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ADS1158 PCB layout

Pablo Nicolás Núñez Pölcher
Prodigy 60 points
Other Parts Discussed in Thread: ADS1158, REF3125

Hello,

I am currently working on a DAQ board project using the ADS1158. So far, every doubt I had was answered either by the datasheet or by several of the application notes concerning TI ADCs. However, there is one thing with which I am stuck, and that is the use of separate analog and digital ground planes. No matter how hard I look for answers, this seems to be something that depends on a personal taste, and being an inexperienced engineer doesn't help much.

Straight down to my question... specifically for the ADS1158, should I use separate analog and digital ground planes, or does a single ground plane suffice, provided that
I use adequate filtering for my analog supplies? What would be the best way to lay out a board for the ADS1158?

The power supply on the board consists of an LM2673S switching regulator (Fosc = 260kHz) which provides 7V to a REG104-3.3, a TLV70025, and to an LM2575S configured as an inverter (Fosc = 52kHz) which provides -3V to a TPS72325 regulator. The output of the LM2575S is filtered by an LC filter (22uH, 100uF). The ripple at the output of both switching regulators (on a custom-built eval board) doesn't exceed 10mVpp (measured with a Fluke 105B scope).

The +2.5V/-2.5V from the TLV70025 and the TPS72325 provide the analog supply to the ADS1158 and the REF5040 laid out exactly like the example in the datasheet. The output of the REG104-3.3 powers the rest of the board (LPC2478 microcontroller, DP83848IVV Ethernet PHY, MAX3243UI drvr/rcvr, MCP2551 CAN transceiver).

USB Vbus lines, as well as the grounded shields, are filtered with ferrite beads (Murata BLM21PG221SN1D, 220 ohm @ 100 MHz).

Thank you very much.
Best regards,
Pablo

 

  • 0
    TI__Mastermind 43915 points

    Hi Pablo, 

    Generally, if the part is susceptible to noise,  we split the ground layer / plane to isolate the digital portion of the circuit from the analog portion. Splitting the layer or plane will help with making sure that the return currents from the Analog inputs, Ref, etc. will not coincide with the return currents from the digital control signals. The big worry is that additional noise will be coupled into the analog inputs if the return current paths of a digital control signal (such as SCLK) and an analog input cross. Generally, additional care is taken in higher bit converters (24-bit 32-bit, etc.). The ADS1158 is a 16-bit converter and you should not have any additional noise issues if you decide to not split the plane / layer. Just make sure, in your layout, you do not have any analog and digital signals running side by side or crossing one another. On the EVM, we do not split the ground plane, we just make sure that the signals do not cross. 

    As for the rest of your circuit, I have not tested all that much with using the REF5040 and dual supplies. I see in the datasheet we show an example of using the REF3125 using split supplies. I am not sure how the REF50xx parts will react with dual supplies. Let me get back to you on that. However, you should be able to use the REF5025 and power it from a single supply (use the 3.3V signal you have from the REF104) to get the full range. From my understanding, your ADS1158 is powered from +/-2.5V supplies. So, using the REF5025 will allow you to reach the absolute input voltage range (AVDD+100mV and AVSS-100mV). Keep in mind that the full-scale input voltage that the ADC can reach is +/-1.06Vref.  

    Regards,

    Tony Calabria 

     

  • 0 in reply to Tony Calabria
    TI__Mastermind 43915 points

    Hi Pablo,

    To follow up, you are able to use the REF50xx parts in a split supply +/-2.5V. Just remember that the REF5040 will be 4.096V referenced to -2.5V. 

    I still think simply using the REF5025 in a single supply would be a better fit for your application. You are able to reach the entire input voltage range using it. 

    Regards,

    Tony

  • 0 in reply to Tony Calabria
    Prodigy 60 points

    Hi Tony,

    thank you very much for you prompt answer. I will take a close look to my layout then, to see if there is a real need for a separate ground plane. I have taken special care to keep analog and digital signals from crossing or running side by side so, at a first glance, I'd say I'll stick to the solid ground plane alternative.

    I will also replace REF5040 for REF5025, as you say. It will also prove easier to route than the bipolar supply operated REF5040.

    Regards,

    Pablo