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I am trying to measure battery's current and voltage for a battery charging/monitoring project. I have read all about current sensing (including high side & low side sensing). And i have decided to use Shunt resistors for current measurement as they are accurate as compared to other current measuring devices. My battery would be a Li-Ion Battery, and the maximum rating of this battery stand would be (4.3V , 40A).

However i am confused on how to measure the voltage and current using an ADC, i-e whether it should be measured single endedly or differentially. A very rough sketch of my circuit is given below. (This ADC would be interfaced with a microcontroller)

enter image description here

The battery can be seen connected to a buck converter for the charging purpose. And and ADC can be seen as well.

(Please note that my sketches might not be accurate, but i mean all what i have written here and in the diagrams)

What i think is that, if i try to measure my Battery's voltage and current in this way (shown in the image below), my voltage would be differential(since battery's negative terminal is not directly grounded, theres a shunt in between) so i have to feed it to a differential input ADC, whereas the current is to be measured singe endedly, as one leg of the shunt is grounded. enter image description here

And if i try to measure my Battery's voltage and current in this way (shown in the image below), my voltage would be singe ended (since the battery's negative terminal is directly grounded), and my current measurement would have to be done differentially (as my shunt is placed in between my supply and the battery).

enter image description here

Now, i am no expert in ADCs, but as far as i have read about them (also their data sheets), if an ADC has both single and differential ended inputs, we can use it as a single ended input ADC OR we can use it as a differential ended input ADC. Which means we cannot use it as both single and differential input at the same time.

Which brings me to my question. What could be a solution to it ? Shall i use 2 different ADCs, one for single ended input and the other for differential ended input ? Or can i measure both the current and voltage differentially and feeding them both to a single ADC configured as a differential ended input ADC ? P.S i am not looking forward to use a single to differential ended AMP , as i am supposed to measure these quantities with as high accuracy as possible, and introducing such AMP would decrease my system's measurement accuracy.

So it leaves the question, whether i can measure both the quantities differentially ? like given in the picture below, which is just feeding the voltage measurement connections to the '+'and '-' input of a differential ended input ADC. As the Battery's negative terminal in this case would be at ground potential, so can it be fed to '-' terminal of a differential input ADC ? (Since i dont have much knowledge in field of electronics, i dont know if it would be possible or not, or what i am asking here is totally stupid)

You helful comments would be really appreciated,

Thankyou.

Thankyou.

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...as far as i have read about them (also their data sheets), if an ADC has both single and differential ended inputs, we can use it as a single ended input ADC OR we can use it as a differential ended input ADC. Which means we cannot use it as both single and differential input at the same time.

This is not always true. For example, I have used ADS1015 on a couple of projects recently. On this chip, whenever you switch the channel being read you also have the option to switch between single-ended and differential measurement. (This is not an endorsement of this chip for your project. Just an example of a chip that doesn't have the limitation you thought was universal)

Furthermore, even if you had a device that had to be configured as single-ended or differential for all channels at the same time, nothing prevents you from using ground as one of the inputs to a differential channel. So you could just configure it as differential and move on with your design. The only thing you'd lose is the opportunity to use the 4th input pin for some other purpose.

Another option, if you plan to use external signal conditioning, you can do differential to single-ended conversion in the signal conditioning circuit, and your ADC will never know the signals are anything but single-ended. This essentially makes the amplifiers shown in your diagrams to be external devices rather than internal to the ADC chip (and add some filtering in their feedback networks to reduce noise).

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  • \$\begingroup\$ If i can use ground as one of the inputs to a differential channel, then my problem is solved. Nothing to worry about. Thanks alot for your answer! \$\endgroup\$ – yiipmann Jun 30 '15 at 16:10
  • \$\begingroup\$ @yiipmann, I would double check the datasheet of your ADC to make sure it's okay with inputs very near ground, but it should be okay or you should be able to find an ADC where it's okay. \$\endgroup\$ – The Photon Jun 30 '15 at 16:35
  • \$\begingroup\$ Can you please tell one more thing, a friend of mine told me that since my signal values would be positive only, and ill be using a differential input ADC, ill be loosing 1 bit of resolution (for e.g for a 12bit ADC i would be using 11 bits of resolution). He didnt know the exact reason for that but he said that its like that. Can you please tell me if its correct ? If yes then WHY ? \$\endgroup\$ – yiipmann Jul 3 '15 at 15:24
  • \$\begingroup\$ @yiipmann, yes that's basically right. If the input range is from -2.5 to +2.5 and you only use half that range, you effectively lose 1 bit of resolution. \$\endgroup\$ – The Photon Jul 3 '15 at 16:29
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I think all your proposed solutions are possible good solutions.

If implemented properly I think it does not matter that much if you go for a full differential or partly single-ended solution. But in general differential circuits are less sensitive to external disturbances.

Do make sure that the (differential) amplifiers have the right voltage gain such that you will be using the ADC's full range.

Another potential issue, since you will be using a switching converter, there will be switching noise on the measured current and voltage. Using a low-pass filter between the shunt resistor/ battery and amplifier input might be sufficient to suppress this noise enough. Some averaging of the measured values from the ADC might also help and improve accuracy.

And kudo's for doing your homework and you already know a lot more than many answer-seekers on this forum ! :-)

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  • \$\begingroup\$ Thanks alot for your answer. Will consider using the LPF, will look in to it. \$\endgroup\$ – yiipmann Jun 30 '15 at 16:09
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What value are you using for a sense resistor? A lot of 4.3V LiIon battery chargers/monitors use 10mOhm; at 40A, you would generate 400mV and burn 10mOhm * (40A)^2 = 16W. Seems pretty wasteful, not to mention it will be expensive to pay for a precision resistor at that rating.

Get a resistor small enough that the voltage across it is negligible; then you can measure both the battery voltage and current single ended.

schematic

simulate this circuit – Schematic created using CircuitLab

You can also calibrate out the drop on the sense-resistor with VBAT' = VBAT - IBAT * RSNS.

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  • \$\begingroup\$ Only concern I see here is current measurement accuracy/noise. 4mV sensed is 40A, so to scale that to a 0-5v ADC input would require a gain of 1250. Do-able, but noise and any EMI will be amplified also. Consider shielding and guard traces, and low-pass filter the output. \$\endgroup\$ – rdtsc Jun 30 '15 at 22:20
  • \$\begingroup\$ The sense resistor i am planning to use at 40A are these and they have 0.6667mOhms resistance with a drop of 50mV at full rating, so i guess i wont be burning much power. However i will look into LPF for noise cancellation \$\endgroup\$ – yiipmann Jul 1 '15 at 13:16

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