Hi,
Setup:
- Format SPI Mode
- High Performance Mode
- OSR = 32
- Filter Wideband Band 1
- Vdd, AVdd = 3.3 V, AGND placed to common ground plane
- fsmode (1k gnd), format (1k gnd), reset (1k pull-up)
we encounter a strange behavior at AC input (or sudden dV/dt), in which the ADS suddenly stops converting and DRDY as observed appears to become High Z as shown in figure below. Reseting the ADC resumes the operation.
- yellow: DRDY
- blue spi
- orange: spi CS
when DRDY yellow drops below some threshold then master things it's triggering and it's restarting the spi reading state machine ... that's why you see false triggering >0 s on the x-axis. The interesting point of observation is at time cca -220 us.
Common mode voltage of the inputs is 1.6 V, signal is stable and smooth driven by ADA4945, 10R resistors at the output driving 4.7 nF across the + and - , and 470 pF from + to gnd, and - to gnd; the gain error due to 10R resistors is still to be evaluated and the low impedance input (5k) in HR mode, but this is now not the issue of this post (probably).
The problem can be stimulated already at relatively small signal input, i.e. 100 mV AC. For DC and slow changes it works without issues.
We have checked all the pins for any transients but at least we were unable to catch any. The issue is immediately reproducible.
IF we remove this 10R resistors ADS works perfectly. The noise in this scope plot is due to the bad connection of the probes; however this noise is same in working (with AIN pins floating) and non-working (AIN pins connected to ADA4945) scenarios.
Any idea what to check, that makes (disables) ADS from converting and high-Z-ing the DRDY ?
We have also replaced the part in the case the first was damaged but it didn't fix the issue.
Kind regards,
Uros
