Datasheet

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SBOS301A − MAY 2004 − REVISED MARCH 2007
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16
SETTING THE REFERENCE CURRENT
When the LOG114 is used to compute logarithms, ei-
ther I
1
or I
2
can be held constant to become the refer-
ence current to which the other is compared.
If I
REF
is set to the lowest current in the span of the signal
current (as shown in the front page figure), V
LOGOUT
will
range from:
V
LOGOUT
+ 0.375 log
10
ǒ
I
1
min
I
1
max signal
Ǔ
^ 0V
to some maximum value:
V
LOGOUT
+ 0.375 log
10
ǒ
I
1
min
I
1
max signal
Ǔ
While convenient, this approach does not usually result
in best performance, because I
1
min accuracy is difficult
to achieve, particularly if it is < 20nA.
A better way to achieve higher accuracy is to choose
I
REF
to be in the center of the full signal range. For
example, for a signal range of 1nA to 1mA, it is better
to use this approach:
I
REF
+ I
SIGNAL
min 1mAń1nA
Ǹ
+ 1mAdc
than it is to set I
REF
= 1nA. It is much easier and more
precise (that is, dc accuracy, temperature stability, and
lower noise) to establish a 1mA dc current level than a
1nA level for the reference current.
The reference current may be derived from a voltage
source with one or more resistors. When a single resis-
tor is used, the value may be large depending on I
REF
.
If I
REF
is 10nA and +2.5V is used:
R
REF
= 2.5V/10nA = 250MΩ
A voltage divider may be used to reduce the value of the
resistor, as shown in Figure 5. When using this method,
one must consider the possible errors caused by the
amplifier input offset voltage. The input offset voltage of
amplifier A
1
has a maximum value of 4mV in a ±5V sup-
ply system, and a maximum value of 7mV in a +5V sup-
ply system. Resistor temperature stability and noise
contributions should also be considered.
A
1
+
R
2
R
1
+5V
R
3
V
REF
=100mV
R
3
>> R
2
I
REF
−
V
OS
1
Figure 5. T-Network for Reference Current.
V
REF
may be an external precision voltage reference, or
the on-chip 2.5V voltage reference of the LOG114.
I
REF
can be derived from an external current source,
such as that shown in Figure 6.
2N2905
I
REF
R
REF
2N2905
+15V
−
15V
I
REF
=
6V
R
REF
3.6k
Ω
6V
IN834
Figure 6. Temperature-Compensated Current Source.
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