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參數(shù)資料
| 型號： | AN-242 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | LJT 18C 18#20 SKT RECP |
| 中文描述： | 應用新的精密運算放大器 |
| 文件頁數(shù)： | 2/16頁 |
| 文件大小： | 267K |
| 代理商： | AN-242 |
Table I. Typical characteristics of the
LM11 for T
j
e
25
§
C and V
S
e
g
15V. Operation is
specified down to V
S
e
g
2.5V.
Parameter
Conditions
Input Offset Voltage
Input Offset Current
Input Bias Current
Input Noise Voltage 0.01 Hz
s
f
s
10 Hz
Input Noise Current 0.01 Hz
s
f
s
10 Hz
Long Term Stability
T
j
e
25
§
C
Offset Voltage Drift
b
55
§
C
s
T
j
s
125
§
C
Offset Current Drift
b
55
§
C
s
T
j
s
125
§
C
Bias Current Drift
b
55
§
C
s
T
j
s
125
§
C
Voltage Gain
V
OUT
e
g
12V,
I
OUT
e
g
0.5 mA
V
OUT
e
g
12V,
I
OUT
e
g
2 mA
Common-Mode
b
12.5V
s
V
CM
s
14V
Rejection
Supply-Voltage
g
2.5V
s
V
S
sg
20V
Rejection
Slew Rate
Supply Current
Value
100
m
V
500 fA
25 pA
8
m
Vpp
1 pApp
10
m
V
1
m
V/
§
C
20 fA/
§
C
500 fA/
§
C
1,200V/mV
300V/mV
130 dB
118 dB
0.3V/
m
s
300
m
A
As might be expected, the low bias currents were obtained
with some sacrifice in noise. But the low frequency noise
voltage is still a bit less than a FET amplifier and probably
more predictable. The latter is important because this noise
cannot be tested in production. Long term measurements
have not indicated any drift in excess of the noise. This is
not the case for FETs.
It is worthwhile noting that the drift of offset voltage and
current is low enough that DC accuracy is noise limited in
room-temperature applications.
bias current compensation
The LM11 can operate from M
X
source resistances with
little increase in the equivalent offset voltage, as can be
seen in Figure 2. This is impressive considering the low ini-
tial offset voltage. The situation is much improved if the de-
sign can be configured so that the op amp sees equal resist-
ance on the two inputs. However, this cannot be done with
all circuits. Examples are integrators, sample and holds, log-
arithmic converters and signal-conditioning amplifiers. And
even though the LM11 bias current is low, there will be
those applications where it needs to be lower.
Referring back to Figure 1, it can be seen that the bias
current drift is essentially linear over a
b
50
§
C to
a
100
§
C
range. This is a deliberate consequence of the input stage
design. Because of it, relatively simple circuitry can be used
to develop a compensating current.
TL/H/7479–2
Figure 2. The LM11 operates from M
X
source resistanc-
es with little DC error. With equal source re-
sistances, accuracy is essentially limited by
low frequency current noise.
Bias current compensation is not new, but making it effec-
tive with even limited temperature excursions has been a
problem. An early circuit suggested for bipolar ICs is shown
inFigure 3a. The compensating current is determined by the
diode voltage. This does not vary as rapidly with tempera-
ture as bias current nor does it match the usual non-lineari-
ties.
With the improved circuit inFigure 3b, the temperature coef-
ficient can be increased by using a transistor and including
R2. The drop across R2 is nearly constant with temperature.
The voltage delivered to the potentiometer has a 2.2 mV/
§
C
drift while its magnitude is determined by R2. Thus, as long
as the bias current varies linearly with temperature, a value
for R2 can be found to effect compensation.
TL/H/7479–3
a. original circuit
TL/H/7479–4
b. improved version
Figure 3. Bias-current compensation. With the improved version, the temperature coefficient of the compensating
current can be varied with R2. It is effective only if bias current has linear, negative temperature coefficient.
2
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