DG406/407
Vishay Siliconix
APPLICATIONS HINTS
Sampling speed is limited by two consecutive events: the
transition time of the multiplexer, and the settling time of the
sampled signal at the output.
t
TRANS
is given on the data sheet. Settling time at the load
depends on several parameters: r
DS(on)
of the multiplexer,
source impedance, multiplexer and load capacitances,
charge injection of the multiplexer and accuracy desired.
The settling time for the multiplexer alone can be derived
from the model shown in Figure 5. Assuming a low
impedance signal source like that presented by an op amp or
a buffer amplifier, the settling time of the RC network for a
given accuracy is equal to nτ:
% ACCURACY
0.25
0.012
0.0017
# BITS
8
12
15
N
6
9
11
The maximum sampling frequency of the multiplexer is:
1
-
f
s
= ---------------------------------------------------------
N
(
t
SETTLING
+ t
TRANS
)
where N = number of channels to scan
t
SETTLING
= nτ = n x r
DS(on)
x C
D(on)
For the DG406 then, at room temp and for 12-bit accuracy,
using the maximum limits:
1
f
s
= ------------------------------------------------------------------------------------------------------
-
-12
-12
16
(
9
×
100
Ω ×
10 F
)
+
300
×
10 s
or
f
s
= 694 kHz
(3)
(1)
(2)
From the sampling theorem, to properly recover the original
signal, the sampling frequency should be more than twice
the maximum component frequency of the original signal.
This assumes perfect bandlimiting. In a real application
sampling at three to four times the filter cutoff frequency is a
good practice.
Therefore from equation 2 above:
1
-
f
c
=
--
×
f
s
=
173 kHz
4
r
DS(on)
V
OUT
R
S
= 0
C
D(on)
(4)
Figure 5. Simplified Model of One Multiplexer Channel
From this we can see that the DG406 can be used to sample
16 different signals whose maximum component frequency
can be as high as 173 kHz. If for example, two channels are
used to double sample the same incoming signal then its
cutoff frequency can be doubled.
www.vishay.com
10
Document Number: 70061
S-71009–Rev. I, 14-May-07
VISHAY [ VISHAY TELEFUNKEN ]
