LOG101 Datasheet, PDF (10/13 Pages) Burr-Brown (TI) – Precision LOGARITHMIC AND LOG RATIO AMPLIFIER

+15V to +60V
500Ω
I_SHUNT
5kΩ
5kΩ
+5V
Irx = 1µA to 1mA
Receiver
APD
I to V
Converter
10Gbits/sec
INA168
SOT23-5
2
1
I_OUT
I_OUT = 0.1 • I_SHUNT
+5V
4
1
Q₁
Q₂
100µA
25kΩ
REF3025
2.5V
A₁
8
LOG101
SO-8
5
–5V
C_C
1.2kΩ
1kΩ
3
A₂
OPA703
6
V_OUT = 2.5V to 0V
FIGURE 14. High Side Shunt for Avalanche Photodiode (APD) Measures 3-Decades of APD Current.
The individual component of error is:
∆K = gain accuracy (0.15%, typ), as specified in the
specification table.
I_B1 = bias current of A₁ (5pA, typ)
I_B2 = bias current of A₂ (5pA, typ)
N = log conformity error (0.01%, 0.06%, typ)
0.01% for n = 5, 0.06% for n = 7
V_OSO = output offset voltage (3mV, typ)
n = number of decades over which N is specified:
Example: what is the error when
I₁ = 1µA and I₂ = 100nA
(10)
V_OUT
=
(1±
0.0015)
log
10 ^−6
10 ^−7
− 5 • 10^−12
− 5 • 10^−12
± (2)(0.0001)5 ± 3.0mV
= 1.005055V
(11)
Since the ideal output is 1.000V, the error as a percent of
reading is
% error = ^0.005055 • 100% = 0.5%
(12)
1
For the case of voltage inputs, the actual transfer function is
V_OUT = (1V)(1± ∆K)log
V₁
R₁
–
I_B1
±
E_OS1
R₁
V₂
R₂
– I_B2
± ^EOS2
R₂
± 2Nn ± V_OSO
(13)
Where
E_{OS1 and} E_OS2
R₁
R₂
are considered to be zero for large
values of resistance from external input current sources.
10
www.ti.com
LOG101
SBOS242B