LOG112AIDRE4 Datasheet, PDF (9/20 Pages) Texas Instruments – Precision LOGARITHMIC AND LOG RATIO AMPLIFIERS

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 = logλ₁´/ λ₁
(3)
If D₁ and D₂ are matched A ∝ (0.5V) logI₁/ I₂ (4)
10nA to 1mA (+3.3V Back Bias)
Photodiode
+3.3V⁽¹⁾
1kΩ
+5V
1/2
OPA2335
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.
100kΩ
100kΩ
100kΩ
+5V
1.5kΩ
1/2
OPA2335
100kΩ
1.5kΩ
10nA to 1mA
Pin 1 or Pin 14
LOG112
FIGURE 8. Precision Current Inverter/Current Source.
I₁
1
C_C
V+ = +5V
6
Q₁
Q₂
R₁
R₂
V_LOGOUT
5
+IN3
34
–IN3
LOG112
I₂
A₁
R_REF
14
+2.5V
8
V_REF
100kΩ
150kΩ
+1.5V
V_REF
11
13
V_{REF – GND}
V_CM
A₂
10
GND
A₃
7
V_O3
9
V– = –5V
FIGURE 9. Extending Input Current Level and Improving Accuracy by Applying a Common-Mode Voltage.
LOG112, 2112
9
SBOS246D
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