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| LOG114_12 Datasheet, PDF (12/29 Pages) Burr-Brown (TI) – Single-Supply, High-Speed, Precision LOGARITHMIC AMPLIFIER | |||
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LOG114
SBOS301A â MAY 2004 â REVISED MARCH 2007
www.ti.com
R7
100kâ¦
R8
316kâ¦
I1
Q1
REF3040
IµA
or
4 I1
REF3240
R1
4.096V
Reference
RREF
1.62Mâ¦
5
VCM
(1)
IN
A1
Input current
from photodiode
or current source
I2
Photodiode(4)
3 I2
Q2
R3
A2
R5
100kâ¦
R6
66.5kâ¦
9
VLOGOUT(2)
10 11
+IN4 âIN4
LOG114
R2
A3
R4
A4
VO4(3) 12
+IN5 13
A5
VO5 15
+2.5V
16 VREF
2.5VREF
VREF GND
V
1
8
+5V
1000pF
Vâ Com
6
7
+
10µF
âIN5
1
VCom = +2.5V
NOTE: (1) In singleâsupply configuration, VCM IN must be connected to ⥠1V.
(2) VLOGOUT = 0.375 Ã log(I1/I2) + 2.5V.
(3) VO4 = â0.249 Ã log(I1/I2) + 1.5V.
(4) The cathode of the photodiode is returned to VREF resulting in zero bias across it. The cathode
could be returned to a voltage more positive than VCM IN to create a reverse bias for reducing
photodiode capacitance, which increases speed.
Figure 2. Single-Supply Configuration Example for Measurement Over Eight Decades.
12
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