CD4051 Analog Multiplexer Actual Voltage Range

[pacmann]

I want to use CD4051 to expand how many analog signals to read with a single A2D pin on the microcontroller.
Voltage range from the analog inputs to the 4051 are 0-3.3v. If VDD connects to 3.3v and VEE and VSS connects to Gnd on the 4051, will it be able to output the full 0-3.3v range on the Common pin or is there a dead zone below or above a voltage that will be clipped? Is there a parameter in the data sheet to clarify this?

Thx

[Kleinstein]

The CD4051 will work all the way the the supply range and even a little (e.g. 300 mV) above and below. Than the internal diode would engage.
This is normal behavior with modern CMOS switches. The data-sheets may show anlog signal range, but on what details are in the DS can vary between manufacturers and DS version.

With only 3.3. V the CD4051 is relatively high resistance. 74HC4051 may be the better choise with low voltage.

[pacmann]

Thanks for the details! 74HC4051 is a good suggestion!
I have some CD4051 around and wanted to use them up before buying new. Do you think since the "output" of CD4051 connects to the A2D pin that a 1000 ohm series resistance will attenuate the signal? Depends on A2D pin impedance I guess.

Cheers!

[Kleinstein]

It denpends on the ADC, wether it can accept some 1-3 kOhms of series resistance. At least what I have found the specs are for 5 /10/15 V only with the CD4051, though it is supposed to work from 3 V on.

[Zero999]

Do you have a higher power supply voltage available? If you can bump the supply voltage to the CD4051 up to 4.5V, the resistance will drop. I wouldn't recommend going any higher, otherwise it won't work properly, with 3.3V logic. If you have a 5V supply, use a diode and a pull-down resistor to create 4.4V.

[Bassman59]

It denpends on the ADC, wether it can accept some 1-3 kOhms of series resistance. At least what I have found the specs are for 5 /10/15 V only with the CD4051, though it is supposed to work from 3 V on.

Buffering the switch output with an op-amp will help with driving the ADC input.