Hi Mitch -

Do you have anything connected to the other channels or are they just float=
ing? Try connecting them to ground via 1k resistors.

Stephen


-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of=
 Mitch
Sent: Friday, 4 September 2015 6:44 AM
To: Microcontroller discussion list - Public.
Subject: [EE] ADS7828 Multi-channel Sample Problem

I am fairly new to multi-channel ADC sampling.

I have a ADS7828 and trying to sample all 8 channels - just measure all of =
them once a second.

I found that if I sample only one channel, I get very little noise and the =
values look reasonable.

But if I sample more than just 1 channel, it seems like the value of one sa=
mple has some kind of "leakage" to the value of the next. By increasing the=
 voltage on channel 1, I actually measure a voltage increase on channel 2. =
The "leak" occurs all the way down the chain.

So what should be something like:

1024  0  0  0  0  0  0  0
Is giving:
1024 1024 950 870 780 690 600 550

Sometime it's even showing a bigger rollover than that:
1024 1024 1024 1022 1020 1017 1015 1012


1 Hz from each channel every second is well below the ability of the chip, =
but the data sheet doesn't address anything like this. WTF am I doing wrong=
? Is it an error in my I2C communication? The actual wiring? Bad chip?

I'm sending Single-ended input mode, internal ref off, ADC alway on, with a=
n external reference of 5V:
0b1xxx01xx

Any help would be appreciated.
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