Help adding ADI amp to battery current sensing

Hello,

I would like to add a differential amp to my battery current sensing.  This is due to the resistor in the ground path being to large and creating a negative offset in my analog channels of my data acquisition product.  This battery charge IC (DS2715) senses voltage for charging via this resistor in one direction and discharging when the current flow reverses.  This has me worried that I may need a negative rail for the amp I'm trying to add in.  I would really like to avoid adding yet another power supply especially just for this amp.  So I guess my questions are as follows.

Questions:

1. What low power amp (the bandwidth needs are very low - I'm assuming full differential is required) would be recommended.  I was looking at the AD8476 amp. 

2. Would I need a negative power supply to handle both polarities (created by charging and discharging)?

3. If the polarity switching is an issue could I just push the dc offset up instead of having a negative supply?

I"m just looking for some general guidance on how to implement one of your amplifiers into this circuit to help bring down that resistor R82.  Anything comments are helpful.  Schematics attached.

Thanks!

DS2715-amp-question.zip

  • 0
    •  Analog Employees 
    on May 30, 2019 9:38 AM

    Hi.

    Thank you for your interest in using ADI products.

    First, please let me know if my understanding of your question is correct - You want to monitor the voltage across the sense resistor and since the current flowing through it is changing direction, the voltage across it, with respect to one resistor terminal, also reverses. And since your data converter, I'm assuming an ADC, cannot accept inputs lower than its reference potential, you wanted to use a differential amplifier to translate the voltage across Rsense such that it's lowest potential would be the reference potential of the ADC.

    If my understanding is correct, I would like to recommend that you use a current sense amplifier with bidirectional Vin instead of a differential amplifier. You may select the part number for your application from the table below or in this link: https://www.analog.com/en/parametricsearch/11082

    Common mode range would depend on the voltage across the sense resistor. VS span is the difference between the amplifier's positive and negative supply voltages, which would determine the output voltage span.

    Thank you,
    Shine

  • Hi Shine,

    Thank you for the great reply and recommendation!  I will start looking at the current sense amps you've recommended.  You are very close regarding the application - I should have elaborated.  This is actually for a battery charge control application.  The charge IC is the DS2715 which takes the voltage across the sense resistor as you have drawn.  It then uses that to regulate charge current.  In discharge mode the current switches direction and the charge controller allows the load to draw charge from the battery.  So instead of driving an ADC this will be driving the differential current sense input of the DS2715.  

    My charge current is ~100mA and my load current is lower.  Unfortunately this charge control IC requires a large resistor in the ground path (428mR) which I am trying to get down to a lower resistance.  I was thinking of using a gain of 10 or more so I can shrink the resistance.  

  • After looking at the amps you recommended I do have some more questions.  I'm in need of a low power solution since this will be connected to the battery until the protection circuitry disconnects the battery from all other circuitry until a recharge cycle is initiated.  

    I notice that the only low power current sense amps are single ended.  

    For example, I like the AD8218, but how would I turn this into a bidirectional differential interface to drive my charge controller that has a DC offset that is ~100 to 150mV?

    I can't use the single ended output because I would then have to tie one of the DS2715 since inputs to the battery negative net on one of the sides of the resistor and would therefore loose the intended offset and bidirectional functionality I think.  I hope this makes sense.

    Because of this am I forced to implement another amp to convert the signal from single-ended to differential and are there any ideal low power analog amps in ADI's catalog for this?

  • 0
    •  Analog Employees 
    on Jun 3, 2019 12:50 AM in reply to m3atwad

    Hi.

    Please give me the specifications of your battery. I would like to recommend a full system ADI solution for you. Or you may search in this link: https://www.analog.com/en/products/power-management/battery-management.html

    Thanks,
    Shine

  • Hi Shine!

    My battery pack is 5x series 1.2V NiMH cells with a capacity of 500mAh.  The max load is 100mA and min load will be < 500uA.  I am charging the battery pack with the above IC with a charge current of ~100mA average.  I believe this current from the switcher charger is a ramp and approaches 150mA peak.  Maybe 200mA peak.  I don't have access to each cell - only access across the pack.

    While on this topic I"m using an LTC fuel gauge PN LTC4150.  This will also need an amp to gain up and buffer the Rsns to the LTC4150.  Maybe we an use the same amplifier configuration?