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ADS8699: Help for the design check

Part Number: ADS8699
Other Parts Discussed in Thread: REF5045

Hi TI Technical Support Team,

I have designed a small 4 layer board with 2 18bit 100kHz ADS8699 and I am planning to use it to read voltage and current values from the device under test. The top and bottom layer are used for routing signals. Internal planes are GND planes. The GND plane is solid and I did not make  two seperate AGND and DGND. My voltage range is 0-5V range. I am converting current to voltage values and the voltage range corresponding to current values is between 1.5V and 3.5V. I have attached the schematic part of the ADC. I have following questions regarding the schematic.

1. I am not planning to use RVS, ALARM, RST pins of the device. Are the connections I made in the schematic correct, and also is there big issue in PCB layout? 

2. I am using external 5V reference (ADR5045 0.2% accuracy) for the ADC. It is shunt reference. In the datasheet, the reference schematic uses series voltage reference REF5045. Does using shunt reference will make huge negative effect in reading the signals?

3. Is the filtering part for the ADC inputs correct?

Thank you very much for the help,


  • Hi Ahmet,

    Thanks for choosing ADS8699 in your design. Please see my feedback below:


    1. Firstly, ADR5045 is a 5V voltage reference, the ADS8699 requires 4.096V reference voltage, so the ADR5045 can not be used for this ADS8699. Also, the internal voltage reference on the ADS8699 has higher initial accuracy and lower drift performance than ADR504x series,  0.02% on ADS8699 vs. 0.2% on ADR5045, 7ppm/°C max reference drift on ADS8699 vs. 100ppm/°C max reference drift on ADR5045, I do not see any reason you would not use the internal voltage reference on the ADS8699. 

    2. Precision ADC requires a clean power supply to achieve specified performance, LM78M05 voltage regulator is definitely not a good power device which can meet the requirement. A low noise LDO, such as TPS7A4700, is a good power device for precision ADC especially 18-bit ADS8699.

    3. You do not have any connection on the SDO pin, you need this signal because the SDO is used to get your conversion data. Otherwise, you will not be able to get your conversion data.

    4. You can use different /CS signal for both ADCs as you are doing now, also you can use daisy-chain mode to retrieve the data from both ADCs. Small value series resistors are recommended for the SPI data bus, please check the EVM schematic ADS8681EVM.

    5. If a high input impedance is desired, you can use a buffer for the voltage measurement channel (U4A). If 1Mohm input impedance can meet your requirement, the U4A buffer is not necessary because ADS8699 has a 1Mohm input impedance and also it does have an internal ADC driver.

    6. A reset signal is expected to reset the ADC after power up.

    7. Since you use a 5V as a power supply for ADC's DVDD, make sure your microcontroller has a 5V logic for digital bus.

    8. A same value resistor as R4 and R6 can be used on AIN_GND pin. See the EVM schematic above.

    9. You need a transimpedance amplifier (TIA) for current-to-voltage conversion on the 2nd ADC's input, not a buffer. See some application notes from TI:

        What do you know about transimpedance amplifiers-1

        Transimpedance considerations for high-speed amplifiers

        Build a programmable gain transimpedance amplifier using the OPA3S328

    PCB:  leave more space between ADCs and the power supply circuit in case any noise/interference from the power to the ADCs.

    Answers to your questions:

    1. Both RVS and ALARM are output and can be floating if you do not use them. A RST is recommended to reset the ADC.

    2. See my feedback about your reference above.

    3. Filter design depends on what the bandwidth you want in your system, the ADS8699 has a 15kHz BW internal LPF. You can design an external lower BW LPF if you want. The series resister is helpful to limit input current to the ADC.

    Best regards,


  • Hi Dale,

    Thank you very much for the detailed review and suggestions. This helped me a lot. I revised the schematic based on your recommendations and suggestions (expect power supply part because it was out of stock in digikey so I moved to another low noise 5V LDO ADP7142)

    Best regards,


  • Hi Ahmet,

    Great, thank you! For the LDO, you can go to the Power Management forum and check with the TI professional support team.

    Best regards,