[sdiy] Limiting current through 74-series 4051

[Magnus Danielson]

On 11/13/2011 11:20 PM, Mark Rivera wrote:
> Hi all,
>
> I need to limit the current going through the switches of a 74-series
> 4051 (0V Gnd/Vee, 5V Vcc). I am using the 4051 as a demux.. 8 DC
> inputs to one output. The output is feeding into a single-supply,
> rail-to-rail (0-5V) opamp with voltage-follower configuration.
>
> The input sources (pots as voltage dividers) can be anywhere between
> Gnd and Vcc.
>
>
> Is there any difference between these two setups:
>
> 1. voltage sources ->  8 resistors ->  inputs ->  output ->  opamp in+
>
> 2. voltage sources ->  inputs ->  output ->  1 resistor ->  opamp in+
>
>
> Theoretically, I think there's not, but I wonder about having lots of
> current available at the inputs versus having restricted current at
> the inputs. Does it affect the lifetime or performance of the 4051?
>
> (Ultimately, opamp output goes into an ADC.)

Wait... does your op-amp loads your circuit that much????

Except for capacitor loading I don't see much needs of current flowing 
there at all.

I am sure that your op-amp circuit would behave better if there was a 
little connection to ground all the time, in between the channels, you 
don't want it floating around too long at least.

There is a benefit in locating the resistors on the source side if you 
also terminate the signals towards a virtual ground. That way the source 
impedance of the CMOS analog muxes has very low variance with the 
signal, especially if the resistors is 10-100 times the switch 
impedance... which is about 100-200 Ohms (don't recall numbers, but it 
does vary with the supply current and voltage of the terminals). 
Locating the switches close to 0 V during opeating time and making their 
internal impedance of less importance you have moved out quite a bit of 
non-linear problems.

But then I think others have better experience than me in working these 
issues.

Cheers,
Magnus