參數資料
型號： LOG114AIRGVT
廠商： TEXAS INSTRUMENTS INC
元件分類： 模擬運算功能
英文描述： LOG OR ANTILOG AMPLIFIER, PQCC16
封裝： 4 X 4 MM, GREEN, PLASTIC, QFN-16
文件頁數： 6/29頁
文件大小： 637K
代理商： LOG114AIRGVT
LOG114
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 I1 or I2 as the signal input pin. For this
example, I2 is used. Choosing I1 as the reference
current makes the resistor network around A4
simpler. (Note: Current only flows into the I1 and I2
pins.)
2.
Select the magnitude of the reference current.
Since the signal (I2) spans eight decades, set I1 to
1
A four decades above the minimum I2 value,
and four decades below the maximum I2 value.
(Note that it does not have to be placed in the
middle. If I2 spanned seven decades, I1 could be set
three decades above the minimum and four
decades below the maximum I2 value.) This
configuration results in more swing amplitude in the
negative direction, which provides more sensitivity
(
VO4 per I2) when the current signal decreases.
3.
Using Equation (1) calculate the expected range of
log outputs at VLOGOUT:
For I
2 + 10mA :
V
LOGOUT + 0.375
log
I
1
I
2
+ 0.375
log
1
mA
10mA
+* 1.5V
For I
2 + 100pA :
V
LOGOUT + 0.375
log
I
1
I
2
+ 0.375
log
1
mA
100pA
+) 1.5V
Therefore, the expected voltage range at the output
of amplifier A3 is:
* 1.5V v V
LOGOUT v) 1.5V
This result would be fine in a dualsupply 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, VCom to use for centering
the output between (V) + 0.6V and (V+) 0.6V,
which is the full-scale output capability of the A3
amplifier. Choosing VCom = 2.5V, and recalculating
the expected voltage output range for VLOGOUT using
Equation (2), results in:
) 1V v V
LOGOUT v) 4V
5.
The A4 amplifier scales and offsets the VLOGOUT
signal for use by the ADC using the equation:
V
O4 +*SFACTOR
V
LOGOUT ) VOFFSET
The A4 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
≤ VO4 ≤ +2.5V
This output results in a 2V range for the 3V VLOGOUT
range, or 2V/3V scaling factor.
6.
When I2 = 10mA, VLOGOUT = +1V, and VO4 = 2.5V.
Using the equation in step 5:
V
O4 +*SFACTOR
V
LOGOUT ) VOFFSET
2.5V
+*2V 3V(1V) ) V
OFFSET
Therefore, VOFFSET = 3.16V
The A4 amplifier configuration for VO4 = 2/3(VLOGOUT) +
3.16 is seen in Figure 4a.
The overall transer function is:
V
O4 +*0.249
log
I
1
I
2
) 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, RP, 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 VOFFSET is 3.3V. This circuit
configuration is shown in Figure 4b.
(8)
(9)
(10)
(11)
(12)
(13)
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