LOG101 Datasheet, PDF (5/13 Pages) Burr-Brown (TI) – Precision LOGARITHMIC AND LOG RATIO AMPLIFIER

TYPICAL CHARACTERISTICS (Cont.)
At T_A = +25°C, V_S = ±5V, and R_L = 10kΩ, unless otherwise noted.
17
15
13
11
9
7
5
3
1
–1
100pA
LOG CONFORMITY vs INPUT CURRENT
+85°C
+75°C
–40°C to +25°C
1nA 10nA 100nA 1µA 10µA 100µA 1mA
Input Current (I₁ or I₂)
LOG CONFORMITY vs TEMPERATURE
350
300
7 Decades
(100pA to 1mA)
250
6 Decades
200
(1nA to 1mA)
150
5 Decades
100
(1nA to 100µA)
50
0
–40 –30 –20 –10 0 10 20 30 40 50 60 70 80 90
Temperature (°C)
APPLICATION INFORMATION
The LOG101 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 shows the basic connections required for operation
of the LOG101. 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. Connecting the capacitors as close to the LOG101
as possible will contribute to noise reduction as well.
I₁
I₂
V+
10µF
1000pF
4
1
6
3
LOG101
V_OUT
8
5
C_C
10µF
1000pF
V–
FIGURE 1. Basic Connections of the LOG101.
INPUT CURRENT RANGE
To maintain specified accuracy, the input current range of the
LOG101 should be limited from 100pA to 3.5mA. Input currents
outside of this range may compromise LOG101 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 logging transistor.
On ±5V supplies, the total input current (I₁ + I₂) is limited to
4.5mA. Due to compliance issues internal to the LOG101, to
accommodate larger total input currents, supplies should be
increased.
Currents smaller than 100pA will result in increased errors due
to 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.
R₂
10kΩ
V+
R₁
1MΩ
V–
5
1
I₁
LOG101
8
I₂
R₁'
> 1MΩ
4
V–
R₂'
10kΩ
C_C
V+
FIGURE 2. Bias Current Nulling.
3
V_OUT
6 ^GND
LOG101
SBOS242B
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