The anti-alias boards in the QIL AA chassis have been replaced with newer ones I found in the EE shop (serial numbers S1200217, S1200274, S1200275, S1200277).

The new boards (D070081-v4) have input buffers and a reasonably high input impedance (20k), unlike the old boards (D070081-v1).  However, according to the DCC revision notes, they may suffer from some excess low frequency noise caused by LT1492 opamps.  If it becomes a problem for us, we can replace those opamps.

The low input impedance of the original boards explains the anomalous ADC/DAC loopback measurement Jon made several months ago.  It should now be close to 0.5 ADC ct per DAC ct.  I have checked the DC gain for the first few channels, but have not exhaustively tested the new boards.  (Perhaps Jon has a script to automate this?)