LOG102 Datasheet, PDF (6/15 Pages) Texas Instruments – LOGARITHMIC AND LOG RATIO AMPLIFIER

APPLICATION INFORMATION
R₂
10kΩ
The LOG102 is a true logarithmic amplifier that uses the
V–
V+
base-emitter voltage relationship of bipolar transistors to
compute the logarithm, or logarithmic ratio of a current ratio.
With two uncommitted on-chip operational amplifiers, the
LOG102 provides design flexibility and simplicity.
Figure 1 shows the basic connections required for operation
R₁
1MΩ
6
1
5
I₁
LOG102
V_OUT
of the LOG102 with a gain factor. In order to reduce the
14
10
influence of lead inductance of power supply lines, it is
recommended that each supply be bypassed with a 10µF
I₂
R₁'
> 1MΩ
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tantalum capacitor in parallel with a 1000pF ceramic capaci-
tor, as shown in Figure 1. Connecting the capacitors as close
to the LOG102 as possible will contribute to noise reduction
V–
R₂'
10kΩ
C_C
V+
as well.
FIGURE 2. Bias Current Nulling.
V+
10µF
1
10
14
I₁
I₂
1000pF
6
LOG102
9 11
C_C
V_OUT = G • V_LOGOUT
12
7
4
5
3
8
V_LOGOUT
V_OUT
R₁
R₂
1000pF
10µF
V–
Amplifier A₄ not being used.
Unused amplifiers should
have positive inputs grounded
and negative inputs tied to
their respective outputs.
FIGURE 1. Basic Connections with Output Gain Factor of the
LOG102.
INPUT CURRENT RANGE
To maintain specified accuracy, the input current range of the
LOG102 should be limited from 1nA to 1mA. Input currents
outside of this range may compromise LOG102 performance.
Input currents larger than 1mA result in increased nonlinearity.
An absolute maximum input current rating of 10mA is included
to prevent excessive power dissipation that may damage the
logging transistor.
On ±5V supplies the total input current (I₁ + I₂) is limited to
1.1mA. Due to compliance issues internal to the LOG102, to
accommodate larger total input currents, supplies should be
increased.
Currents smaller than 1nA will result in increased errors due
the input bias currents of op amps A₁ and A₂ (typically 5pA).
The input bias currents may be compensated for, as shown in
Figure 2. The input stages of the amplifiers have FET inputs,
with input bias current doubling every 10°C, which makes the
nulling technique shown practical only where the temperature
is fairly stable.
SETTING THE REFERENCE CURRENT
When the LOG102 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_LOGOUT is expressed as:
V_LOGOUT = (1V) • 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 = 250MΩ
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
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I_REF
A₁
R₂
R₃ >> R₂
FIGURE 4. T Network for Reference Current.
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LOG102
SBOS211A