LOG100 Datasheet, PDF (7/9 Pages) Burr-Brown (TI) – Precision LOGARITHMIC AND LOG RATIO AMPLIFIER

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 LOG100 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. Frequency response
decreases as the gain increases.
GENERAL INFORMATION
INPUT CURRENT RANGE
The stated input range of 1nA to 1mA is the range for
specified accuracy. Smaller or larger input currents may be
applied with decreased accuracy. Currents larger than 1mA
result in increased nonlinearity. The 10mA absolute maxi-
mum is a conservative value to limit the power dissipation
in the output stage of A₁ and the logging transistor. Currents
below 1nA will result in increased errors due to the input
bias currents of A₁ and A₂ (1pA typical). These errors may
be nulled. See Optional Adjustments section.
FREQUENCY COMPENSATION
Frequency compensation for the LOG100 is obtained by
connecting a capacitor between pins 7 and 14. The size of
the capacitor is a function of the input currents as shown in
the Typical Performance Curves. For any given application,
the smallest value of the capacitor which may be used is
determined by the maximum value at I₂ and the minimum
value of I₁. Larger values of C_C will make the LOG100 more
stable, but will reduce the frequency response.
SETTING THE REFERENCE CURRENT
When the LOG100 is used as a straight log amplifier I₂ is
constant and becomes the reference current in the expression
I
V_OUT = K log
1
I_REF
(21)
I_REF can be derived from an external current source (such as
shown in Figure 4), or it may be derived from a voltage
source with one or more resistors.
When a single resistor is used, the value may be quite large
when I_REF is small. If I_REF is 10nA and +15V is used
R_REF =
15V
10nA
= 1500MΩ.
+15V
R_REF
2N2905
6V
IN834
I_REF
2N2905
3.6kΩ
6V
I_REF = _RREF
–15V
FIGURE 4. Temperature-Compensated Current Reference.
A voltage divider may be used to reduce the value of the
resistor. When this is done, one must be aware of possible
errors caused by the amplifier’s input offset voltage. This is
shown in Figure 5.
In this case the voltage at pin 14 is not exactly zero, but is
equal to the value of the input offset voltage of A₁, which
ranges from zero to ±5mV. V_T must be kept much larger
than 5mV in order to make this effect negligible. This
concept also applies to pin 1.
R₁
V_T
R₃
V_OS
+–
14
I_REF
A₁
V_REF
R₂
FIGURE 5. “T” Network for Reference Current.
OPTIONAL ADJUSTMENTS
The LOG100 will meet its specified accuracy with no user
adjustments. If improved performance is desired, the follow-
ing optional adjustments may be made.
INPUT BIAS CURRENT
The circuit in Figure 6 may be used to compensate for the
input bias currents of A₁ and A₂. Since the amplifiers have
FET inputs with the characteristic bias current doubling
every 10°C, this nulling technique is practical only where
the temperature is fairly stable.
R₂
10kΩ
+V_CC
–V_CC
R₁
1kMΩ
9
1
7
I₁
LOG100
14
10
I₂
R₁'
1kMΩ
6
5 43
–V_CC
R₂'
10kΩ
C_C
+V_CC
FIGURE 6. Bias Current Nulling.
+
V_OUT
–
OUTPUT OFFSET
The output offset may be nulled with the circuit in Figure 7.
I₁ and I₂ are set equal at some convenient value in the range
of 100nA to 100µA. R₁ is then adjusted for zero output
voltage.
®
7
LOG100