Information

Texas Instruments Signal Chain Guide 2013 | 27
Get more information: www.ti.com/product/LOG114
The LOG114 is specifically designed for measuring low-level and wide dynamic
range currents in communications, lasers, medical and industrial systems. The device
computes the logarithm or log-ratio of an input current or voltage relative to a reference
current or voltage (logarithmic transimpedance amplifier).
High precision is ensured over a wide dynamic range of input signals on either Bipolar
(±5 V) or single (+5 V) supply. Special temperature drift compensation circuitry is includ-
ed on-chip. In log-ratio applications, the signal current may be from a high impedance
sour
ce such as a photodiode or resistor in series with a low impedance voltage source.
The reference current is provided by a resistor in series with a precision internal voltage
reference, photo diode or active current source.
LOG114 functional block diagram
Key Features
• Advantages:
Tiny for high-density systems
Precision on one supply
Fast over eight decades
Fully tested function
• Twoscalingamplifiers
• Wideinputdynamicrange:
eight decades, 100 pA to 10 mA
•
2.5Vreference
• Stableovertemperature
• Lowquiescentcurrent:10mA
• Dualorsinglesupply:±5V,+5V
• Specifiedtemperaturerange:
−5°C to +75°C
•
Package:smallQFN-16(4mmx4mm)
Applications
• Oneterbium-dopedfiberoptic
amplifiers (EDFA)
•
Laseropticaldensitymeasurement
• Photodiodesignalcompressionamps
• Log,log-ratiofunctions
• Analogsignalcompressioninfrontof
analog-to-digital converters
•
Absorbancemeasurement
Amplifiers and Comparators
Logarithmic Amplifiers
Single-Supply, High-Speed, Precision Logarithmic Amplifier
LOG114
Q
1
200
Ω
R
1
(1)
1250
Ω
R
2
R
5
R
6
200
Ω
R
3
(1)
R
REF
1250
Ω
R
4
V
O4
(3)
−
IN
4
+IN
4
V
LOGOUT
+IN
5
−
IN
5
V
O5
V
CM IN
I
1
4
I
1
and I
2
are current inputs
from a photodiode
or other current source
11
10
9
(2)
14
5
3
15
13
12
16
V
REF
2.5 V
V
−
ComV
REF GND
Q
2
A
3
(4)
A
1
A
2
A
4
LOG114
A
5
I
REF
I
2
V+
REF
NOTES: (1) Thermally dependent R
1
and R
3
provide temperature compensation.
(2) V
LOGOUT
= 0.375 × log(I
1
/I
2
).
(3) V
O4
= 0.375 × K × log(I
1
/I
2
)
K = 1 + R
6
/R
5
.
(4) Differential Amplifier (A
3
) Gain = 6.25
1 8 6 7
Logarithmic Amplifiers
Device
Scale
Factor
(V/Decade)
Input
Current
Range
(nA)
(min)
Input
Current
Range
(mA)
(max)
Conformity
Error
(Initial 5
Decades)
(%)
(max)
Conformity
Error
(Initial 5
Decades)
(%/°C)
(typ/temp)
Bandwidth
(kHz)
V
S
(V)
(min)
V
S
(V)
(max)
I
Q
Per
Ch.
(mA)
(max)
Reference
Type
Auxiliary
Op Amps
HiRel
Avail. Package Price*
LOG101 1 0.1 3.5 0.2 0.0001 38 9 36 1.5 External — N SO-8 6.95
LOG102 1 1 1 0.3 0.0002 38 9 36 2 External 2 N SO-14 7.25
LOG104 0.5 0.1 3.5 0.2 0.0001 38 9 36 1.5 External — N SO-8 6.95
LOG112 0.5 0.1 3.5 0.2 0.0001 38 9 36 1.75 2.5 V Internal 1 N SO-14 7.90
LOG2112 0.5 0.1 3.5 0.2 0.0001 38 9 36 1.75 2.5 V Internal 1/Ch N SO-16 11.35
LOG114 0.375 0.1 10 0.2 0.001 5000 5 10 15 2.5 V Internal 2 N QFN-16 7.90
*Suggested resale price in U.S. dollars in quantities of 1,000. See www.ti.com/hirel for HiRel options.