Opto Isolated ADC input needed for mega128

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#1
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Hi Folks,
I'd like to add an isolated 10 bit ADC input to a mega128 project. I've not done a lot with ADCs.
It just needs a single channel and the sample rate can be very slow. I'm measuring 10-40 volt battery voltage and trying for about 100mv accuracy.
Isolation is wanted for noise and safety reasons, I don't want to share grounds.

I was looking at using a 10 bit chip from Maxim or LTC with an spi interface and to use opto couplers on the spi to provide the isolation.

But... the ADC chip costs about $5, and we need three opto channels and three pins on the m128.

To make it simpler/cheaper I was thinking of just using a attiny15 (or some other 8 pin part with adc), do continuous conversions and send data via a single opto channel to a single pin on the micro. Some simple timing based serial protocol to mark data start.

Anybody got a cheaper/simpler solution?

thanks,
-carl

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Directly opto-isolating the analog voltage is hard, particularly at 10 bit accuracy.

Your scheme is a pretty good idea, actually. You would want to use SPI rather than I2C because I2C requires 2-way comm on one of the lines and, again, thats hard for opto. You may want to run the serial fairly slow because most optos that don't take too much power are not very fast.

One of your challenges will be how to power the slave micro/ADC. It may be low enough power consumption that you can derive the power from the source you are measuring.

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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I'm just a software weenie & will probably get shot down, but here goes--

We measure all kinds of stuff with AVR A/Ds. The general approach is a series resistor, and a double-diode (one pointing to Vcc, one pointing from Gnd). And remember that the ADC inputs are very high impedance.

If you don't want to share the ground, use a differential channel or make one with an op amp circuit in front of the AVR.

We've done mains up to 3-phase 600VAC industrial. Sure we blow up a few protos & have to measure/tweak the circuits sometimes. But kinda the combination of the high impedance, AVRs not being very fragile, and reasonable/proper precautions & circuit design make it very doable for the res you desire.

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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As for the power supply. You might want to take a look at www.tracopower.com at the TME series isolated DC-DC converters.
minimum power consumption is 20mA.

As to the AD. It might be interesting to use an AVR for it. Just put one AVR at he "hot" side and the other at the safe side. Tiny 26 has 8 channels and should cost about $3. You could transfer data over UART over optocouplers.

SPI (as mentioned) also works fine

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You may find usefull information int the datasheets of the Agilent HCNR200 / HCNR 201 and Vishay IL300 analog opto-couplers.

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hi

try IL717 and other multichannel isolators from www.nve.com,but they are very fast (30mbps)
bye

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Thanks for the input gents.

Getting islolated power is not an issue, I should have mentioned that in the original post.

High linearity opto couplers are intersting. I knew of them, but thought they would be more expensive. Surprisingly the HCNR200 is only a couple of bucks. Have to keep that in mind for the future.

But I'm pretty sure Lee's idea of using differential inputs should work for me. Forgot the m128 has them. I did mention that I've not dona a lot of ADC design :)?

And that does seem to be the simplest. The voltages involved are not that high, and connecting via high enough value resistors should give me enough isolation for my needs. Now just to make it work.

I still like the idea of using a tiny26 or equiv isolated by optos, If I needed more IO on the 'hot' side I'd probably go that route.

Thanks again.

-carl

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Surely the cheapest way is to connect one side of an optocoupler through resistors to drop voltage to the battery then other side to the ADC. Then create a lookup table for the input/output voltages.

Ralph Hilton

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Just read an advert from Analog Devices about a new "Isolated" ADC they are offering. I didn't look at details or cost.

Jim Wagner

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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I am using AD7741 V to F converter, Traco TMA0505s and 6n137 optocoupler.
Alexander.

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I am using AD7741 V to F converter, Traco TMA0505s and 6n137 optocoupler.
Alexander.