AD8305ACP-R2 datasheet(15/24 Pages) AD

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AD8305

Rev. B | Page 15 of 24

CALIBRATION

The AD8305 has a nominal slope and intercept of 200 mV/decade

and 1 nA, respectively. These values are untrimmed, and the

slope alone may vary as much as 7.5% over temperature. For

this reason, it is recommended that a simple calibration be done

to achieve increased accuracy.

IPD (A)

1.4

1.2

1.0

0.8

0.6

0.4

0.2

–1

–2

–3

IDEAL OUTPUT

CALIBRATED ERROR

MEASURED OUTPUT

UNCALIBRATED ERROR

10n

10µ

100µ

10m

100n

1µ

Figure 36. Using Two-Point Calibration to Increase Measurement Accuracy

Figure 36 shows the improvement in accuracy when using a two

point calibration method. To perform this calibration, apply

two known currents, I1 and I2, in the linear operating range

between 10 nA and 1 mA. Measure the resulting output, V1 and

V2, respectively, and calculate the slope m and intercept b.

m = (V1 − V2)/[log10(I1) − log10(I2)]

(7)

b = V1 – m × log10(I1)

(8)

The same calibration is performed with two known optical

powers, P1 and P2. This allows for calibration of the entire

measurement system while providing a simplified relationship

between the incident optical power and VLOG voltage.

m = (V1 − V2)/(P1 − P2)

(9)

b = V1 − m × P1

(10)

The uncalibrated error line in Figure 36 is generated assuming

that the slope of the measured output was 200 mV/ decade

when in fact it was actually 194 mV/decade. Correcting for this

discrepancy decreased measurement error up to 3 dB.

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