Electronics > Beginners
better way for adc multiplexing
hsn93:
hi,
im trying to build industrial analog front end that can be configured by software to accept (4-20mA) or (0-10V / 1-5V / etc) on same connector pin
where accuracy doesnt matter that much
output to be connected with a filtering cap 10nF or so..
for switching i dont like to use relays due to relaiblity issues so
im thinking about SSR like AQY282S (with TVS with Voltage @Ipp < 60v)
also becasue there is 2 ssr per configuration it means 2x resistance =~5 ohm (ie 5% of 4-20mA lost?)
but this would require 4 ssr for 1 channel .. they arent cheap (1.5$ each).
also it will require a not gate for 2/4 ssr.. overall i think there should be simpler way i didnt think about?
so what would be the alternative robust configurable analog front end?
I'm wondering what way being used in PLCs.
and if the above way is good enough or not ?
pwlps:
If this is for signal multiplexing you don't need any high-current SSR like AQY282S, for your application I would suggest an SPDT switch like this one or similar (e.g. ADG1633, is cheaper):
https://www.analog.com/en/products/adg1634.html?doc=ADG1633_1634.pdf#
Brutte:
--- Quote ---industrial analog front end that can be configured by software to accept (4-20mA) or (0-10V / 1-5V / etc) on same connector pin
--- End quote ---
Maybe 74HC4051 and provide 8 different frontend circuits. One for 4-20mA, one for 0-10V one for 1-5V etc. Then you can select which one is fed through in software.
pwlps:
--- Quote from: Brutte on January 21, 2020, 10:29:46 am ---
--- Quote ---industrial analog front end that can be configured by software to accept (4-20mA) or (0-10V / 1-5V / etc) on same connector pin
--- End quote ---
Maybe 74HC4051 and provide 8 different frontend circuits. One for 4-20mA, one for 0-10V one for 1-5V etc. Then you can select which one is fed through in software.
--- End quote ---
74HC4051 has a >100Ohm on-resistance (vs. 5 Ohm for ADG16xx), so ok if you can afford a voltage drop of 2V at 20mA. It seems you have to pay much more if you want a very low on resistance.
Kleinstein:
If one does not have a higher supply at hand, one may not be able to use the simple CMOS switches. These are limited to signals inside there supply. One could still use CMOS switches after the divider. It should be OK to always have the divider (possibly higher resistors) connected. So one could likely get away without a switch there. So it would be mainly about connecting the 150 Ohms or similar resistor for the 20 mA signal.
If the signals are only positive, one could use just a single divider chain of some 25 K , 10 K and 150 Ohms and have a N-Channel MOSFET (e.g. 2N7000) in parallel to the 25 K and 10 K resistors. The control off the FET would however need some 8-10 V to turn it on. A low R Photomos could be an option here.
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