An314 – Cirrus Logic AN314 User Manual
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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