LOG114 Datasheet, PDF (11/30 Pages) Burr-Brown (TI) – Single-Supply, High-Speed, Precision LOGARITHMIC AMPLIFIER

LOG114
www.ti.com
APPLICATIONS INFORMATION
OVERVIEW
The LOG114 is a precision logarithmic amplifier that is
capable of measuring currents over a dynamic range of
eight decades. It computes the logarithm, or log ratio,
of an input current relative to a reference current ac-
cording to equation (1).
ǒ Ǔ V_LOGOUT + 0.375
log₁₀
I₁
I₂
(1)
The output at V_LOGOUT can be digitized directly, or scaled
for an ADC input using an uncommitted or external op
amp.
An offsetting voltage (V_Com) can be connected to the
Com pin to raise the voltage at V_LOGOUT. When an
offsetting voltage is used, the transfer function
becomes:
V_LOGOUT + 0.375
ǒ Ǔ log₁₀
I₁
I₂
) V_Com
(2)
SBOS301A − MAY 2004 − REVISED MARCH 2007
Either I₁ or I₂ can be held constant to serve as the refer-
ence current, with the other input being used for the in-
put signal. The value of the reference current is selected
such that the output at V_LOGOUT (pin 9) is zero when the
reference current and input current are equal. An on-
chip 2.5V reference is provided for use in generating the
reference current.
Two additional amplifiers, A₄ and A₅, are included in the
LOG114 for use in scaling, offsetting, filtering, threshold
detection, or other functions.
BASIC CONNECTIONS
Figure 1 and Figure 2 show the LOG114 in typical dual
and single-supply configurations, respectively. To re-
duce 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 these capacitors as close to the LOG114
V+ supply pin to ground as possible improves supply−
related noise rejection.
R₇
100kΩ
R₈
56.2kΩ
I_REF
Q₁
1µF
4 ^I1
R₁
5 ^VCM IN
A₁
R_REF
2.5MΩ
Input Signal
100pA to 10mA
3
I₂
Q₂
R₃
A₂
R₅
100kΩ
9
V_LOGOUT(1)
10 11
+IN₄ −IN₄
R₆
66.5kΩ
LOG114
R₂
A₃
R₄
A₄
V_{O4(2) 12}
+IN_{5 13}
A₅
V_{O5 15}
16 V_REF
2.5V_REF
V_{REF GND}
1
V+
8 _1000pF
V−
6 _1000pF
10µF
+
+5V
10µF
+
−5V
Com
7
−IN₅
14
NOTE: (1) V_LOGOUT = 0.375 × log(I₁/I₂)
(2) V_O4 = −0.249 × log(I₁/I₂) + 1.5V
Figure 1. Dual Supply Configuration Example for Best Accuracy Over Eight Decades.
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