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| 型号： | PA94 |
| 英文描述： | Operational Amplifier |
| 中文描述： | 运算放大器 |
| 文件页数： | 4/4页 |
| 文件大小： | 70K |
| 代理商： | PA94 |
APEX MICROTECHNOLOGY CORPORATION
5980 NORTH SHANNON ROAD
TUCSON, ARIZONA 85741
USA
APPLICATIONS HOTLINE: 1 (800) 546-2739
GENERAL
Please read Application Note 1 "General Operating Consid-
erations" which covers stability, supplies, heat sinking, mount-
ing, current limit, SOA interpretation, and specification inter-
pretation. Visit www.apexmicrotech.com for design tools that
help automate tasks such as calculations for stability, internal
power dissipation, current limit; heat sink selection; Apex
’
s
complete Application Notes library; Technical Seminar Work-
book; and Evaluation Kits.
CURRENT LIMIT
For proper operation, the current limit resistor (R
) must be
connected as shown in the external connection diagram. The
minimum value is 3.5 ohm, however for optimum reliability the
resistor value should be set as high as possible. The value is
calculated as follows; with the maximum practical value
of 30 ohms.
.7
R
LIM
=
I
LIM
COMMON MODE INPUT RANGE
Operational amplifiers are usually designed to have a com-
mon mode input voltage range that approximates the power
supply voltage range. However, to keep the cost as low as
possible and still meet the requirements of most applications
the common mode input voltage range of the PA94 is re-
stricted. The input pins must always be a least 30V from either
supply voltage but never more than 500V. This means that the
PA94 cannot be used in applications where the supply volt-
ages are extremely unbalanced. For example, supply voltages
of +800V and
–
100V would not be allowed in an application
where the non-inverting pin is grounded because in normal
operation both input pins would be at 0V and the difference
voltage between the positive supply and the input pins would
be 800V. In this kind of application, however,
supply voltages
+500V and -100V does meet the input common mode voltage
range requirements since the maximum difference voltage
between the inputs pins and the supply voltage is 500V (the
maximum allowed). The output has no such restrictions on its
voltage swing. The output can swing within 24V of either supply
voltage regardless of value so long as the total supply voltage
does not exceed 900V.
INPUT PROTECTION
Although the PA94 can withstand differential input voltages
up to
±
20V, additional external protection is recommended. In
most applications 1N4148 or 1N914 signal diodes are suffi-
cient (D1, D2 in Figure 1a). In more demanding applications
where low leakage or low capacitance are of concern 2N4416
or 2N5457-2N5459 JFETs connected as diodes will be re-
quired (Q1, Q2 in Figure 1b). In either case the input differential
voltage will be clamped to
±
.7V. This is sufficient overdrive to
produce maximum power bandwidth. Note that this protection
does
not
automatically protect the amplifier from excessive
common mode input voltages.
POWER SUPPLY PROTECTION
Unidirectional zener diode transient suppressors are recom-
mended as protection on the supply pins. The zeners clamp
transients to voltages within the power supply rating and also
clamp power supply reversals to ground. Whether the zeners
are used or not, the system power supply should be evaluated
for transient performance including power-on overshoot and
power-off polarity reversal as well as line regulation.
Conditions which can cause open circuits or polarity rever-
sals on either power supply rail should be avoided or protected
against. Reversals or opens on the negative supply rail is
known to induce input stage failure. Unidirectional transzorbs
prevent this, and it is desirable that they be both electrically and
physically as close to the amplifier as possible.
STABILITY
The PA94 is stable at gains of 100 or more with a NPO
(COG) compensation capacitor of 2.2pF. The compensation
capacitor, Cc, in the external connections diagram must be
rated at 1000V working voltage and mounted closely to pins 4
and 6 to prevent spurious oscillation. A compensation capaci-
tor less than 2.2pF is not recommended.
EXTERNAL COMPONENTS
The compensation capacitor Cc must be rated for the total
supply voltage. An NPO (COG) capacitor rated a 1kV is
recommended.
Of equal importance are the voltage rating and voltage
coefficient of the gain setting feedback resistor. Typical volt-
age ratings of low wattage resistors are 150 to 250V. Up to 500
V can appear across the feedback resistor. High voltage rated
resistors can be obtained. However a 1 megohm feedback
resistor composed of five 200k resistors in series will produce
the proper voltage rating.
CAUTIONS
The operating voltages of the PA94 are potentially lethal.
During circuit design develop a functioning circuit at the lowest
possible voltages. Clip test leads should be used for "hands
off" measurements while troubleshooting.
FIGURE 1. OVERVOLTAGE PROTECTION
PA94
+V
S
–
IN
+IN
Z1
Z2
D2
D1
12
10
1
2
+V
S
–
IN
+IN
Z1
Z2
12
10
1
2
PA94
Q1
Q2
A.
B.
–
V
S
–
V
S
PA94
OPERATING
CONSIDERATIONS
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