Datasheet
"#$$%
SBOS301A − MAY 2004 − REVISED MARCH 2007
www.ti.com
11
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).
V
LOGOUT
+ 0.375 log
10
ǒ
I
1
I
2
Ǔ
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 log
10
ǒ
I
1
I
2
Ǔ
) V
Com
Either I
1
or I
2
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, A
4
and A
5
, 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.
Q
1
R
1
R
2
R
5
100k
Ω
R
6
66.5k
Ω
R
7
100k
Ω
R
8
56.2k
Ω
R
3
R
REF
2.5M
Ω
R
4
V
O4
(2)
−
IN
4
+IN
4
V
LOGOUT
(1)
+IN
5
−
IN
5
V
O5
V
CM IN
I
1
4
1110
9
14
5
3
15
13
12
16
V
REF
2.5V
REF
V−
Com
V
REF GND
Q
2
A
3
A
1
A
2
A
4
A
5
I
REF
1
µ
F
Input Signal
100pAto 10mA
I
2
V+
1
86 7
+5V
++
−
5V
1000pF
1000pF
10
µ
F10
µ
F
LOG114
NOTE: (1) V
LOGOUT
= 0.375
×
log(I
1
/I
2
)
(2) V
O4
=
−
0.249
×
log(I
1
/I
2
)+1.5V
Figure 1. Dual Supply Configuration Example for Best Accuracy Over Eight Decades.
(1)
(2)