Datasheet

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SBOS301A − MAY 2004 − REVISED MARCH 2007
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13
DESIGN EXAMPLE FOR DUAL-SUPPLY
CONFIGURATION
Given these conditions:
D V+ = 5V and V− = −5V
D 100pA ≤ Input signal
D The stage following the LOG114 is an analog-to-
digital converter (ADC) with +5V supply and
+2.5V reference voltage, so V
O4
swings from
+0.5V to +2.5V.
1. Due to LOG114 symmetry, you can choose either
I
1
or I
2
as the signal input pin. Choosing I
1
as the
reference makes the resistor network around A4
simpler. (Note: Current must flow into pins 3 (I
1
) and
pin 4 (I
2
).)
2. Select the magnitude of the reference current.
Since the signal (I
2
) spans eight decades, set I
1
to
1µA − four decades above the minimum I
2
value.
(Note that it does not have to be placed in the
middle. If I
2
spanned seven decades, I
1
could be set
three decades above the minimum and four
decades below the maximum I
2
value.) This
configuration results in more swing amplitude in the
negative direction, which provides more sensitivity
(∆V
O4
per ∆I
2
) when the current signal decreases.
3. Using Equation (1) calculate the expected range of
log outputs at V
LOGOUT
:
For I
2
+ 10mA :
V
LOGOUT
+ 0.375 log
ǒ
I
1
I
2
Ǔ
+ 0.375 log
ǒ
1mA
10mA
Ǔ
+*1.5V
For I
2
+ 100pA :
V
LOGOUT
+ 0.375 log
ǒ
I
1
I
2
Ǔ
+ 0.375 log
ǒ
1mA
100pA
Ǔ
+)1.5V
Therefore, the expected voltage range at the output
of amplifier A
3
is:
* 1.5V v V
LOGOUT
v)1.5V
4. The A
4
amplifier scales and offsets the V
LOGOUT
signal for use by the ADC using the equation:
V
O4
+*S
FACTOR
ǒ
V
LOGOUT
Ǔ
) V
OFFSET
The A
4
amplifier is specified with a rated output swing
capability from (V−) +0.5V to (V+) − 0.5V.
Therefore, choose the final A
4
output:
0V ≤ V
O4
≤ +2.5V
This output results in a 2.5V range for the 3V V
LOGOUT
range, or 2.5V/3V scaling factor.
5. When I
2
= 10mA, V
LOGOUT
= −1.5V. Using the
equation in step 5:
V
O4
+*S
FACTOR
ǒ
V
LOGOUT
Ǔ
) V
OFFSET
0V +*2.5Vń3V(*1.5V) ) V
OFFSET
Therefore, V
OFFSET
= 0V
The A
4
amplifier configuration for V
O4
= −2.5/3(V
LOGOUT
)
+ 0V is seen in Figure 3.
The overall transer function is:
V
O4
+*0.249 log
ǒ
I
1
I
2
Ǔ
) 1.5V
A
4
Internal A
4
Output Amplifier
R
5
100k
Ω
V
REF
+2.5V
V
LOGOUT
V
O4
=
−
2/3 (V
LOGOUT
)
I
2
100pA
10mA
A
4
amplifierusedtoscaleandoffsetV
LOGOUT
for 0V to 2.5V output.
R
6
82.5k
Ω
R
7
100k
Ω
R
8
37.4k
Ω
V
O4
0V +2.5V
+5V
−
5V
Figure 3. Operational Amplifier Configuration for
Scaling the Output Going to ADC Stage.
(3)
(4)
(5)
(6)
(7)