LTC1867 Technical Support

Hi, Need support for customer's question. 

Cistomer is manufacturer for glove testing machine and would like to develop one of machine function to read the output from a pressure sensor. 

The usage per machine is as follow.

1) ADCxxxx(waiting recommend, may be LTC1867)

2) Instrumentation amplifier (AD8237)

3) DAC (AD5621)

For LTC1867 and LTC1867L, please clarify what is the effect due to high gain error and power supply sensitivity as indicated in the table below. 

Refer to page 3 of https://www.analog.com/media/en/technical-documentation/data-sheets/LTC1863-1867.pdf

Thanks

John 

    •  Analog Employees 
    on Nov 6, 2019 4:38 PM over 1 year ago

    Hi John,

    Ideally, the ADC response is a straight line from ground(zero-scale) to REFCOMP-1LSB (full-scale). If the ADC has gain error, full-scale will be reached either below or above an input voltage of REFCOMP- 1LSB.

    Power supply sensitivity indicates how much the output code can change with changes in supply voltage (Vdd).

    AD8237 may not be a good choice to drive the LTC1867. SAR ADCs have a capacitor dumped on the analog input at the beginning of every sample period. The ADC driver must recover from this transient on the input and settle to within 1LSB before the start of the next conversion ideally. Failure to settle will result in gain error and possibly linearity error. Linearity error happens when the ADC response deviates from that ideal straight line. You might consider using the AD8421 to drive the ADC.

  • Hi Ghoover,

    Thanks for the support. 

    I got the latest update from customer design. 

    Is this combination solution good to propose for design below? 

    1. AD8237 + AD7949
    • Performance better and price lower, but AD7949 is QFN package.
    1. AD8421 + LTC1867
    • Performance lower and price higher. LTC1867 is SSOP package.

    Thanks

    John

    •  Analog Employees 
    on Nov 12, 2019 3:32 PM over 1 year ago in reply to John pg

    Hi John,

    The AD8237 will have the same problem driving the AD7949 that it would driving the LTC1867. AD7949 will also produce glitches at the analog input that the AD8237 will be very slow to recover from.

    Your schematic doesn't make sense to me. You show the amplifier driving the MCU instead of the ADC.

  • Thanks Ghoover for the assist. 

    Here is revised block diagram after review with customer. 

    Do you have better recommendation for 

    1) ADC: LTC1867 / AD7949 / ???
    2) Instrumentation amplifier: AD8421 / AD8237 / ??
    3) DAC: AD5621 / ???
    4) Op-amp: ???

    ADC requirement as follow:
    - 4 channels
    - Minimum 14-bit resolution
    - Is there an option to use either single ended or differential input?
    - The sampling rate required is <1ms.

    Looking forwards your next support. 

    •  Analog Employees 
    on Nov 13, 2019 4:38 PM over 1 year ago in reply to John pg

    1) Either ADC should work for your application. Both can do single-ended or differential

    2)If you run the ADC at only 1ksps or even 10ksps, either instrumentation amp should work

    3)AD5621 is only 12-bit, if that's ok

    4) you haven't said what op amp requirement are