Datasheet

LOG112, 2112
8
SBOS246D
www.ti.com
FIGURE 7. Precision Current Inverter/Current Source.
Figure 5 shows a low-level current source using a series
resistor. The low offset op amp reduces the effect of the
LOG112 and LOG2112’s input offset voltage.
FREQUENCY COMPENSATION
Frequency compensation for the LOG112 is obtained by
connecting a capacitor between pins 5 and 14. Frequency
compensation for the LOG2112 is obtained by connecting a
capacitor between pins 2 and 5, or 15 and 12. The size of the
capacitor is a function of the input currents, as shown in the
Typical Characteristic curves (
Minimum Value of Compensa-
tion Capacitor
). For any given application, the smallest value
of the capacitor which may be used is determined by the
maximum value of I
2
and the minimum value of I
1
. Larger
values of C
C
make the LOG112 and LOG2112 more stable,
but reduce the frequency response.
In an application, highest overall bandwidth can be achieved
by detecting the signal level at V
OUT
, then switching in
appropriate values of compensation capacitors.
NEGATIVE INPUT CURRENTS
The LOG112 and LOG2112 function only with positive input
currents (conventional current flows into input current pins).
In situations where negative input currents are needed, the
circuits in Figures 6, 7, and 8 may be used.
FIGURE 5. Current Source with Offset Compensation.
1MΩ
+2.5mV
100kΩ
–2.5V
+2.5V
OPA335
100Ω
I
1
= 2.5nA to 1mA
I
2
= 2.5nA
V
LOGOUT
C
C
14
1
910
V–
LOG112
6
5
V+
8
V
REF
Chopper Op Amp
GND
FIGURE 6. Current Inverter/Current Source.
Q
B
National
LM394
I
IN
I
OUT
D
1
OPA703
D
2
Q
A
FREQUENCY RESPONSE
The frequency response curves seen in the Typical Charac-
teristic curves are shown for constant DC I
1
and I
2
with a
small-signal AC current on one input.
The 3dB frequency response of the LOG112 and LOG2112 are
a function of the magnitude of the input current levels and of the
value of the frequency compensation capacitor. See Typical
Characteristic curve,
3dB Frequency Response for details.
The transient response of the LOG112 and LOG2112 are
different for increasing and decreasing signals. This is due to
the fact that a log amp is a nonlinear gain element and has
different gains at different levels of input signals. Smaller
input currents require greater gain to maintain full dynamic
range, and will slow the frequency response of the LOG112
and LOG2112.
1.5kΩ
Photodiode
10nA to 1mA
10nA to 1mA
+5V
+5V
1kΩ
+3.3V
(1)
1/2
OPA2335
1/2
OPA2335
BSH203
TLV271 or 1/2 OPA2335
(+3.3V
Back Bias)
LOG112
Pin 1 or Pin 14
NOTE: (1) +3.3V bias is an arbitrary ac level < 5V that also
appears on the −IN through the op amp where it
applies a reverse bias to the photodiode.