TCF6000

4

MOTOROLA ANALOG IC DEVICE DATA

The use of Cin is not mandatory, and is not recommended

where the lines to be protected are used for output or for both

input and output. For digital input lines, the use of a small

capacitor in the range of 50 pF to 220 pF is recommended as

this will reduce the rate of rise of voltage seen by the

TCF6000 and hence the possibility of overshoot.

In the case of the analog inputs, such as that from the

pressure sensor, the capacitor Cin is necessary for devices

such as the MC6805S2 shown, which present a low

impedance during the sampling period. The maximum value

for Cin is determined by the accuracy required, the time taken

to sample the input and the input impedance during that time,

while the maximum value is determined by the required

frequency response and the value of Rin.

Thus for a resistive input A/D connector where:

Ts

RD

Vin

k

Q1

Q2

ID

= Sample time (seconds)

= Device input resistance (

Ω)

= Input voltage (V)

= Required accuracy (%)

= Charge on capacitor before sampling

= Charge on capacitor after sampling

= Device input current (A)

Q1–Q2 =

k

Q1

100

Thus:

but,

and,

Q1

Q1–Q2

= Cin Vin

= ID • Ts

so that,

ID Ts =

k

• Cin–Vin

100

and,

Cin (min) =

ID • Ts

Vin • k

Farad

so,

Cin (min) =

100

• Ts

k

• RD

Farad

The calculation for a sample and hold type converter is

even simpler:

k

CH

= Required accuracy (%)

= Hold capacitor (Farad)

Cin (min) =

100

• CH

k

Farad

For the MC6805S2 this comes out at:

Cin (min) =

100.25 pF

0.25

= 10 nF for 1/4% accuracy