Need Multiplexer Solution

Hello,

For a project, I need to switch 5V DC through one of eight different
resistors using the digital outputs of a small USB data acuisition
device. I'm thinking of using a 74LS138 3 to 8 line decoder. Will this
work? I need the resistance of the chip to be 50 Ohms or less, as
anything higher will affect the project. I've come up with a design
using a relay network, but if there is a better way to do it with a
multiplexer chip, I really wnat to go that route. The resistances are
50-300 Ohms, and the 5 VDC will be coming from USB power. Thanx.

gm

 

I should add to this that the output will carry up to 20 milliamps

 

Lord Garth wrote:

greenman_11_11@yahoo.com> wrote in message
news:1113356821.415507.52620@f14g2000cwb.googlegroups.com...
Hello,

For a project, I need to switch 5V DC through one of eight
different
resistors using the digital outputs of a small USB data acuisition
device. I'm thinking of using a 74LS138 3 to 8 line decoder. Will
this
work? I need the resistance of the chip to be 50 Ohms or less, as
anything higher will affect the project. I've come up with a
design
using a relay network, but if there is a better way to do it with a
multiplexer chip, I really wnat to go that route. The resistances
are
50-300 Ohms, and the 5 VDC will be coming from USB power. Thanx.

gm


No, the 138 is not going to work for you. The 138 has active low
outputs
so the output is one of eight low also, TTL is a current sinking
logic
family so the 74LS138 will sink current to ground far more
effectively
than it can source 'something near' 5 volts as an output. You could
use the 4022 counter since it has decoded outputs that are active
high.
It sounds to me like you really want a switch like the 4066. This
old
part has an on resistance of about 80 ohms. Worse would be the 4067
which has a 125 ohm on resistance. There are much better analog
switches but these are in surface mount packages and only one switch
per:

http://www.fairchildsemi.com/ds/FS%2FFSA1156.pdf
or
http://www.fairchildsemi.com/ds/FS%2FFSA4157.pdf

Watch the power supply on these to insure they will operate on 5
volts,
Have the type of switch configuration you need and have TTL
compatible
inputs. The two links above are very low resistance devices... .9
ohms
and 1 ohm respectively. The first is a SPST N.O. switch while the
second
is a SPDT. Either will operate on a 5 volts supply. Be aware of the
current
limitations through each switch.

Thanks for the help.

If I put my device between the 138 and ground, and the eight resistors
between the "outputs" and the 5V, will this destroy the device? Just
curious. But I think I may have a new solution, which is to use an
octal line driver with the voltage source going from the outputs to
ground. I can put my device between the ground pin and ground. I'll
need eight lines to drive it, but thats OK.

 

Excellent suggestion, thank you very much. Another question: Is there
a digital potentiometer available that will carry a wiper current up to
25 ma continuously? If so, I could replace all those discreet
components with the pot, and get 256 resistor values instead of 8,
which would be nice for this project. The output of my circuit will
draw up to 20 ma of current at the lowest resistor value. I looked at
the AD8400 and it will only allow a max current of 5 ma when the wiper
is set at the lowest (50 ohm) setting. Is there another device similar
to the AD8400 that is designed to carry more current? Thanks for your
help.

gm