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MSP430F427A SD16 RC Filter and Grounding

Sebastian Delaile
Prodigy 130 points
Other Parts Discussed in Thread: MSP430F427A

I am looking at putting a single pole RC filter (with around 100Hz cutoff I was thinking and design of ()) on the input to the SD16 channels of a MSP430F427A, and was wondering about the drawbacks of doing this. In the family datasheet it recommends a external RC filter however in another TI application note (ftp.ti.com/.../sbaa111_Understanding_MSC12xx_ADC_Input.pdf) it suggests there are some offset and delay issues associated with putting an RC filter on the un-buffered input of a delta-sigma ADC, do you think that this would be an issue for the SD16?

Another question I wanted to ask was concerning the use of ground as one of the inputs to the ADC, as I am measuring the voltage drop across a precision current shunt (for a coulomb counting battery monitor) which is connected to battery negative and the other end is used as system ground. I have read in these forums that using ground as an input to the differential ADC can make it quite noisy, is there an obvious workaround to this situation?

Thanks

  • 0
    Guru 46710 points

    Sebastian Delaile said:
    it suggests there are some offset and delay issues associated with putting an RC filter on the un-buffered input of a delta-sigma ADC

    Any filter shall be considered as delay line causing signal phase offset.

    Sebastian Delaile said:
    do you think that this would be an issue for the SD16?

    Of course not. Actually SD16 will be happy to see low impedance signal source (capacitor) on it's input. Phase shift (signal delay) will have impact in AC sampling __application__ like AC power meter. Even in power meter RC filters are used, just phase errors are compensated in software.

    Your battery is DC signal source, so you shall not worry about signal phase delays in your application. Use filter as in EDN article and you will be fine.

    Sebastian Delaile said:
    Another question I wanted to ask was concerning the use of ground as one of the inputs to the ADC, as I am measuring the voltage drop across a precision current shunt (for a coulomb counting battery monitor) which is connected to battery negative and the other end is used as system ground. I have read in these forums that using ground as an input to the differential ADC can make it quite noisy

    It is not ADC which is "made quite noisy". It's improper engineering. Just don't connect negative ADC input to local ground (of the chip) but wire both differential pairs to precision shunt. By connecting negative pair of differential ADC to local (and noisy ground) you simply destroy all the beauty of differential ADC! You shall learn about four terminal sensing to understand why.

  • 0 in reply to Ilmars
    Prodigy 130 points
    so if my shunt PCB is connected to the Controller PCB using a cable, run two 'grounds' in separate lines, use one for PCB ground, and connect the other to the differential input of the ADC? Also, I want to make other measurements of the battery (shunt voltage, battery voltage, and mid-point battery voltage), is it noisy for them to also share a ground? The accuracy of these other measurements is significantly less significant. Thanks.
  • 0 in reply to Sebastian Delaile
    Guru 46710 points

    Sebastian Delaile said:
    so if my shunt PCB is connected to the Controller PCB using a cable, run two 'grounds' in separate lines, use one for PCB ground, and connect the other to the differential input of the ADC?

    Yes. Just I would call "ground line" connected to adc input as ground sense line, obviously will not connect it to anything else but ADC input(s) and associated filter components.

    Sebastian Delaile said:
    Also, I want to make other measurements of the battery (shunt voltage, battery voltage, and mid-point battery voltage), is it noisy for them to also share a ground?

    I cannot tell about noise of your circuit, thou if you have differential ADC it would be good to use it properly for all the measurements- to have best precision you can get out of components used. Why would you need to substract voltage drop on shunt and voltage drop on ground power wires from battery voltage measurement if you have everything that's needed to measure just battery voltage? After all to precisely measure battery, midpoint and shunt you don't need to run three pairs, but just four wires in total considering that shunt is connected to one of the battery terminals.

  • 0 in reply to Ilmars
    Guru 23000 points
    In addition to Ilmars, in which I fully agree, when 4-terminal sensing and wants to use a ‘precision shunt’ then use also a 4-terminal shunt, this will give you ‘high precision’.

    And add protection (diodes) to the ADC connections, in case one of the power connections fails then full current flows through the MSP.
  • 0 in reply to Leo Bosch
    Prodigy 130 points
    I am using the 3020-01103-0 shunt by Murata (www.murata-ps.com/.../dpm_shunts.pdf), which appears to be 4-terminal, however doesn't explicitly say that, is that what you are referring to? I was going to have a fast-acting fuse on the 12V line to the Controller PCB, do you think that that protection is sufficient? Or also protection diodes. Also if protection diodes are necessary, would you be able to point me in the direction of an application note detailing the process? Thanks
  • 0 in reply to Sebastian Delaile
    Guru 23000 points
    You wants to go big!

    Yes correct this is a 4-terminal shunt, the schematic in the pdf is clear about how to connect. The little two screws to the ADC and the big nuts on top to your main current.

    In my previous post I forgot to add; The Data Sheet recommends a capacitor at Vref but I would advise you to read this as ‘must add’, this will reduce noise.

    When the Ground connection fails (loosen screw or equal), or the shunt blows you need to protect the MCU-ADC. How depends on the maximal voltage and current you expect.
    But it looks like here you need to use the filter resistance to adjust to this case.
  • 0 in reply to Leo Bosch
    Guru 23000 points
    Your ADC has PGA which you can use to amplify the signal. Calculate it so that you get the lowest possible voltage across the shunt, which reduces shunt heat and so increases accuracy.
  • 0 in reply to Ilmars
    Prodigy 130 points
    Just thinking about the RC filter design, considering that I am running at 1Mhz Sampling with an oversampling rate of 256, my data output rate would then be 3906(1000000/256), is this correct in saying that this is the frequency that I want to reject? Considering that I am not particularly interested in measuring transients, would an RC filter with a cutoff of like 100Hz be feasible? say 10uF between the input pins with 0.1uF between each pin and ground, and 160ohm series resistance before the capacitors?
    Thanks
  • 0 in reply to Sebastian Delaile
    Guru 46710 points

    Sebastian Delaile said:
    Just thinking about the RC filter design, considering that I am running at 1Mhz Sampling with an oversampling rate of 256, my data output rate would then be 3906(1000000/256), is this correct in saying that this is the frequency that I want to reject? Considering that I am not particularly interested in measuring transients, would an RC filter with a cutoff of like 100Hz be feasible?

    You shall start with signal you measure, not sampling rate. Knowing bandwidth of the signal - you design filter & it' s cutoff frequency. Then according to your application but mainly Nyquist theorem you choose sample rate. Battery measurement don't need 3906 samples/sec! This is HUGE overkill unless you are building some instrument which measures not battery but fast changing power consumption of external device.

    Further reading

  • 0 in reply to Ilmars
    Prodigy 130 points
    Ok cool, I was planning on taking some samples like once a second and then sleeping the remainder of the time and was thinking it was more power efficient to use a high sample rate when doing this to maximize sleeping time. But mainly I was just concerned about RC filter affecting the ADC reading.

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