Datasheet

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LOG102
7
SBOS211B
FIGURE 6. Current Inverter/Current Source.
1MΩ
+2.5mV
100kΩ
–2.5V
+2.5V
OPA335
100Ω
I
1
= 2.5nA to 1mA
I
2
= 2.5nA
V
LOGOUT
C
C
1GΩ to 2.5kΩ
REF3025
14
5
1
910
5
V–
LOG102
6
2.5V
V+
V+
Chopper Om Amp
FIGURE 5. Current Source with Offset Compensation.
Figure 5 shows a low-level current source using a series
resistor. The low offset op-amp reduces the effect of the
LOG102’s input offset voltage.
FIGURE 7. Precision Current Inverter/Current Source.
Q
B
National
LM394
I
IN
I
OUT
D
1
OPA703
D
2
Q
A
1.5kΩ
Photodiode
10nA to 1mA
10nA to 1mA
+5V
+5V
1.5kΩ
+3.3V
+3.3V
1/2 OPA2335
1/2 OPA2335
OPA2335
1
2
BSH203
TLV271 or
Back Bias
LOG102
Pin 1 or Pin 14
NEGATIVE INPUT CURRENTS
The LOG102 will function only with positive input currents
(conventional current flow into pins 1 and 14). In situations
where negative input currents are needed, the circuits in
Figures 6, 7, 8, and 9, may be used.
VOLTAGE INPUTS
The LOG102 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 (14) applies to this configura-
tion.
FREQUENCY RESPONSE
The 3dB frequency response of the LOG102 is a function of
the magnitude of the input current levels and of the value of
the frequency compensation capacitor. See Typical Charac-
teristic Curves for details.
The frequency response curves are shown for constant DC
I
1
and I
2
with a small signal AC current on one of them.
The transient response of the LOG102 is different for in-
creasing 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 LOG102.
FREQUENCY COMPENSATION
Frequency compensation for the LOG102 is obtained by
connecting a capacitor between pins 5 and 14. The size of
the capacitor is a function of the input currents, as shown in
the Typical Characteristic Curves (Minimum Value of Com-
pensation 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
will make the LOG102 more stable, but will
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.
As seen on front page diagram, the voltage output of V
LOGOUT
can be scaled by increasing or decreasing the resistor ratio
connected to pins 4 and 7. The gain, G, can be set according
to the following equation:
G = 1 + R
2
/R
1
(2)