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AN201

High-Performance Multiplexing with the DG408

Vishay Semiconductors 

AN201

Vishay Siliconix

VC

(Capacitor

Voltage)

Switch

Delay

(tON)

Acquisition

Time (tA)

Capacitor

Charging

Error

Band

Sample

Command

time

FIGURE 11. Acquisition Time of a Sample/Hold System

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

tA = ton + n (RS + rDS(on)) x CH

where n is the number of time constants (determined by the

accuracy required).

RS is the source resistance of the analog input while

CH is the hold capacitor.

If

CH = 100 pF

RS = 50 W

n = 10

then for the DG508A,

tA = 1.5 ms + 10 (50 + 450)

tA = 1.5 ms + 500 ns

tA = 2 ms.

100 pF

Using the same conditions for the DG408,

tA = 250 ns + 10 (50 +100) x 100 pF

tA = 250 ns + 150 ns

tA= 400 ns.

Thus, by using the DG408, the acquisition time is improved by

5 times.

Document Number: 70600

05-Aug -99

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

VO = Qi/CH

where

VO is the offset error

Qi is the injected charge

CH is the hold capacitor.

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,

VO = 50 pC 1 nF = 50 mV

and for the DG408,

VO = 20 pC 1 nF = 20 mV.

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).

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