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參數資料
| 型號： | 6204I |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Rail-to-Rail Input and Output, Ultralow 1.9n Root Hz Noise, Low Power Op Amps |
| 中文描述： | 軌到軌輸入和輸出，超低1.9n根赫茲噪聲，低功耗運算放大器 |
| 文件頁數： | 16/24頁 |
| 文件大小： | 587K |
| 代理商： | 6204I |
LT6202/LT6203/LT6204
16
620234fa
Input Protection
There are back-to-back diodes, D1 and D2, across the
+ and – inputs of these amplifiers to limit the differential
input voltage to
±
0.7V. The inputs of the LT6202/LT6203/
LT6304 do not have internal resistors in series with the
input transistors. This technique is often used to protect
the input devices from over voltage that causes excessive
currents to flow. The addition of these resistors would
significantly degrade the low noise voltage of these ampli-
fiers. For instance, a 100
resistor in series with each
input would generate 1.8nV/
√
Hz of noise, and the total
amplifier noise voltage would rise from 1.9nV/
√
Hz to
2.6nV/
√
Hz. Once the input differential voltage exceeds
±
0.7V, steady state current conducted though the protec-
tion diodes should be limited to
±
40mA. This implies 25
of protection resistance per volt of continuous overdrive
beyond
±
0.7V. The input diodes are rugged enough to
handle transient currents due to amplifier slew rate over-
drive or momentary clipping without these resistors.
Figure 2 shows the input and output waveforms of the
amplifier driven into clipping while connected in a gain of
A
V
= 1. When the input signal goes sufficiently beyond the
power supply rails, the input transistors will saturate.
When saturation occurs, the amplifier loses a stage of
phase inversion and the output tries to change states.
Diodes D1 and D2 forward bias and hold the output within
a diode drop of the input signal. In this photo, the input
signal generator is clipping at
±
35mA, and the output
transistors supply this generator current through the
protection diodes.
With the amplifier connected in a gain of A
V
≥
2, the output
can invert with very heavy input overdrive. To avoid this
inversion, limit the input overdrive to 0.5V beyond the
power supply rails.
ESD
The LT6202/LT6203/LT6204 have reverse-biased ESD
protection diodes on all inputs and outputs as shown in
Figure 1. If these pins are forced beyond either supply,
unlimited current will flow through these diodes. If the
current is transient and limited to one hundred milliamps
or less, no damage to the device will occur.
Noise
The noise voltage of the LT6202/LT6203/LT6204 is equiva-
lent to that of a 225
resistor, and for the lowest possible
noise it is desirable to keep the source and feedback
resistance at or below this value, i.e. R
S
+ R
G
||
R
FB
≤
225
.
With R
S
+ R
G
||
R
FB
= 225
the total noise of the amplifier
is: e
n
=
√
(1.9nV)
2
+ (1.9nV)
2
= 2.7nV. Below this resistance
value, the amplifier dominates the noise, but in the resis-
tance region between 225
and approximately 10k
, the
noise is dominated by the resistor thermal noise. As the
total resistance is further increased, beyond 10k, the noise
current multiplied by the total resistance eventually domi-
nates the noise.
The product of e
n
√
I
SUPPLY
is an interesting way to gauge
low noise amplifiers. Many low noise amplifiers with low
e
n
have high I
SUPPLY
current. In applications that require
low noise with the lowest possible supply current, this
product can prove to be enlightening. The LT6202/LT6203/
LT6204 have an e
n
,
√
I
SUPPLY
product of 3.2 per amplifier,
yet it is common to see amplifiers with similar noise
specifications have an e
n
√
I
SUPPLY
product of 4.7 to 13.5.
For a complete discussion of amplifier noise, see the
LT1028 data sheet.
Figure 2. V
S
=
±
2.5V, A
V
= 1 with Large Overdrive
LT6202/03/04 F02
APPLICATIU
W
U
U
OV
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