CD74HC4051: The crosstalk of 74HC4051

Justin,

To find the best device for your system would you be able to describe what you need a switch to do in your system? 

A quick was to check for the TI part that matches with competition is the ti.com cross reference search tool.

Both the TI part CD74HC4051PWR and the onsemi part MC74HC4051ADTR2G have the same function 1-channel 8:1 switch, same pinout, and same TSSOP package.  

The one semi device is the HC version so its power supply is 2-6V while the HCT TI device is 4.5-5.5V.  To be comparable with the HC version you will need to use the CD74HC4051.    

I'm not sure what you are referencing about a thread with a crosstalk issue.  Would you be able to provide the link of the thread you are asking about? 

There is some cross talk information for the CD74HCXXXX family of devices in the application note below. 

TI has a new MUX508 device that has good performance for an 8:1 switch.  

Thank you, 

Adam 

Hi Adam,

The switch is used in an analog front end for a heart monitor to select signals to connect to an adc. 

Here is a link to an earlier TI thread from someone who tried the cd74hc4051 in their circuit and experienced high noise levels.

e2e.ti.com/.../2297000

The data you supplied appears comparable to the Onsemi part but the biggest problem we've had with this type of switch is the switched signal to momentarily short to ground during changes of state.

Note that this doesn't mean that break before make design will necessarily solve the problem since the signal isn't shorting to inputs together.

This is a legacy design so switching to the mux508, which has a different pinout and is much more expensive isn't an option.

Justin

Justin,

Thank you for providing the post reference. We don't know if there was high noise levels or how high due to our device or something else in the system because there wasn't any follow up information.

I'm not familiar with seeing signal switches shorting to ground while they change state. Do you have a schematic of your system? Is there something on the bus that could pull down the bus to ground while the switch changes to a high impedance state?

Thank you,
Adam

Here's a partial circuit for a similar part (74hc4053) and a plot of the signal and here is an excerpt from an email by another engineer who looked at this earlier who is no longer available. Note there are series resistors on the analog inputs so I don't see how external circuits can account for the plot.

I wonder if you are aware of all the subtleties of these FET switches. I'm not sure how much similarity there is between the 74HC4000 series and the original RCA 4000 series. But see Hor and Hill, pg 148 (2nd ed)  "A related bad habit is the propensity of some switches (e.g., the 4066) to short the input to ground momentarily during changes of state." Note in the scope photo the spike goes downward (to ground) but there is no corresponding upward spike on the alternate transition.

Have you ever studied the schem of these bad boys? The original 4016 was a better analog switch. The 4066 improved it to make it a better digital switch, lower on resistance, double buffered the inputs to the switches (dble buffering added to all "4000B" series per mil requirements versus 4000A series, incidentally). Now, to produce smooth ON resistance across voltage range, the substrates of the N and P fet switches are not returned to the supplies. They float, with a very complicated circuit to drive the substrates. I tried to figure it out once, and abandoned the project before complete comprehension. Thus under some conditions, it is possible to forward bias the diode from the the drain or source of the FETs to its substrate, creating a low impedance path from an I or O terminal to some other potential. So these switches work well when they are on and when they are off, but the transition is problematical.

Perhaps the 74HC4000 series share the idiosyncracies of their ancestors, and in this type of analog application you are relying essentially on un-spec'd characteristics. so one brand of switch might work better than another. the Maxim might work better.