L9700(1990) 查看數據表（PDF） - STMicroelectronics

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L9700
(Rev.:1990)

HEX PRECISION LIMITER

STMicroelectronics 

Figure 3 : Typical Application.

L9700

The minimum value for CIN is determined by the ac-

curacy required, the time taken to sample the input

and the input impedance during that time, while the

maximum value is determined by the required fre-

quency response and the value of RS.

Thus for a resistive input A/D connector where :

TS = Sample time (Seconds)

RD = Device input resistance (Ohms)

VIN = Input voltage (Volts)

k = Required accuracy (%)

Q1 = Charge on capacitor before sampling

Q2 = Charge on capacitor after sampling

ID = Device input current (Amps)

Thus :

k ⋅ Q1

Q1– Q2 = 100

but

Q1 = CIN VIN

and

Q1– Q2 = ID – TS

so that

and

so

k ⋅ CIN – VIN

ID TS =

100

ID ⋅ TS

CIN (min) = VIN ⋅ k Farad

100 ⋅ TS

CIN (min) = k ⋅ RD Farad

The calculationfor a sample and hold type convertor

is even simpler :

k = Required accuracy (%)

CH = Hold capacitor (Farad)

100 ⋅ CH

CIN (min) = k

Farad

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