How to keep 2 ADC inputs isolated

[rjconway]

Hi,

I have two isolated 1-4VDC signals that have to be kept isolated. They are readings from pH and ORP meters and each need their own sperate common. So both these signals need to be integrated into one 40x2 PICAXE.

I was thinking of getting LM231 voltage-to-freq converters and try and read a frequency via an opto into a DI pin. I was also thinking I might be able to use two PICAXE 08M read the adc and output this to the master picaxe via serial output or a pwn signal, again via a simple opto isolator. I want to try and keep the parts count down and maintain accuracy.

ANybody have any idea's how I can keep these two 1-4VDC signals isolated and read them into a PICAXE.

rob

 

[mikie_121]

So you're looking for two differential inputs then. A differential input simply compares one input to another input (and not to ground).

An op amp would work here. Try an LM308 or LM311 (these are just the two I use the most, there are millions(?) of other op amps that would work.)
You would connect the meters to the inputs and then readadc the op amp outputs.

 

[rjconway]

not quite. The two measurements are independant, I just cannot connect the common of each signal together. Thus feeding both signals into one PICAXE40 means connecting both of the input -VE to the PICAXE -VE, which is what I need to avoid.

 

[eclectic]

not quite. The two measurements are independant, I just cannot connect the common of each signal together. Thus feeding both signals into one PICAXE40 means connecting both of the input -VE to the PICAXE -VE, which is what I need to avoid.

Use Opto-isolators / Infraout and infrain / Radio ................?

e

 

[ylp88]

not quite. The two measurements are independant, I just cannot connect the common of each signal together. Thus feeding both signals into one PICAXE40 means connecting both of the input -VE to the PICAXE -VE, which is what I need to avoid.

In the method mikie mentioned, you don't connect the input -ve pins to a common point. Look up connecting opamps as a differential input or, better yet, as an instrumentation amplifier.

ylp88

 

[MFB]

I think your idea about using a 08M and opto-isolator per channel would provide the lowest component count. The common (3rd) PICAXE could send a request pulse to both 08M chips, that imitated an ADC read and the data then sent serially via an opto -isolator. However, one 08M chips would have to delay its transmission to avoid both being received by the common PICAXE at the same time. The collectors of the two opto-isolator could be wired together and fed to a common serial input line.

 

[KMoffett]

If the two voltages don't need to be read too frequently or at "exactly" the same time, you could just use a small DPDT relay to alternate between V1/Com1 and V2/Com2.

Just a thought.

Ken

 

[rjconway]

Differential amp..Thanks I now understand how this could work and I could use it as the second stage of my actual pH meter circuit, I'll need a bit more of a think about it.

08M I'll look at infraout however think I like the idea of sending a DI to both 08M's and for them to send the info, one having a small delay. I assume I would just use serout command, like the idea of connecting both otpo's outputs to one serin

 

[ylp88]

08M I'll look at infraout however think I like the idea of sending a DI to both 08M's and for them to send the info, one having a small delay. I assume I would just use serout command, like the idea of connecting both otpo's outputs to one serin

You'd need separate power supplies if you want complete isolation if using optocouplers or IR. You might have problems trying to isolate the systems using serial such as RS232 as you still typically need a common return conductor.

ylp88

 

[MFB]

No need for infra commands. Just use sertxd or serout to send and serin to receive. The opto-isolators will have no problem with these TTL level signals (but keep a check on for any inversions and use T/N transmissions as necessary). You will of course need seperate supplies for each opto emitter but the opto receivers can use the common PICAXE supplies.

 

[rjconway]

I have the power supplies sorted. I thought they are expensive however ebay and china to my $$ rescue. a 12VDC input 5V output DC-DC isolator AUD$13.00

Thansk for your help guy's.

 

[RobertN]

Take a look at flying capacitor sample and holds, they offer good isolation and can easily be implimented with a bilateral switch like 4066 and good quality capacitors. If you like to go high end, Linear and others offer switching blocks. A 08M output could easily clock the switch function. Also, isolated + and - power sources can be made with switching caps.