10
PS8185C 10/06/99
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PS398/PS399
Precision 8-Ch, Diff. 4-Ch, 17V Analog Multiplexers
The table below shows how many time constants (m
τ
) are
needed to reach an accuracy of one LSB.
τ
= R
ON
x C
COM(ON)
Bits
8
12
15
Accuracy (%)
0.25
0.012
0.0017
m
6
9
11
Now, lets calculate what the maximum sampling rate for the
PS398. Assume a 12-bit accuracy and room temperature
operation.
Figure 10. Equivalent model of one multiplexer channel
V
IN
R
S
V
S
C
COM
(ON)
R
ON
C
L
In equation (1) above, n = 8, t
TRANS
= 150ns, t
SETTLING
= 9
τ
,
τ
= 100ohm x 54pF
________ 1
8 [150ns + 9(100ohm
x 54pF)]
or f
S
= 630kHz.
Assuming a x4 oversampling rate, the maximum sampling speed
for the PS398 would be 630
÷
4 = 157kHz.
S
Pericom Semiconductor Corporation
2380 Bering Drive San Jose, CA 95131 1-800-435-2336 Fax (408) 435-1100 http://www.pericom.com
