Datasheet

AD8305
Rev. B | Page 18 of 24
REVERSING THE INPUT POLARITY
Some applications may require interfacing to a circuit that
sources current rather than sinks current, such as connecting to
the cathode side of a photodiode. Figure 40 shows the use of a
current mirror circuit. This allows for simultaneous monitoring
of the optical power at the cathode, and a data recovery path
using a transimpedance amplifier at the anode. The modified
Wilson mirror provides a current gain very close to unity and a
high output resistance. Figure 41 shows measured transfer
function and law conformance performance of the AD8305 in
conjunction with this current mirror interface.
2.5V
0.1µF
5V
0V
0V
1nF
200kΩ
MAT03
MAT03
DATA PATH
5V
0.1µF
TIA
I
IN
≈ I
PD
10nA TO 1mA
I
PD
1kΩ
1nF
OUTPUT
V
OUT
= 0.2 ×
log
10
(I
PD
/1nA)
AD8305
12
11
10
9
1
2
3
4
578
COMM COMM COMM COMM
16 131415
VSUM
VRDZ
VREF
IREF
INPT
VOUT
SCAL
BFIN
VLOG
VNEG VNEG VPOS
6
03053-039
Figure 40. Wilson Current Mirror for Cathode Interfacing
I
PD
(A)
1.4
0
V
LOG
(V)
1.2
1.0
0.8
0.6
0.4
0.2
1.00
0.75
0
1.6
0.50
–1.00
–0.75
–0.50
–0.25
0.25
+5V
+3V
5V
5V
+5V
+3V
ERROR; dB (10mV/dB)
1n 10n 100n 1µ 10µ 100µ 1m 10m
0
3053-040
Figure 41. Log Output and Error Using Current Mirror with Various Supplies