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INA851: Supply for AFE

Theo Keinarth
Intellectual 325 points
Part Number: INA851
Other Parts Discussed in Thread: ADS127L21, TPS7A49, TPS7A47, TPS7A39, TPS7A57, TPS7A96

Tool/software:

Hi TI Team

I'm currently working on an isolated AFE for +/-20mA current loops. As an ADC I plan to use the ADS127L21 in combination with the INA851 and an TPS26611 to limit the current.

Now I'm not so sure on how to supply all those different components. My Push-Pull transformer gives me +/-14V. Can I take that voltage and give it directly to the INA851 or should I use a LDO like TPS7A39 or LT3097 to reduce noise? For the ADCs AVDD and IOVDD I will use 2x LT3045 or TPS7A49. Are there any lower noise alternatives from TI except for the TPS7A47 that can handle VIN>30V and are smaller? I want at least 20Bit noise free at 1kHz using the FIR3 filter. Preliminary circuit below:

Thank you in advance.

Regards Theo

  • 0
    TI__Genius 15400 points

    Hi Theo, 

    I do recommend that you use an LDO in between the transformer and INA851 to minimize the noise at the higher frequency caused by the transformer. The INA851 has PSRR curve across frequency to help illustrate how effectively the device can suppress this ripple but using an LDO instead would be the recommended design route. 

    I don't see any major issues with the INA851 part of the thread, so I will pass the thread along to the LDO team to comment on your questions on the LDO. 

    Thank you!

    Regards,
    Ashley

  • 0 in reply to Ashley Kang
    Intellectual 325 points

    Hi Ashley

    Tanks for your help. For LDOs I will use the LT3097 and LT3045 for now. They seem to be outstanding in terms of performance and are really small in size.

    Hope that TI will offer some alternatives in the future.

    Regards Theo

  • 0 in reply to Theo Keinarth
    TI__Expert 6500 points

    Hi Theo,

    For low noise high-performance LDOs we have available TPS7A96 (with 0.46uVRMS from 10Hz to 100kHz) and TPS7A57 (5A LDO with high PSRR). These two are low input voltage LDOs, so generally it is common to step down the voltage close to the desired output and using these LDOs to clean up the supply.

    Here is a quick spec view:

      TPS7A96 TPS7A57
    Iout(Max)(A) 2 5
    Vin(Max)(V) 5.7 6
    Vin(Min)(V) 1.7 0.7
    Vout(Max)(V) 5.5 5.2
    Vout(Min)(V) --- 0.5
    Fixed output options(V) --- ---
    PSRR @ 100
    KHz(dB)    		 60 60
    Noise(uVrms) 0.5 2.1
    Accuracy(%) 1 1
    Dropout voltage
    (Vdo)(Typ)(mV)    		 250 75
    Iq(Typ)
    (mA)    		 15 5
    Output capacitor type      Ceramic ---
    Thermal resistance
    θJA(°C/W)    		 46.1 40.3
    Operating temperature
    range(C)    		 -40 to 125 -40 to 125
    Package size: mm2 9 mm2: 3 x 3 (WSON|10) 9 mm2: 3 x 3 (WQFN|16)
    Rating Catalog Catalog
    Features Adjustable current limit,Enable,Foldback Overcurrent protection,Output discharge,Power good,Soft start Adjustable current limit, Enable, Foldback overcurrent protection, Output discharge, Power good, Soft start, Thermal shutdown

    Regards,

    Andres

  • 0 in reply to Andres Sagredo
    Intellectual 325 points

    Hi Andreas

    I do only have +14V and -14V supply voltage so I need an LDO that is capable of handling those voltages. For positive voltages the TPS7A47 seems to be the only option and for negative voltages its the TPS7A39 from TI. Since I don't need a lot of current the LT3097 and LT3045 seem to be the better part in my case.

    Thanks for your help.

    Regards Theo