LOG102 Datasheet, PDF (7/15 Pages) Texas Instruments – LOGARITHMIC AND LOG RATIO AMPLIFIER

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.
V+
REF3025 ^2.5V
I₁ = 2.5nA to 1mA
1GΩ to 2.5kΩ
+25mV
100kΩ I₂ = 2.5nA
10MΩ
+2.5V
100Ω
V+
6
1
LOG102
5
V_LOGOUT
14
5
9 10
C_C
V–
OPA335 Chopper Om Amp
–2.5V
FIGURE 5. Current Source with Offset Compensation.
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
I₁ and I₂ 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 I₂ and the minimum value of I₁. 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 + R₂/R₁
(2)
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.
I_IN
Q_A
D₁
OPA703
Q_B
National
LM394
D₂
I_OUT
FIGURE 6. Current Inverter/Current Source.
+3.3V
+5V
TLV271
or
1
2
OPA2335
1/2 OPA2335
1.5kΩ
1.5kΩ
+5V
10nA to 1mA Back Bias
1/2 OPA2335
BSH203
+3.3V
Photodiode
LOG102
10nA to 1mA
Pin 1 or Pin 14
NOTE: OPA2335 Available Q2 2002
FIGURE 7. Precision Current Inverter/Current Source.
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.
LOG102
SBOS211A
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