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參數(shù)資料
| 型號： | LOG104AIDRE4 |
| 廠商： | TEXAS INSTRUMENTS INC |
| 元件分類： | 模擬運算功能 |
| 英文描述： | LOG OR ANTILOG AMPLIFIER, 0.01 MHz BAND WIDTH, PDSO8 |
| 封裝： | ROHS COMPLIANT, PLASTIC, MS-012AA, SOIC-8 |
| 文件頁數(shù)： | 10/13頁 |
| 文件大小： | 244K |
| 代理商： | LOG104AIDRE4 |
www.ti.com
LOG104
6
SBOS243C
FIGURE 6. Current Inverter/Current Source.
1M
+2.5mV
100k
–2.5V
+2.5V
OPA335
100
I
1 = 2.5nA to 1mA
I
2 = 2.5nA
V
OUT
C
1G
to 2.5k
REF3025
8
3
1
56
V–
LOG104
4
3
2.5V
V+
Chopper Op Amp
GND
FIGURE 5. Current Source with Offset Compensation.
Figure 5 shows a low-level current source using a series
resistor. The low offset op-amp reduces the effect of the
LOG104’s input offset voltage.
FREQUENCY RESPONSE
The frequency response curves seen in the Typical Charac-
teristics Curves are shown for constant DC I1 and I2 with a
small-signal AC current on one input.
The 3dB frequency response of the LOG104 is a function of
the magnitude of the input current levels and of the value of the
frequency compensation capacitor. See Typical Characteristic
Curve,
3dB Frequency Response for details.
The transient response of the LOG104 is different for in-
creasing and decreasing signals. This is due to the fact that
a log amp is a nonlinear gain element and has different gains
Q
B
National
LM394
I
IN
I
OUT
D
1
OPA703
D
2
Q
A
2N2905
I
REF
R
REF
2N2905
+15V
–15V
I
REF =
6V
R
REF
3.6k
6V
IN834
FIGURE 3. Temperature Compensated Current Source.
SETTING THE REFERENCE CURRENT
When the LOG104 is used to compute logarithms, either I1 or
I2 can be held constant and becomes the reference current to
which the other is compared.
VOUT is expressed as:
VOUT = (0.5V) log (I1/I2)(1)
IREF can be derived from an external current source (such as
shown in Figure 3), or it may be derived from a voltage
source with one or more resistors. When a single resistor is
used, the value may be large depending on IREF. If IREF is
10nA and +2.5V is used:
RREF = 2.5V/10nA = 250M
A
1
+
R
2
R
1
+5V
R
3
V
REF = 100mV
R
3 >> R2
I
REF
–
V
OS
1
FIGURE 4. T Network for Reference Current.
A voltage divider may be used to reduce the value of the
resistor (as shown in Figure 4). When using this method, one
must consider the possible errors caused by the amplifier’s
input offset voltage. The input offset voltage of amplifier A1
has a maximum value of 1.5mV, making VREF a suggested
value of 100mV.
(2)
at different levels of input signals. Smaller input currents
require greater gain to maintain full dynamic range, and will
slow the frequency response of the LOG104.
FREQUENCY COMPENSATION
Frequency compensation for the LOG104 is obtained by
connecting a capacitor between pins 3 and 8. The size of the
capacitor is a function of the input currents, as shown in the
Typical Characteristic Curves (
Minimum Value of Compen-
sation Capacitor). For any given application, the smallest
value of the capacitor which may be used is determined by
the maximum value of I2 and the minimum value of I1. Larger
values of CC will make the LOG104 more stable, but will
reduce the frequency response.
In an application, highest overall bandwidth can be achieved
by detecting the signal level at VOUT, then switching in
appropriate values of compensation capacitors.
NEGATIVE INPUT CURRENTS
The LOG104 will function only with positive input currents
(conventional current flows into pins 1 and 8). In situations
where negative input currents are needed, the circuits in
Figures 6, 7, and 8 may be used.
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