Analog multiplex switch help needed

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cdenk
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Analog multiplex switch help needed

Post by cdenk » Fri Jul 28, 2006 4:41 pm

Application is wanting to multiplex 4 DC ( 9 to 12 volts) voltage inputs (I can do the voltage divider or OP-Amp design thing) to one available ADC (0 to 5 VDC) PLC input. I have a Vishay DG408 CMOS analog multiplexer and have the following pins connected as single supply:
V- > Gnd.

Gnd > Gnd.

V+ > +12VDC, .1 mfd bypass capacitor to GND.

Enable and address
EN, A0, A1, A2, A3 > SPDT switches, ground and all through one 470 ohm to +5VDC

Inputs
S1 > +5 VDC
S2 > +2.5 VDC
S3 > GND.
S4, S5, S6, S7, S8 > individually 2K ohm to GND.

Output D > 10K to Ground.

When powered on, in a few seconds the chip gets very hot.

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Bob Scott
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Post by Bob Scott » Fri Jul 28, 2006 7:46 pm

Your connections look good. Recheck the pin dumbering with the data sheet. The data sheet lists both DG408 and DG409 devices. Their pinouts differ.

If the IC has warmed up after incorrect connection, it's best to discard it and use a new one.

Regards,
Bob :cool:

cdenk
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Post by cdenk » Fri Jul 28, 2006 7:57 pm

Thanks for the reply, I was aware of the 2 different chips on the same sheet, I have a couple of others of the chip, will try swapping next, just want to try to prevent messing us another chip. :) This was goint to be my test circuit, with the 3 different input voltages, to see what happened, and the wire in the actual voltages. It's for a standyby generator controller, 2 battery voltages and utility and generator voltages (use 9VAC out wall wart that's then rectified).

cdenk
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Post by cdenk » Sat Jul 29, 2006 8:07 am

Wonders will never end after sleeping on it and waking up in the morning. Checked the wiring once again including verifying the resistor values with ohmeter, nothing found goofy. Swapped the chip for another new one fresh out of Mouser's package, still gets hot when powered up. Pulled all the wiring and resistors off and started with basic power and ground, and then adding other items to circuit one at a time to determine what is causing heating. Everything in place and chip stays cool to touch.:) Swapped chips back to the original one, and no heating, with same output results.

It appears the chip output appears to be not linear, the following input/output:
0 v > 0 v
2.46 v > 1.62 v
4.94 v > 4.90 v

Wasn't expecting quite that much, but can handle that in the PLC with Basic programming.

cato
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Post by cato » Sun Jul 30, 2006 11:29 am

What are the respective output resistances of your 3 signal sources? I'm guessing that the 2.5V signal has significantly more resistance associated with it than the other two signals. Accordingly, it is being divided down by the input resistance of the multiplexer/

cdenk
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Post by cdenk » Sun Jul 30, 2006 12:06 pm

The 3 inputs were 5 vdc, ground, and middle of (2) 5.6K resistor voltage divider. Since have connected to PLC's ADC with 10K pot input to switch with one of the ADC channels, and another ADC channel at the switch output. Tracking was very good of the 2 ADC channels. :)

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