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參數(shù)資料
| 型號： | LPC661IM |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 運動控制電子 |
| 英文描述： | Low Power CMOS Operational Amplifier |
| 中文描述： | OP-AMP, 6300 uV OFFSET-MAX, 0.35 MHz BAND WIDTH, PDSO8 |
| 封裝： | PLASTIC, SOP-8 |
| 文件頁數(shù)： | 7/13頁 |
| 文件大?。?/td> | 311K |
| 代理商： | LPC661IM |
Application Hints
AMPLIFIER TOPOLOGY
The topology chosen for the LPC661 is unconventional
(compared to general-purpose op amps) in that the tradi-
tional unity-gain buffer output stage is not used; instead, the
output is taken directly from the output of the integrator, to al-
low rail-to-rail output swing. Since the buffer traditionally de-
livers the power to the load, while maintaining high op amp
gain and stability, and must withstand shorts to either rail,
these tasks now fall to the integrator.
As a result of these demands, the integrator is a compound
affair with an embedded gain stage that is doubly fed forward
(via C
and C
) by a dedicated unity-gain compensation
driver. In addition, the output portion of the integrator is a
push-pull configuration for delivering heavy loads. While
sinking current the whole amplifier path consists of three
gain stages with one stage fed forward, whereas while
sourcing the path contains four gain stages with two fed
forward.
The large signal voltage gain while sourcing is comparable
to traditional bipolar op amps, for load resistance of at least
5 k
. The gain while sinking is higher than most CMOS op
amps, due to the additional gain stage; however, when driv-
ing load resistance of 5 k
or less, the gain will be reduced
as indicated in the Electrical Characteristics. The op amp
can drive load resistance as low as 500
without instability.
COMPENSATING INPUT CAPACITANCE
Refer to the LMC660 or LMC662 datasheets to determine
whether or not a feedback capacitor will be necessary for
compensation and what the value of that capacitor would be.
CAPACITIVE LOAD TOLERANCE
Like many other op amps, the LPC661 may oscillate when
its applied load appears capacitive. The threshold of oscilla-
tion varies both with load and circuit gain. The configuration
most sensitive to oscillation is a unity-gain follower. See the
Typical Performance Characteristics.
The load capacitance interacts with the op amp’s output re-
sistance to create an additional pole. If this pole frequency is
sufficiently low, it will degrade the op amp’s phase margin so
that the amplifier is no longer stable at low gains. The addi-
tion of a small resistor (50
to 100
) in series with the op
amp’s output, and a capacitor (5 pF to 10 pF) from inverting
input to output pins, returns the phase margin to a safe value
without interfering with lower-frequency circuit operation.
Thus, larger values of capacitance can be tolerated without
oscillation. Note that in all cases, the output will ring heavily
when the load capacitance is near the threshold for
oscillation.
Capacitive load driving capability is enhanced by using a pull
up resistor to V
+
(Figure 3). Typically a pull up resistor con-
ducting 50 μA or more will significantly improve capacitive
load responses. The value of the pull up resistor must be de-
termined based on the current sinking capability of the ampli-
fier with respect to the desired output swing. Open loop gain
of the amplifier can also be affected by the pull up resistor
(see Electrical Characteristics).
PRINTED-CIRCUIT-BOARD LAYOUT
FOR HIGH-IMPEDANCE WORK
It is generally recognized that any circuit which must operate
with less than 1000 pA of leakage current requires special
layout of the PC board. When one wishes to take advantage
of the ultra-low bias current of the LPC661, typically less
than 0.04 pA, it is essential to have an excellent layout. For-
tunately, the techniques for obtaining low leakages are quite
simple. First, the user must not ignore the surface leakage of
the PC board, even though it may sometimes appear accept-
ably low, because under conditions of high humidity or dust
or contamination, the surface leakage will be appreciable.
To minimize the effect of any surface leakage, lay out a ring
of foil completely surrounding the LPC661’s inputs and the
terminals of capacitors, diodes, conductors, resistors, relay
terminals, etc. connected to the op-amp’s inputs. See Figure
4 To have a significant effect, guard rings should be placed
on both the top and bottom of the PC board. This PC foil
must then be connected to a voltage which is at the same
voltage as the amplifier inputs, since no leakage current can
flow between two points at the same potential. For example,
a PC board trace-to-pad resistance of 10
12
, which is nor-
mally considered a very large resistance, could leak 5 pA if
the trace were a 5V bus adjacent to the pad of an input. This
would cause a 100 times degradation from the LPC660’s ac-
tual performance. However, if a guard ring is held within
5 mV of the inputs, then even a resistance of 10
11
would
cause only 0.05 pA of leakage current, or perhaps a minor
DS011227-6
FIGURE 1. LPC661 Circuit Topology
DS011227-7
FIGURE 2. Rx, Cx Improve Capacitive Load Tolerance
DS011227-24
FIGURE 3. Compensating for Large
Capacitive Loads with A Pull Up Resistor
www.national.com
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相關PDF資料 |
PDF描述 |
|---|---|
| LPC661IN | Low Power CMOS Operational Amplifier |
| LPC662 | Low Power CMOS Dual Operational Amplifier |
| LPC662AIM | Low Power CMOS Dual Operational Amplifier |
| LPC662AIN | Low Power CMOS Dual Operational Amplifier |
| LPC662AMD | Low Power CMOS Dual Operational Amplifier |
相關代理商/技術參數(shù) |
參數(shù)描述 |
|---|---|
| LPC661IM | 制造商:TELCOM SEMICONDUCTOR 功能描述:IC OP AMP CMOS SMD SOIC8 661 |
| LPC661IM/NOPB | 功能描述:運算放大器 - 運放 Sgl CMOS Opamp RoHS:否 制造商:STMicroelectronics 通道數(shù)量:4 共模抑制比（最小值）:63 dB 輸入補償電壓:1 mV 輸入偏流（最大值）:10 pA 工作電源電壓:2.7 V to 5.5 V 安裝風格:SMD/SMT 封裝 / 箱體:QFN-16 轉換速度:0.89 V/us 關閉:No 輸出電流:55 mA 最大工作溫度:+ 125 C 封裝:Reel |
| LPC661IMX | 功能描述:運算放大器 - 運放 RoHS:否 制造商:STMicroelectronics 通道數(shù)量:4 共模抑制比（最小值）:63 dB 輸入補償電壓:1 mV 輸入偏流（最大值）:10 pA 工作電源電壓:2.7 V to 5.5 V 安裝風格:SMD/SMT 封裝 / 箱體:QFN-16 轉換速度:0.89 V/us 關閉:No 輸出電流:55 mA 最大工作溫度:+ 125 C 封裝:Reel |
| LPC661IMX/NOPB | 功能描述:運算放大器 - 運放 RoHS:否 制造商:STMicroelectronics 通道數(shù)量:4 共模抑制比（最小值）:63 dB 輸入補償電壓:1 mV 輸入偏流（最大值）:10 pA 工作電源電壓:2.7 V to 5.5 V 安裝風格:SMD/SMT 封裝 / 箱體:QFN-16 轉換速度:0.89 V/us 關閉:No 輸出電流:55 mA 最大工作溫度:+ 125 C 封裝:Reel |
| LPC661IN | 制造商:Rochester Electronics LLC 功能描述:- Bulk |
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