• State Resolved
  • Locked Locked
  • Replies 1 reply
  • Subscribers 47 subscribers
  • Views 310 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

PGA855: PGA855, INA851 and ADS981X

Micha Keck
Prodigy 30 points
Part Number: PGA855INA851
Other Parts Discussed in Thread: INA851

Hello,

I am designing a circuit in which a differential input signal is amplified with the INA851 and the PGA855 and then digitized with the ADS981X A/D converter. The amplifiers are supplied with +-15V, the DIffernet input signal is maximum +-13V. The smallest one is 4.7mV. The question is how I have to interconnect the three devices with respect to the pins VOCM and FDA. Can I set VOCM of the INA851 and PGA855 to ground? And how high do I choose the capacitors between the output and FDA?

Thanks a lot



  • +1
    TI__Guru 50886 points

    HI Micha,

    The ADS9810 accept input ranges of ±12V,  ±10, ±7V, ±5V and supports single-ended and differential inputs, where the positive and negative inputs accept voltages from -14 to +14V.  

    The VOCM on the INA851 and PGA855 sets the output common-mode voltage level. Hence, assuming the PGA855 and INA851 are powered with bipolar ±15V input (Vs+, Vs-) and bipolar ±15V output (LVDD+, LVSS-) supplies, connecting VOCM to GND will allow you the largest output voltage range.  On the INA851 connect the CLAMP+ and CLAMP- to the supplies if these are not used.

    The CFB feedback capacitors at the oPGA855 and INA851 output stage are optional, and are set according to your signal bandwidth requirements. The feedback capacitor, CFB, is in parallel with the PGA855 output-stage 5-kΩ feedback resistors to implement additional noise filtering. The internal resistors have ±15 % absolute resistance variation, and this variation must be taken in to account when implementing noise filtering. 

    For example, CFB is set to 25 pF, it provides a typical f–3dB corner frequency of 1 MHz. The estimated minimum f–3dB corner frequency for this circuit is approximately 938 kHz when accounting for the feedback-resistor variation.

    Hence, set the CFB capacitor while accounting for your signal bandwidth requirement, allowing margin for the ±15 % absolute resistance variation, according to your signal bandwidth requirement.

    If you need us to to provide suggestions for the CFB components, please let us know the signal bandwidth requirement in the application, and provide a preliminary schematic defining the supplies, and specific gains you intend to use on the INA851 and PGA855.

    Thank you and Regards,

    Luis