Datasheet

www.ti.com
LOG104
7
SBOS243C
FIGURE 7. Precision Current Inverter/Current Source.
1.5kΩ
Photodiode
10nA to 1mA
10nA to 1mA
+5V
+5V
1.5kΩ
+3.3V
(1)
+3.3V)
1/2 OPA2335
1/2 OPA2335
OPA2335
1
2
BSH203
TLV271 or
(Back Bias
LOG104
Pin 1 or Pin 8
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.
OPERATION ON SINGLE SUPPLY
Many applications do not have the dual supplies required to
operate the LOG104. Figure 10 shows the LOG104 config-
ured for operation with a single +5V supply.
VOLTAGE INPUTS
The LOG104 gives the best performance 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.
FIGURE 8. Precision Current Inverter/Current Source.
1.5kΩ
+5V
+5V
1.5kΩ
1.5kΩ
100kΩ
10nA to 1mA
LOG104
10nA to 1mA
100kΩ
100kΩ
+3.3V
(1)
1/2 OPA2335
1/2 OPA2335
Photodiode
Back Bias
Pin 1 or Pin 8
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.
APPLICATION CIRCUITS
LOG RATIO
One of the more common uses of log ratio amplifiers is
to measure absorbance. A typical application is shown in
Figure 9.
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)
DATA COMPRESSION
In many applications the compressive effects of the logarith-
mic transfer function are useful. For example, a LOG104
preceding a 12-bit A/D converter can produce the dynamic
range equivalent to a 20-bit converter.
FIGURE 9. Absorbance Measurement.
I
2
I
1
8
1
5
6
3
V
OUT
4
V+
V–
C
C
LOG104
D
2
D
1
Sample
λ
1
λ
1
´
λ
1
Light
Source
8
1
3
6
4
5
1
2
35
4
–5V
1µF
LOG104
1µF
I
1
I
2
V
OUT
C
C
1µF
TPS
(1)
Single Supply +5V
(1) TPS60402DBV negative charge pump.
FIGURE 10. Single +5V Power-Supply Operation.