Odd results from ADG608 multiplexer

[jimbojohn]

Hi dear enthusiasts,

I am trying to use the following circuit (in the attachments) to switch between multiple resistors using a multiplexer. Initially I had different values of resistors attached to every channel, but since I was getting very strange and HUGE!! results, I grounded all the channels except the first channel and just connected a 120 ohm resistor. I am measuring resistor readings of magnitudes of Mega ohms and I have no idea how such huge values of resistance are showing up at all. I am not sure what I am doing wrong and I really really appreciate any help.

[johnsold]

Your unspecified strange results may result from a combination of two things:

1. I have no idea how the ADG608 model works. Some models for switching devices tend to be very different from the actual circuit configurations. This can produce strange simulation results.

2. When using the multimeter in ohmmeter mode you have a floating source. Depending on the model for the ADG608, there may not be a well defined path to ground from the source in the DMM.  Simulators do not llike floating circuits. 

Try placing a Voltage Source V1 between Y2 and ground and a load resistor RL from Y1 to ground.  Measure the current from the voltage source and the voltage at Y1. Then calculate the resistance. This should be a more realistic circuit configuration.  It should also be expandable to resistors R2 through R8 and Sources V2 through V8 to look at both open and closed switch behavior.

Lynn

[jimbojohn]

Hi Lynn

Thanks you for your reply. I did what you mentioned based on my understanding of it and I am still getting wierd results in Mega ohms. I apologise if I am interpretting you inccorectly, but let me know what you think. I have attached the schematics.

[johnsold · Accepted solution]

You cannot measure resistance directly in an active (powered) circuit. You must measure the voltage and current at the relevant points and use Ohm's Law to calculate the resistance. SImulators are jsut like real circuits in this regard.

You also need to use voltages and currents which are compatible with the devices in the circuit. With 12 V applied to Y2 you will get ~100 mA flowing through S1 of the ADG608 when it is enabled.  I did not look up the specs but suspect that that is too much current.

I only have a few minutes now and cannot draw up a circuit for you. Connect V1 to Y2 through an ammeter. This will let you measure the current through the mux. Also measure the voltage at Y1.  Then the switch resistance Rs = (V1 - V(Y1))/I(V1).

Lynn

[jimbojohn]

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@johnsold wrote:

You cannot measure resistance directly in an active (powered) circuit. You must measure the voltage and current at the relevant points and use Ohm's Law to calculate the resistance. SImulators are jsut like real circuits in this regard.

You also need to use voltages and currents which are compatible with the devices in the circuit. With 12 V applied to Y2 you will get ~100 mA flowing through S1 of the ADG608 when it is enabled.  I did not look up the specs but suspect that that is too much current.

I only have a few minutes now and cannot draw up a circuit for you. Connect V1 to Y2 through an ammeter. This will let you measure the current through the mux. Also measure the voltage at Y1.  Then the switch resistance Rs = (V1 - V(Y1))/I(V1).

Lynn

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Thank you very very much Lynn. I applied the setting that you were talking about which makes a very good sense and I found out that the internal S1 resistance was roughly 15 ohms. When I put a 120 ohm resistance before S1 next time I got a total resistance of around 135 ohms which makes sense considering the individual resistances of the S1 channels and the 120 ohm resistor. The 12V for V1 was also too much and I was careless about it because I thoought I had already taken that into account and had reduced it, but obviously I had not.

Once again thank you and congratulation for being nominated the Knight of NI.

I have attached my schematic for the sake of interest.

[johnsold]

That is a much more realistic test circuit. It might be a good idea to include a load resistor at the Y1 output to ground.  If the model includes output capacitance, that capacitance might charge up and not discharge.  Also, many analog multiplexers have on resistance which varies with the input voltage. If your application will use voltages significantly different from the 0.5 V, then you may need to repeat the simulations with other voltages.

It is important to remember that circuit simulators are not substitutes for understanding how a circuit works.  And, they are only as good as the models used. The physical ADG608 is built from FETs used as the switches but a simulation model might use Spice switches which have very different transient and leakage behavior.

Lynn

[jimbojohn]

Thank you! It makes sense

[gmack]

ADG608BRU doesn't simulate properly in Multisim 13 (and likely not 12), but ADG406BN does.  See attachments 

[jimbojohn]

Much appreciated!!!

[gmack]

You're welcome.  If you only need 8 channnels, either the ADG408 or ADG508 should work too (provided you don't expect (any of them) to disable or switch channels nearly as fast as they should).  It's interesting that Analog Device's new free SPICE simulator (www.analog.com/ADIsimPE) has problems with the ADG608 too.  "*** ERROR *** The circuit is too large to simulate with this version - Too many analog nodes".  It seems to me that NI should either fix the simulator's problem with the ADG608 or remove it from the database!  Will they?