TCF6000 Datasheet, PDF (4/6 Pages) Motorola, Inc – PERIPHERAL CLAMPING ARRAY

TCF6000
The use of Cin is not mandatory, and is not recommended
where the lines to be protected are used for output or for both
Thus:
 _Q1–Q2
=
k Q1
100
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
but,
Q1= Cin Vin
and, Q1–Q2= ID • Ts
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
so that,
ID Ts
=
k • Cin–Vin
100
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
and,
Cin (min) =
ID • Ts
Vin • k
Farad
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:
so,
Cin (min) =
100 • Ts
k • RD
Farad
The calculation for a sample and hold type converter is
Ts = Sample time (seconds)
RD= Device input resistance (Ω)
Vin= Input voltage (V)
even simpler:
k = Required accuracy (%)
CH= Hold capacitor (Farad)
k = Required accuracy (%)
Q1 = Charge on capacitor before sampling
Q2 = Charge on capacitor after sampling
ID = Device input current (A)
Cin (min) =
100 • CH
k
Farad
For the MC6805S2 this comes out at:
Cin (min) =
100.25
0.25
pF
=
10
nF
for
1/4%
accuracy
4
MOTOROLA ANALOG IC DEVICE DATA