AD8305ACP-REEL7 Analog Devices Inc, AD8305ACP-REEL7 Datasheet - Page 18

• Manufacturer Part Number: AD8305ACP-REEL7
• Description: IC,Converter, Other/Special/Miscellaneous,LLCC,16PIN
• Manufacturer: Analog Devices Inc
• Type: Logarithmic Converterr

Specifications of AD8305ACP-REEL7

• Rohs Status: RoHS non-compliant
• Design Resources: Interfacing ADL5315 to Translinear Logarithmic Amplifier (CN0056) Interfacing ADL5317 High Side Current Mirror to a Translinear Logarithmic Amplifier in an Avalanche Photodiode Power Detector
• Applications: Fiber Optics
• Mounting Type: Surface Mount
• Package / Case: 16-LFCSP

AD8305
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.
I
PD
MAT03
MAT03
5V
Figure 40. Wilson Current Mirror for Cathode Interfacing
1nF
TIA
1kΩ
0.1µF
200kΩ
10nA TO 1mA
1nF
I
IN ≈
I
PD
2.5V
0V
0V
DATA PATH
1
2
3
4
0.1µF
VRDZ
VREF
IREF
INPT
COMM COMM COMM COMM
VSUM
16
5
VNEG VNEG VPOS
AD8305
15
6
14
7
5V
13
8
VLOG
VOUT
SCAL
BFIN
V
log
OUT
10
12
11
10
9
OUTPUT
= 0.2
(I
PD
/1nA)
×
Rev. B | Page 18 of 24
Figure 41. Log Output and Error Using Current Mirror with Various Supplies
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
1n
+3V
+5V
10n
5V
100n
1µ
I
PD
(A)
10µ
100µ
1m
5V
+5V
+3V
10m
1.00
0.75
0.50
0.25
–0.25
–0.50
–0.75
–1.00
0