Datasheet

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SBOS301A − MAY 2004 − REVISED MARCH 2007
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15
A
4
OPA335
Internal A
4
Output Amplifier External Output Amplifier
+5V
R
5
100k
Ω
V
REF
+2.5V
V
LOGOUT
V
LOGOUT
V
O4
=
−
2/3 (V
LOGOUT
) + 3.16
I
2
100pA
10mA
V
OUT
=
−
2.5/3 (V
LOGOUT
)+3.3
a) A
4
amplifier used to scale and offset V
LOGOUT
for 0.5V to 2.5V output. b) OPA335 amplifier used to scale and offset V
LOGOUT
for 0V to 2.5V output.
NOTE: (1) See OPA335 data sheet for use of R
P
connected to
−
5V to achieve 0V output.
R
6
66.5k
Ω
R
7
100k
Ω
R
8
316k
Ω
R
5
100k
Ω
V
REF
+2.5V
R
6
82.5k
Ω
R
P
(1)
−
5V
R
7
100k
Ω
R
8
267k
Ω
V
O4
0.5V
2.5V
I
2
100pA
10mA
V
OUT
0.5V 2.5V
Figure 4. Operational Amplifier Configuration for Scaling and Offsetting the Output Going to ADC Stage.
ADVANTAGES OF DUAL−SUPPLY OPERATION
The LOG114 performs very well on a single +5V supply
by level-shifting pin 7 (Com) to half-supply and raising
the common-mode voltage (pin 5, V
CM IN
) of the input
amplifiers. This level−shift places the input amplifiers in
the linear operating range. However, there are also
some advantages to operating the LOG114 on dual ±5V
supplies. These advantages include:
1) eliminating the need for the +4.096V precision
reference;
2) eliminating a small additional source of error arising
from the noise and temperature drift of the level−shifting
voltage; and
3) allowing increased magnitude of a reverse bias
voltage on the photodiode.
COM (PIN 7) VOLTAGE RANGE
The voltage on the Com pin is used to bias the differen-
tial amplifier, A
3
, within its linear range. This voltage can
provide an asymmetrical offset of the V
LOGOUT
voltage.
V
CM IN
(Pin 5)
The V
CM IN
pin is used to bias the A
1
and A
2
amplifier into
its common-mode input voltage range, (V−) + 1.5V to
(V+) − 1.5V.
INPUT CURRENT RANGE
To maintain specified accuracy, the input current range
of the LOG114 should be limited from 100pA to 3.5mA.
Input currents outside of this range may compromise
the LOG114 performance. Input currents larger than
3.5mA result in increased nonlinearity. An absolute
maximum input current rating of 10mA is included to
prevent excessive power dissipation that may damage
the input transistor.