LOG112 Datasheet, PDF (7/16 Pages) Texas Instruments – LOGARITHMIC AND LOG RATIO AMPLIFIERS

APPLICATION INFORMATION
The LOG112 is a true logarithmic amplifier that uses the
base-emitter voltage relationship of bipolar transistors to
compute the logarithm, or logarithmic ratio of a current ratio.
Figure 1 and Figure 2 show the basic connections required
for operation of the LOG112 and LOG2112. In order to
reduce the influence of lead inductance of power-supply
lines, it is recommended that each supply be bypassed with
a 10µF tantalum capacitor in parallel with a 1000pF ceramic
capacitor, as shown in Figure 1 and Figure 2. Connecting
the capacitors as close to the LOG112 and LOG2112 as
possible will contribute to noise reduction as well.
I₁
I₂
V+
10µF
1000pF
6
1
8
V_REF
5
LOG112
V_LOGOUT
11 ^{VREF – GND}
14
9 10 13
V_{CM – IN}
C_C
10µF
1000pF
V–
FIGURE 1. Basic Connections of the LOG112.
V+
I_2A
I_1A
I_1B
I_2B
10µF
1000pF
C_CA
6
2
1
5
V_LOGOUTA
9
V_REF
LOG2112
16
15
12
V_LOGOUTB
11
8
C_CB
10µF
1000pF
V–
FIGURE 2. Basic Connections of the LOG2112.
INPUT CURRENT RANGE
To maintain specified accuracy, the input current range of the
LOG112 and LOG2112 should be limited from 100pA to
3.5mA. Input currents outside of this range may compromise
the LOG112 performance. Input currents larger than 3.5mA
result in increased nonlinearity. An absolute maximum input
current rating of 10mA is included to prevent excessive power
dissipation that may damage the input transistor.
On ±5V supplies, the total input current (I₁ + I₂) is limited to
4.5mA. Due to compliance issues internal to the LOG112 and
LOG2112, to accommodate larger total input currents, supplies
should be increased.
SETTING THE REFERENCE CURRENT
When the LOG112 and LOG2112 are used to compute loga-
rithms, either I₁ or I₂ can be held constant to become the
reference current to which the other is compared.
V_LOGOUT is expressed as:
V_LOGOUT = (0.5V)LOG (I₁/I_REF)
(1)
I_REF can be derived from an external current source (such as
that 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 = 250MΩ
(2)
+15V
R_REF
2N2905
6V
IN834
I_REF
2N2905
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.
+5V
V_REF = 100mV
R₁
R₃
V_OS
+–
1
I_REF
A₁
R₂
R₃ >> R₂
FIGURE 4. T Network for Reference Current.
LOG112, 2112
7
SBOS246C
www.ti.com