Datasheet

LOG112, 2112
9
SBOS246D
www.ti.com
FIGURE 8. Precision Current Inverter/Current Source.
1kΩ
+5V
+5V
1.5kΩ
1.5kΩ
100kΩ
10nA to 1mA
LOG112
10nA to 1mA
100kΩ
100kΩ
(+3.3V Back Bias)
+3.3V
(1)
1/2
OPA2335
1/2
OPA2335
Photodiode
Pin 1 or Pin 14
100kΩ
NOTE: (1) +3.3V bias is an arbitrary dc level < 5V that also
appears on the −IN through the op amp where it
applies a reverse bias to the photodiode.
VOLTAGE INPUTS
The LOG112 and LOG2112 give the best performances with
current inputs. Voltage inputs may be handled directly with
series resistors, but the dynamic input range is limited to
approximately three decades of input voltage by voltage
noise and offsets. The transfer function of Equation 13
applies to this configuration.
ACHIEVING HIGHER ACCURACY WITH HIGHER
INPUT CURRENTS
As input current to the LOG112 increases, output accuracy
degrades. For a 4.5mA input current on ±5V supplies and a
10mA input current on ±12V supplies, total output error can
be between 15% and 25%. Applying a common-mode volt-
age to V
CM
of at least +1V and up to 2.5V, brings the log
transistors out of saturation and reduces output error to
approximately 10%. To avoid forward biasing a photodiode,
return the cathode to the V
CM
pin, as shown in Figure 9. To
reverse bias the photodiode, apply a more positive voltage to
the cathode than the anode.
APPLICATION CIRCUITS
LOG RATIO
One of the more common uses of log ratio amplifiers is
to measure absorbance. See Figure 10 for a typical application.
Absorbance of the sample is A = logλ
1
´/ λ
1
(3)
If D
1
and D
2
are matched A ∝ (0.5V) logI
1
/I
2
(4)
FIGURE 9. Extending Input Current Level and Improving Accuracy by Applying a Common-Mode Voltage.
A
2
A
1
Q
1
Q
2
C
C
I
1
V
REF – GND
V
O3
V+ = +5V
GND
LOG112
A
3
R
2
R
1
V
REF
100kΩ
+2.5V
150kΩ
R
REF
+IN3
V
LOGOUT
–IN3
V– = –5V
V
CM
I
2
V
REF
6534
7
9101311
8
14
1
+1.5V