Need 2>1 analog mux for higher-than-VCC signals

[Chancy99]

Well dang it. Didn't quite read the datasheet closely enough.

I have a car datalogger design that uses the DG333A quad 2>1 mux switch to multiplex the 16 input signals to 8 opamps for signal scaling for the ADC. The part I didn't notice was the voltage limitation on the signal inputs ...

They can only range just above/below VCC and GND. Since the datalogger is a 5V system, that kind of clobbers it for measuring vehicle 12V and above signals The DG333A was chosen for the nice low 25ohm R-on value, and the fact that it had 4 switches in one package.

So, before I set about redesigning the analog input section, does anyone know of a quad (or any) 2>1 mux switch that can handle higher input voltages ? Those inputs are fed via 180k resistors, so the total current is quite small.

If not, I have to rethink the analog input design strategy. The requirement are :

16 inputs
jumper selectable input ranges (0-5V, 0-14V and 9-14V)
feed to 8 ADC inputs

I suppose I could put the opamps up front, and feed their outputs to the mux switches, but that doubles the opamp footprint, along with the attendant scaling resistors etc

Dean.

 

[nbucska]

Put an R-R attenuator into each input line. Clamp
it to e.g. 4.5 V with Schottky-s or 4 V with regular
diodes.



<nbucska@pcperipherals.com>

 

[Lewish]

You might look at Supertex for high-voltage muxes.

 

[Chancy99]

Cool - thanks for the tips. What exactly is meant by an R-R attenuator ? How will that affect measuring a signal that ranges from 9-14V ?

Supertex has some high-voltage switches, but don't appear to hav ethe 2>1 muxes. I'll keep looking.

Thanks -

Dean.

 

[nbucska]

Vout = Vin*R1/(R1+R2) ; using 12 V ---> 4V then the attenuation is 1/3 i.e. R2 = 2*R1. Clamp it to less than 5V.
Select R1 and R2 according the load.

<nbucska@pcperipherals.com>

 

[Chancy99]

Oh, got it. You're just talking about a divider. Doh.

D.

 

[geomit]

You are almost on the same track I am.
Try looking at the HCF4051B from STMicroelectronics.
It is an 8:1 MUX but you could easily cascade two of them together.