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4

GENERAL

Please read Application Note 1 "General Operating Con-

siderations" which covers stability, supplies, heat sinking,

mounting, current limit, SOA interpretation, and specification

interpretation. 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

Workbook; and Evaluation Kits.

CURRENT LIMIT

For proper operation, the current limit resistor (R

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

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 restricted.

The input pins must always be a least 30V from either supply

voltage but never more than 450V. This means that the PA94

cannot be used in applications where the supply voltages

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

+450V 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 450V (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 sufficient

(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 required

(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 reversals

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 capaci-

tor, 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 capacitor 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 rec-

ommended.

Of equal importance are the voltage rating and voltage co-

efficient of the gain setting feedback resistor. Typical voltage

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

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PA94

OPERATING

CONSIDERATIONS

This data sheet has been carefully checked and is believed to be reliable, however, no responsibility is assumed for possible inaccuracies or omissions. All specifications are subject to change without notice.

PA94U REV J APRIL 2005 © 2005 Apex Microtechnology Corp.