Datasheet

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.
I
PD
(A)
1.4
0
V
LOG
(V)
1.2
1.0
0.8
0.6
0.4
0.2
4
3
2
1
0
–1
–2
–3
IDEAL OUTPUT
CALIBRATED ERROR
MEASURED OUTPUT
UNCALIBRATED ERROR
ERROR; dB (10mV/dB)
1n 10n 10µ 100µ 1m 10m100n 1µ
03053-035
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, I
1
and I
2
, in the linear operating range
between 10 nA and 1 mA. Measure the resulting output, V
1
and
V
2
, respectively, and calculate the slope m and intercept b.
m = (V
1
− V
2
)/[log
10
(I
1
) − log
10
(I
2
)] (7)
b = V
1
– m × log
10
(I
1
) (8)
The same calibration is performed with two known optical
powers, P
1
and P
2
. This allows for calibration of the entire
measurement system while providing a simplified relationship
between the incident optical power and V
LOG
voltage.
m = (V
1
− V
2
)/(P
1
− P
2
) (9)
b = V
1
− m × P
1
(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.