Other Parts Discussed in Thread: DDC232, DDC264
Hey!
We tied a current source to two channels in order to extend the input current range. It may not be much, but I could double the range if the channels shared the input current well. If I did my math right, Range 7 (350pC) at minimum continuous integration time (400uS at low power) and nominal Vref (4.096V) gives a max channel current of 3.6uA. So Imax of 7.2uA if they shared.
We used channels 1 and 5 which are adjacent on the package, but use different A/Ds internally. We used a Source Meter to inject DC current and tried fairly hard over several cable iterations to get leakage & noise controlled. The DDC118 is on a board of our own design, following TI guidelines reasonably well.
The did share, but noise went way up. Sharing gave two channels within 5% of each other, except at very low currents where sharing was uneven. For an example of noise, at 50nA injected we got:
CH Avg over 50 samples Std Dev
1 125 kADU 261 ADU
1+5 150 kADU 4580 ADU
My question is: Why did noise go up so much when sharing inputs?
The noise would ruin the scheme. The data sheet says all sampling is simultaneous, so we didn't expect switching noise to be a problem.
Thanks in advance!
Paul