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ADS8505: ADS8505 Design question

Wu JW
Intellectual 2935 points
Part Number: ADS8505
Other Parts Discussed in Thread: ADS7805

Hi,

My customer used the reference circuit as followed. The AD input is the output from an amplifier. Customer found our device's accuracy performance can't meet their requirement.

We tried to change the parameters, change the 200ohm to 50ohm, change the two 2.2uF Tantalum cap to 2.2uF ceramic cap. And the performance can meet the customer's requirement.

Is there any problem for these changes? How about the function for the 200ohm resistor and how to choose the value of this resistor? Thanks!

  • 0
    TI__Guru 51436 points

    Hi Wu,

    In the case of the ADS8505, the device will work fine with ceramic capacitors; you may replace the 2.2uF tantalum capacitors in the CAP pin with a ceramic capacitor.   The ADS8505 device was developed to be an exact drop-in replacement of the former ADS7805.  The ADS7805 was developed around 1996 when ceramic capacitors may not have been low cost.  Today, ceramic capacitors are lower cost and offer good performance. The ADS8505 datasheet was kept very similar to the former ADS7805.   However, the ADS8505 does not require the ESR of the tantalum capacitor and will be stable/work using ceramic capacitors.

    Regarding the replacement of the 200 ohm resistor, the characterization/testing of the device is performed using the external resistors (1% tolerance) shown in figure 27 (and/or Figure 28); therefore, the recommended resistor is 200 Ohms.  I suspect the effect of changing the resistance from 200Ohms to 50 ohms is small since the nominal input impedance due to the internal resistors is ~11.5kOhm.  Can you please explain what is the performance issue while using the external 200 ohm resistor?

    Thank you and Best Regards,

    Luis

  • 0 in reply to Luis Chioye
    TI__Intellectual 2935 points
    Hi, Luis,

    My customer's application is IC test machine. They need very high accuracy for the AD's output, and the input signal will be switched by high frequency. They tested our device and the competitor's device - AD976, they found ADS8505's random noise is bigger than AD976.
    I checked the input cap for the AD976 is 22pF, which is 50pF for our device, so I tried to reduce the input resistor to 50ohm, found it will have some improvement.
  • 0 in reply to Wu JW
    TI__Guru 51436 points
    Hi JW,

    The external resistor has a negligible effect on the random noise contribution, since the noise contribution of the 200 ohm series resistor is small. The ADS8505 device presents a 11.5kOhm internal input resistance prior the sample and hold capacitor. Changing the resistance from 200 Ohm to 50 ohms will change the initial full-scale error and zero error of the device. These parameters are characterized using the 200 ohm external series resistance.

    If an schematic is available showing the circuit driving the device, we could review.

    Thanks and Regards,
    Luis
  • 0 in reply to Luis Chioye
    TI__Intellectual 2935 points
    Hi, Luis,

    The SCH should be confidential and I will send a Email to you.