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

TIA

10nA TO 1mA

1nF

OUTPUT

AD8305

12

11

10

9

1

2

3

4

57

8

COMM COMM COMM COMM

16

13

14

15

VSUM

VRDZ

VREF

IREF

INPT

VOUT

SCAL

BFIN

VLOG

VNEG VNEG VPOS

6

Figure 40. Wilson Current Mirror for Cathode Interfacing

1.4

0

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

1n

10n

100n

1µ

10µ

100µ

1m

10m

Figure 41. Log Output and Error Using Current Mirror with Various Supplies