Datasheet > 05F6971

05F6971

更新时间： 2024-02-26 04:53:54

品牌	Logo	应用领域
其他 - ETC		放大器
页数	文件大小	规格书
12页	420K
描述
IC-ELECTROMETER AMPLIFIER

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中文翻译

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数据手册

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05F6971 数据手册

AD549

100µ

10µ

1µ

–

100n

10n

100p

Figure 39. Input Voltage Clamp with Diodes

10p

SAMPLE AND DIFFERENCE CIRCUIT TO MEASURE

ELECTROMETER LEAKAGE CURRENTS

100f

10f

There are a number of methods used to test electrometer leak-

age currents, including current integration and direct current to

voltage conversion. Regardless of the method used, board and

interconnect cleanliness, proper choice of insulating materials

(such as Teflon or Kel-F), correct guarding and shielding tech-

niques and care in physi-cal layout are essential to making accu-

rate leakage measurements.

–5 –4 –3 –2 –1

–

)

DIFFERENTIAL INPUT VOLTAGE – V (V

– V

Figure 36. Input Current vs. Differential Input Voltage

INPUT PROTECTION

The AD549 safely handles any input voltage within the supply

voltage range. Subjecting the input terminals to voltages beyond

the power supply can destroy the device or cause shifts in input

current or offset voltage if the amplifier is not protected.

Figure 40 is a schematic of the sample and difference circuit. It

uses two AD549 electrometer amplifiers (A and B) as current-to

voltage converters with high value (10¹⁰Ω) sense resistors (RSa

and RSb). R1 and R2 provide for an overall circuit sensitivity of

10 fA/mV (10 pA full scale). C_Cand C_Fprovide noise suppres-

sion and loop compensation. C_Cshould be a low leakage poly-

styrene capacitor. An ultralow leakage Kel-F test socket is used

for contacting the device under test. Rigid Teflon coaxial cable

is used to make connections to all high impedance nodes. The

A protection scheme for the amplifier as an inverter is shown in

Figure 37. R_Pis chosen to limit the current through the invert-

ing input to 1 mA for expected transient (less than 1 second)

overvoltage conditions, or to 100 µA for a continuous overload.

Since R_Pis inside the feedback loop, and is much lower in value

than the amplifier’s input resistance, it does not affect the

inverter’s dc gain. However, the Johnson noise of the resistor

will add root sum of squares to the amplifier’s input noise.

Figure 37. Inverter with Input Current Limit

In the corresponding version of this scheme for a follower,

shown in Figure 38, R_Pand the capacitance at the positive input

terminal will produce a pole in the signal frequency response at

a f = 1/2 π RC. Again, the Johnson noise R_Pwill add to the

amplifier’s input voltage noise.

Figure 38. Follower with Input Current Limit

Figure 39 is a schematic of the AD549 as an inverter with an

input voltage clamp. Bootstrapping the clamp diodes at the in-

verting input minimizes the voltage across the clamps and keeps

the leakage due to the diodes low. Low leakage diodes, such as

the FD333’s should be used, and should be shielded from light

to keep photocurrents from being generated. Even with these

precautions, the diodes will measurably increase the input cur-

rent and capacitance.

Figure 40. Sample and Difference Circuit for Measuring

Electrometer Leakage Currents

REV. A

–9–

1...6 7 8910 11 12

中文翻译

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