參數(shù)資料
型號(hào): LTC6241CS8
廠(chǎng)商: LINEAR TECHNOLOGY CORP
元件分類(lèi): 運(yùn)動(dòng)控制電子
英文描述: Dual/Quad 18MHz, Low Noise, Rail-to-Rail, CMOS Op Amps
中文描述: DUAL OP-AMP, 250 uV OFFSET-MAX, 18 MHz BAND WIDTH, PDSO8
封裝: 0.150 INCH, PLASTIC, SOP-8
文件頁(yè)數(shù): 15/24頁(yè)
文件大小: 490K
代理商: LTC6241CS8
LTC6241/LTC6242
15
62412f
APPLICATIU
Proprietary design techniques are used to obtain simul-
taneous low 1/f noise and low input capacitance. Low
input capacitance is important when the amplifier is used
with high source and feedback resistors. High frequency
noise from the amplifier tail current source, I
TAIL
in Fig-
ure 1, couples through the input capacitance and appears
across these large source and feedback resistors. As an
example, the photodiode amplifier of Figure 11 on the last
page of this data sheet shows the noise results from the
LTC6241 and the results of a competitive CMOS amplifier.
The LTC6241 output is the ideal noise of a 1M
Ω
resistor
at room temperature, 130nV√Hz.
W
U
U
Half the Noise
The circuit shown in Figure 3 can be used to achieve even
lower noise voltage. By paralleling 4 amplifiers the noise
voltage can be lowered by √4, or half as much noise. The
√ comes about from an RMS summing of uncorrelated
noise sources. This circuit maintains extremely high input
resistance, and has a 250
Ω
output resistance. For lower
output resistance, a buffer amplifier can be added without
influencing the noise.
Stability
The good noise performance of these op amps can be at-
tributed to large input devices in the differential pair. Above
several hundred kilohertz, the input capacitance rises and
can cause amplifier stability problems if left unchecked.
When the feedback around the op amp is resistive (R
F
), a
pole will be created with R
F
, the source resistance, source
capacitance (R
S
, C
S
), and the amplifier input capacitance.
In low gain configurations and with R
F
and R
S
in even
the kilohm range (Figure 4), this pole can create excess
phase shift and possibly oscillation. A small capacitor C
F
in parallel with R
F
eliminates this problem.
Low Noise Single-Ended Input to Differential Output
Amplifier
The circuit on the first page of the data sheet is a low noise
single-ended input to differential output amplifier, with a
200k input impedance. The very low input bias current
of the LTC6241 allows for these large input and feedback
resistors. The 200k resistors, R1 and R2, along with C1
and C2 set the –3dB bandwidth to 80kHz. Capacitor C3 is
used to cancel effects of input capacitance, while C4 adds
Figure 3. Parallel Amplifier Lowers Noise by 2x
Figure 4. Compensating Input Capacitance
+
C
IN
C
S
6241 F04
R
F
R
S
OUTPUT
C
F
10
6241 F03
+
1/4
LTC6242
1k
1k
10
+
1/4
LTC6242
1k
1k
10
+
1/4
LTC6242
1k
1k
10
+
1/4
LTC6242
1k
1k
+2.5
–2.5
V
IN
V
O
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