4

AN314REV1

AN314

selected input's voltage level and this charge will be dumped into the next input’s source when that switch closes.

If an op-amp or high-impedance source is used for the input to the MUX, there could be an issue with settling when

this charge is transferred through the switch. One of the functions of the anti-alias capacitor on the input side of the

MUX in Figure 3 is to provide a reservoir of charge to the switch-channel capacitance. This is another reason why

the on resistance of the MUX should be as low as possible – so the channel capacitance can be charged quickly.

However, there is an inverse relationship between on resistance and channel capacitance because low on resis-

tance requires larger transistors and larger transistors result in higher channel capacitance. The following table

shows some examples of MUX devices along with their critical parameters. The devices grayed out are not recom-

mended due to the parameters indicated.

Table 1: Critical Parameters of Typical Multiplexers

Notes:

1. Devices in gray are not recommended for this application due to the indicated parameters.

2. Parameters are typical values and are for comparative purposes only.

3. The manufacturer's data sheet should be referenced for min and max values

The third parameter to be considered when selecting a MUX is the switching speed. What needs to be considered

here is the total time from when the system selects another channel to when the conversion can begin. This includes

MUX turn-on time and analog circuitry settling time. For example, if it is desired to convert with the CS556x at its

maximum speed of 50 kSps (CONV held low), then from the time the RDY signal falls until the next conversion be-

gins is 10 MCLKs (at 16 MHz) or 625 nanoseconds. This means that the MUX channel must change as RDY falls

and its turn on time and all analog settling must occur in less than 625 nanoseconds. If a MUX requires

250 nanoseconds to turn on, there is only 325 nanoseconds for the analog circuitry to settle. If more settling time

is required, the CONV pin timing can be delayed to slow down the conversion rate, allowing enough delay after the

MUX channel is changed to ensure complete settling.

The type of multiplexer selected will depend upon whether the converter is a single-ended-input or a differential-

input ADC. The advantage of using a differential ADC rather than one with a single-ended input is an improvement

Part

Number

# of

Poles

# of

Inputs

On/Off

Speed ns

On Res

Ohms

Off

Cap pF

On Cap

pF

Supply

Voltage

ADG706

1

16

50/14

2.5

13

200

1.8 to 5.5

ADG707

2

8

50/14

2.5

13

100

1.8 to 5.5

ADG787

2

2

22/6

2.5

16

40

1.8 to 5.5

ADG884

2

2

53/21

0.28

103

295

1.8 to 5.5

DG406

1

16

134/98

60

6

114

±5 to ±20

DG407

2

8

134/98

60

6

57

±5 to ±20

DG408

1

8

150/150

100

3

37

±5 to ±20

DG409

2

4

150/150

100

3

25

±5 to ±20

HI-506

1

16

250/250

180

10

62

±15

HI-507

2

8

250/250

180

10

40

±15

HI-508

1

8

250/250

180

10

27

±15

HI-509

2

4

250/250

180

10

22

±15

ISL43L220

2

2

12/5

0.23

115

224

1.1 to 4.5

MAX306

1

16

130/55

60

8

140

±4.5 to ±20

MAX307

2

8

130/55

60

8

70

±4.5 to ±20

MAX4617

1

8

7/4.5

8

5

32

2 to 5.5

MAX4618

2

4

7/4.5

8

5

21

2 to 5.5

MAX4635

2

2

12/5

2.5

9

32

1.8 to 5.5