LOG104 Datasheet, PDF (6/13 Pages) Texas Instruments – LOGARITHMIC AND LOG RATIO AMPLIFIER

SETTING THE REFERENCE CURRENT
When the LOG104 is used to compute logarithms, either I₁ or
I₂ can be held constant and becomes the reference current to
which the other is compared.
V_OUT is expressed as:
V_OUT = (0.5V) • log (I₁/I₂)
(1)
I_REF can be derived from an external current source (such as
shown in Figure 3), or it may be derived from a voltage
source with one or more resistors. When a single resistor is
used, the value may be large depending on I_REF. If I_REF is
10nA and +2.5V is used:
R_REF = 2.5V/10nA = 250M
(2)
I_REF
2N2905
+15V
R_REF
2N2905
6V
IN834
3.6kΩ
6V
I_REF = _RREF
–15V
FIGURE 3. Temperature Compensated Current Source.
A voltage divider may be used to reduce the value of the
resistor (as shown in Figure 4). When using this method, one
must consider the possible errors caused by the amplifier’s
input offset voltage. The input offset voltage of amplifier A₁
has a maximum value of 1.5mV, making V_REF a suggested
value of 100mV.
V_REF = 100mV
R₁
R₃
V_OS
+–
+5V
1
I_REF
A₁
R₂
R₃ >> R₂
FIGURE 4. T Network for Reference Current.
V+
REF3025 ^2.5V
I₁ = 2.5nA to 1mA
1GΩ to 2.5kΩ
+25mV
100kΩ I₂ = 2.5nA
10MΩ
+2.5V
100Ω
V+
4
1
LOG104
3
V_OUT
8
3
5
C_C
V–
6 _GND
OPA335 Chopper Op Amp
–2.5V
FIGURE 5. Current Source with Offset Compensation.
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 LOG104.
FREQUENCY COMPENSATION
Frequency compensation for the LOG104 is obtained by
connecting a capacitor between pins 3 and 8. The size of the
capacitor is a function of the input currents, as shown in the
Typical Characteristic Curves (Minimum Value of Compen-
sation 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 LOG104 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.
NEGATIVE INPUT CURRENTS
The LOG104 will function only with positive input currents
(conventional current flows into pins 1 and 8). In situations
where negative input currents are needed, the circuits in
Figures 6, 7, and 8 may be used.
Figure 5 shows a low-level current source using a series
resistor. The low offset op-amp reduces the effect of the
LOG104’s input offset voltage.
FREQUENCY RESPONSE
The frequency response curves seen in the Typical Charac-
teristics Curves are shown for constant DC I₁ and I₂ with a
small-signal AC current on one input.
The 3dB frequency response of the LOG104 is 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 LOG104 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
I_IN
Q_A
D₁
OPA703
Q_B
National
LM394
D₂
I_OUT
FIGURE 6. Current Inverter/Current Source.
6
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LOG104
SBOS243B