Datasheet

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SBOS301A − MAY 2004 − REVISED MARCH 2007
www.ti.com
14
DESIGN EXAMPLE FOR SINGLE-SUPPLY
CONFIGURATION
Given these conditions:
D V+ = 5V
D V− = GND
D 100pA ≤ Input signal ≤ 10mA
D The stage following the LOG114 is an analog to
digital converter (ADC) with +5V supply and
+2.5V reference voltage
1. Choose either I
1
or I
2
as the signal input pin. For this
example, I
2
is used. Choosing I
1
as the reference
current makes the resistor network around A4
simpler. (Note: Current only flows into the I
1
and I
2
pins.)
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,
and four decades below the maximum 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
This result would be fine in a dual−supply system
(V+ = +5V, V− = −5V) where the output can swing
below ground, but does not work in a single supply
+5V system. Therefore, an offset voltage must be
added to the system.
4. Select an offset voltage, V
Com
to use for centering
the output between (V−) + 0.6V and (V+) − 0.6V,
which is the full-scale output capability of the A
3
amplifier. Choosing V
Com
= 2.5V, and recalculating
the expected voltage output range for V
LOGOUT
using
Equation (2), results in:
) 1V v V
LOGOUT
v)4V
5. 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 A4 output:
+0.5V ≤ V
O4
≤ +2.5V
This output results in a 2V range for the 3V V
LOGOUT
range, or 2V/3V scaling factor.
6. When I
2
= 10mA, V
LOGOUT
= +1V, and V
O4
= 2.5V.
Using the equation in step 5:
V
O4
+*S
FACTOR
ǒ
V
LOGOUT
Ǔ
) V
OFFSET
2.5V +*2Vń3V(1V) ) V
OFFSET
Therefore, V
OFFSET
= 3.16V
The A
4
amplifier configuration for V
O4
= −2/3(V
LOGOUT
) +
3.16 is seen in Figure 4a.
The overall transer function is:
V
O4
+*0.249 log
ǒ
I
1
I
2
Ǔ
) 1.5V
A similar process can be used for configuring an
external rail-to-rail output op amp, such as the OPA335.
Because the OPA335 op amp can swing down to 0V
using a pulldown resistor, R
P
, connected to −5V (for
details, refer to the OPA335 data sheet, available for
download at www.ti.com), the scaling factor is 2.5V/3V
and the corresponding V
OFFSET
is 3.3V. This circuit
configuration is shown in Figure 4b.
(8)
(9)
(10)
(11)
(12)
(13)