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ADS8668: ADS8668 floating CV inputs management

Pierluigi Vienna
Intellectual 350 points
Part Number: ADS8668
Other Parts Discussed in Thread: OPA180, TL072, ADS8688

Hi.

I'm wondering what kind of readings i will have when an input is not phisically connected to the ads8668, somebody told me that if a jack is unplugged reading will go to VREF/2 and this is not ok for the design.

Should that be true on my current design? we wish to read a 0 when jack is unplugged.

At present this is the circuit of one CV input with related attenuverter. After this the signal goes to an RC input filter before getting to ADS8668 pin

the attenuverter is meant to:

at knob value 0  multiply incoming signal by -1
at knob value 0.5 multiply incoming signal by 0
at knob value 1 multiply incoming signal by 1

Thanks a lot and regards

  • 0
    TI__Mastermind 35450 points

    Hi Pier,

    I made a TINA simulation  circuit according to your schematic, see attached file. OPA180  is used because it has TINA model and similar bandwidth as TL072.  If the jack is unplugged , this amplifier circuit will output a signal close to zero (27.7uV for OPA180 circuit).  Hence,  the conversion from ADS8668  will be zero as well if the connection form H1_2 to ADC input is good.

    ADS8688 has a constant input resistance 1Mohm(Rin) with analog front-end(AFE) on each channel shown in following Figure 67 in datasheet, this design usually eliminates the requirement to use external amplifier to drive the ADC in most applications. There is a bias signal VB used for internal PGA on ADC, which is around 2.25V for ±2.5VREF input range or 2.5V for 1.25VREF range, this voltage will be shown as conversion code from ADC when the input or sensor is disconnected or floating, but this bias voltage will not be seen if any available signal is shown up on ADC’s input. Since you are using external amplifier before ADC, so this is not the case for your design.

    Thanks.

    OPA180_CV Input.TSC

    Regards,

    Dale