Electrically Isolate Voltage Sensor from Circuit 1

[chumarkrch]

Hello this is my first post.

I am trying to build a voltage sensor for a battery management system. The voltage sensor will simply be a voltage divider across a lithium polymer battery that goes to an ADC on a micro controller. However, I need to isolate the two circuits electrically because it is specified by the rules.

I originally thought to use an optoisolator but I don't think that optoisolators work well with analog voltages.

Please let me know if you have any ideas!

 

[IRstuff]

Have you even done a websearch? There seems to be plenty of hits for "analog" "optoisolator".

TTFN

FAQ731-376

 

[chumarkrch]

I have, and I ordered the 4N25 optocoupler which didn't seem to work for my application. I was made aware that that it was designed to be used in logic applications. So can anyone suggest a type of analog optocoupler that would be good for my application?

 

[waross]

Use an A/D converter before the optocoupler and a D/A converter after the optocoupler.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jimkirk]

These Analog Devices digital isolators might be overkill for your application, but it might give you some ideas.

And I'll second Bill. If you're dealing with lithium batteries, you'll probably want to sense the voltage with 25 mV accuracy or better. Charging and state of charge determination is very sensitive to voltage.

 

[IRstuff]

So get an isolation amp, if you don't know what to order for analog optos:

http://www.analog.com/en/amplifiers-and-comparators/isolation-amplifiers/products/index.html

TTFN

FAQ731-376

 

[itsmoked]

Look at the Linear Tech part that is made for doing this accurately and amongst huge offset voltages.

http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1037,C1786,P86662

Otherwise do it differently. Use a V/F on the battery side and send only the pulse train across the optos. And! Don't use stinkin optos. Use any of the superior magnetic replacements that have arrived in the last few years, to avoid the many problems optos cause.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

Why not just A/D on the battery side and transmit the digital across the isolator right to the processor? I'd think the processor would support something like I2C communications. The more circuitry between the battery and the A/D, the harder to be accurate.

 

[VEBill]

uC with built-in A/D are cheap enough - just put one of them on the battery side of the barrier. Have it toss the digital bits across the barrier.

 

[DHambley]

Look at your isolation spec. Maybe the customer will allow so many uA of leakage or they have some kind of minimum resistance allowed. If this is the case, to get <25mV error on a large stack up, the LT6802 may not be the right choice. It's spec's at 0.25% worse case. That's more than 25mV if you have a large 28V stack. The input current, 10uA, is going to required less than 25kOhms, so it may not be defined as "isolated".
Voltage balancing for Li-Ion is extrememely important for safety as well as battery life. I designed a stack sensor using 0.05% resistors in the Meg-Ohm range with FET input instumentation amps. The worse case errors here were in the range of <10mV. I would suggest doing that.
Otherwise, the suggestions from the other guys to ADC it on the battery side, then transmit the data across with an isolater.