I am trying to develop a system that will interface with a uC that measures battery cell voltages one at a time and records them for later recall. I am trying to figure out how my two leads would get connected to an ADC so that nothing gets fried.

Think of a standard multimeter with each lead on either side of a battery cell (approx 2.1v or so)

Any help wrapping my head around this would be wonderful. I've been searching for the last couple of days and the only references I've found about ADC have only a single input. Where does the other input (the common lead) go?

Thanks for any help!

 

Are your cells tied directly to Vdd, or are you using a regulator? If using a regulator, what is the Vdd to the CPU?

Edit: After re-reading your post, is this a device to measure an external cell voltage, or are the cells actually powering your CPU circuit?

 

The cells are not connected to my device at all (other than the measurement connections). My device will be powered by a separate 12v supply with the appropriate regulator.

That's the main reason this is confusing me. I've measured voltages from the same supply that's powering the uC before. This is for a project at work where I need to know the voltage of battery cells external to my circuit.

 

Ok. So you are trying to measure 2.1V with a CPU, running at, say 5V Vdd.

*If* you know the analog voltage will not exceed Vdd to the CPU, you could actually plumb right into the chip's A/D port...but this is not generally a good idea.

At a minimum, you should have a resistor in series with your CPU pin, like 1K to 10K ohms. This will prevent overvoltage from killing your CPU. A small filter cap from the pin to ground will also help attenuate some noise and possibly protect the CPU from (short) high voltage transients.

You could also make an analog front end that could include static discharge protection, filtering, amplification (or attenuation). It really depends on the application.

What you are trying to do is not hard. It just takes some consideration with respect to the kind of inputs you expect to see (worst case), overall reliability, accuracy, and resolution.

 

You can get ADC chips with differential inputs. This way you simply attach one end of the battery to the in+ and the other to the in-. Take note of previously mentioned input protections, reference voltage in the ADC and maximum pin voltages for the ADC.

We typically use Linear Technology chips for differential inputs (for wheatstone bridge instrumentation rather than battery voltage though). http://www.linear.com/products/analog-to-digital_converters_(adc).

 

You can get ADC chips with differential inputs. This way you simply attach one end of the battery to the in+ and the other to the in-. Take note of previously mentioned input protections, reference voltage in the ADC and maximum pin voltages for the ADC.

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A differential input is not required as long as you can directly connect the battery minus terminal to your ADC input ground, which sounds like the case here. A differential input is only needed if you have a serious ground noise problem or the signal is floating above ground.

 

As per my understanding you want to measure o/p of two cells using single channel ADC. If this is correct then you can use simple 'transistor-relay assembly' to switch the inputs to the ADC.

 

The important question is
Do your cells have a common ground?
if yes then using something like CD4051 will do
if not, are they connected in series? then with a divider to fix the maximum value to VDD of ADC or its maximum input range, you can do it otherwise you need a DPDT relay system to connect each cell in turn to ADC

 

Thanks all for the pointers. I think I have found a chip that will help me out (I don't have the part number handy...). I ordered a couple differential input ADCs to try some stuff out and see what breaks and what doesn't.

 

Thanks all for the pointers. I think I have found a chip that will help me out (I don't have the part number handy...). I ordered a couple differential input ADCs to try some stuff out and see what breaks and what doesn't.

From your first post, I seemed to pick up on two different question:

"Where do I hook the common terminal of the multimeter?"
and
"How do I switch between cells?"

ADCs usually have an analog Vdd and digital Vdd, sometimes not. In either case, the analog ground, or just ground, is where the common lead from the signal goes.

You can either switch a signal input with analog multiplexers, analog switches (4066, as an example), or you can buy an ADC with multiple channels. This is completely overkill (ad7734) but it is an example of a multi-channel chip.

Alternatively, you can use a microcontroller with multiple analog inputs. (16f877a, example)

Good luck!