AN201 Vishay Intertechnology, AN201 Datasheet - Page 7

• Manufacturer Part Number: AN201
• Description: High-performance Multiplexing With The DG408
• Manufacturer: Vishay Intertechnology

To illustrate the improvements in acquisition time made
possible with the DG408/409, consider the following
comparison with the DG508A pin-compatible multiplexer. The
acquisition time is
where n is the number of time constants (determined by the
accuracy required).
R
C
If
then for the DG508A,
Using the same conditions for the DG408,
Thus, by using the DG408, the acquisition time is improved by
5 times.
Document Number: 70600
05-Aug -99
S
H
t
C
R
n = 10
t
t
t
t
t
t
A
A
A
A
A
A
A
is the source resistance of the analog input while
is the hold capacitor.
H
S
= 400 ns.
= t
= 1.5 ms + 10
= 1.5 ms + 500 ns
= 2 ms.
= 250 ns + 10
= 250 ns + 150 ns
= 50 W
= 100 pF
on
+ n
(Capacitor
Voltage)
(R
V
S
C
+ r
(50 + 450)
(50 +100) x 100 pF
DS(on)
Command
Sample
) x C
H
100 pF
FIGURE 11. Acquisition Time of a Sample/Hold System
Switch
Delay
(t
ON
)
Acquisition
Time (t
A
)
For high-accuracy systems, an important consideration is the
sample-to-hold offset error. This error arises when the
switch turns off and charge is injected by the switch onto the
hold capacitor, adding an error to the stored analog voltage.
The error is related to the hold capacitor by
where
As an example of the possible improvement with the DG408,
a comparison is drawn with the DG508A. Assuming a 1-nF
hold capacitor, for the DG508A,
and for the DG408,
Thus, a 2.5-fold improvement in error voltage is provided by
using the DG408.
The ability of the system to store the analog sample when the
switch goes off is referred to as the droop rate. It is measured
as the change in voltage versus time while in the hold mode.
The droop rate depends on bias current of the amplifier, the
leakage of the capacitor, and the off-state leakage of the
multiplexer. The low-leakage performance of the DG408/409
allows a lower droop rate (i.e., a more accurate storage of the
analog sample).
V
V
Q
C
V
V
O
O
O
O
H
i
is the injected charge
Capacitor
Charging
= Q
is the offset error
is the hold capacitor.
= 50 pC 1 nF = 50 mV
= 20 pC 1 nF = 20 mV.
i
/C
H
Vishay Siliconix
Band
Error
time
AN201
www.vishay.com
7