full-scale output voltage. The calculation involves iterating

across a set of all measured immediate LCE values for a given range. The difference of the maximum and

minimum values is then halved and normalized with a division by the output voltage span of the measurement

(the difference of the maximum output voltage and minimum output voltage, typically at the two endpoints of the

data set), to express LCE as a percentage of the full-scale range:

LCE%= LCEmax−LCEmin

2× VLOGOUTmax−VLOGOUTmin ×100%

(6)

The LCE envelope can then be expressed in dB through the following relationship, where the factor of 20 is

associated with amplitude. For expression in terms of optical power, this factor is 10.

LCEdB=20log1−LCE%100%

(7)

8.2 Typical Application

8.2.1 Optical Current Sensing

A common use case for the LOG200 is an optical current sense circuit, using an external photodiode. Figure 8-5

shows an implementation using an InGaAs, PIN photodiode for a λ = 1.31

‑μm application. This design uses ±5‑V

supplies and is intended for use with input currents from 10 nA to 100 µA. Decoupling capacitors are not shown

for brevity. The design can be easily implemented using the LOG200 Evaluation Module board.

100

To Oscilloscope

Photodiode

+

–

+

–

+

–

I2

I1

PD

Bias

IBIAS

VCM

+

–

REFGND

+

–

2.5 V

1.65 V

REF25

REF165

+

–

REFA

OUTA

OUTB

–IN

+IN

VS+

VS–

VS+

1 μA

IREF

I2

I1

2.5 V

2.5 V

5 V

5 V

LOG200

–5 V

Figure 8-5. LOG200 Optical Current Sensing Application

LOG200

SBOSA28 – AUGUST 2023

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