Datasheet

LOG112, 2112
7
SBOS246D
www.ti.com
FIGURE 1. Basic Connections of the LOG112.
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.
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 (I
1
+ I
2
) 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 I
1
or I
2
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 (I
1
/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)
14
1
5
8
6
91013
V–
V+
10µF
LOG112
1000pF
10µF 1000pF
I
1
I
2
V
LOGOUT
V
REF
11
V
REF – GND
V
CM – IN
C
C
15
16
5
6
11 8
V–
V+
10µF
LOG2112
1000pF
10µF
1000pF
I
1B
I
2B
1
2
I
2A
I
1A
V
LOGOUTA
9
V
REF
12
V
LOGOUTB
C
CB
C
CA
FIGURE 2. Basic Connections of the LOG2112.
FIGURE 3. Temperature Compensated Current Source.
2N2905
I
REF
R
REF
2N2905
+15V –15V
I
REF
=
6V
R
REF
3.6kΩ
6V
IN834
FIGURE 4. T Network for Reference Current.
A
1
+
R
2
R
1
+5V
R
3
V
REF
= 100mV
R
3
>> R
2
I
REF
–
V
OS
1
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 A
1
has a maximum value of 1.5mV, making V
REF
a suggested
value of 100mV.