Hi Mitch,

What you're seeing is normal. You have one A/D converter with a
multichannel analog switch ahead of it. The issue you are seeing is due
to the capacitive load the converter has. When you switch to another
input, the converter's capacitor still has the previous channel's
voltage on it.

You need to present a low impedance to each input channel. Since you're
only sampling once per second, just put a "large" capacitor (0.01 to 0.1
uF) to ground from each input (you'll need 8 capacitors).

That will solve most of the channel-to-channel problems by reducing the
AC impedance. You will still have input bias current (DC) errors, but
they will be small compared to what you've been seeing.

Best regards, Bob

On Thu, Sep 3, 2015, at 01:44 PM, Mitch wrote:
> I am fairly new to multi-channel ADC sampling.
>=20
> I have a ADS7828 and trying to sample all 8 channels - just measure all
> of
> them once a second.
>=20
> I found that if I sample only one channel, I get very little noise and
> the
> values look reasonable.
>=20
> But if I sample more than just 1 channel, it seems like the value of one
> sample 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.
>=20
> So what should be something like:
>=20
> 1024  0  0  0  0  0  0  0
> Is giving:
> 1024 1024 950 870 780 690 600 550
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> Sometime it's even showing a bigger rollover than that:
> 1024 1024 1024 1022 1020 1017 1015 1012
>=20
>=20
> 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?
>=20
> I'm sending Single-ended input mode, internal ref off, ADC alway on, with
> an external reference of 5V:
> 0b1xxx01xx
>=20
> Any help would be appreciated.

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