Unstable, voltage reference of AD7682/AD7689

We made two boards for AD7682/AD7689. One of them work perfectly, one of them does not. The voltage reference for it is unstable from time to time. The reference does not export 4.096V any more, but randomly move from 0V to 4.9V.

We use the internal reference of AD7682 as a voltage reference for a DAC. The reference at REF pin is followed by AD4841-2 as a voltage follower. However, the voltage reference is unstable. When we solder out the AD4841, the adc reference works again.

And when we solder the AD4841 back, it continues to work. However, on one day it suddenly does not work again....And the cycle repeats twice in these two weeks...

Is it because the capacitive input for AD4841 is too large?(10uF) Or what could be the reason? What should I check?

The schematic is like following:

Top Replies

Parents
  • Hi,

    well seen, Harry, that the input voltage of the ADA4841 is beyond the limits.

    As 4.096 V is just marginal too high and the thread is not marked as answered, I want to suggest a few more tips

    that Richie could try:

    1. The 10 uF capacitor on the REF output is ok (AD7682 datasheet),
      but is it a ceramic type of high quality (X5R, X7R)?
    2. ADA4841: It is good practice not to leave unused inputs of an opamp floating.
      Please try to wire the 1st opamp of the ADA4841, as well with a reasonable circuitry.
      e.g: make a voltage divider for the 5 V, wire this so created 2.5 V to pin 3 and connect pin 1 & 2
      OR
      connect the REF voltage to both opamps of the ADA4841 (and wire both opamps as voltage follower)
    3. Load the opamp output OUT2 with a resistor in the range of 1 kOhm to 10 kOhm.
    4. Ferrite beads are quite popular nowadays, but they can induce ringing.
      Try to shortcircuit the ferrite bead of the opamps' +5 V supply.
    5. To check, if your 2 boards are really equal, swap the opamps.

    Good luck!

    Reinhard

Reply
  • Hi,

    well seen, Harry, that the input voltage of the ADA4841 is beyond the limits.

    As 4.096 V is just marginal too high and the thread is not marked as answered, I want to suggest a few more tips

    that Richie could try:

    1. The 10 uF capacitor on the REF output is ok (AD7682 datasheet),
      but is it a ceramic type of high quality (X5R, X7R)?
    2. ADA4841: It is good practice not to leave unused inputs of an opamp floating.
      Please try to wire the 1st opamp of the ADA4841, as well with a reasonable circuitry.
      e.g: make a voltage divider for the 5 V, wire this so created 2.5 V to pin 3 and connect pin 1 & 2
      OR
      connect the REF voltage to both opamps of the ADA4841 (and wire both opamps as voltage follower)
    3. Load the opamp output OUT2 with a resistor in the range of 1 kOhm to 10 kOhm.
    4. Ferrite beads are quite popular nowadays, but they can induce ringing.
      Try to shortcircuit the ferrite bead of the opamps' +5 V supply.
    5. To check, if your 2 boards are really equal, swap the opamps.

    Good luck!

    Reinhard

Children
No Data