I've had a similar issue in the past, but it turned out to be external=20
(to the PIC)... ie: due to my calibration resistors on each channel=20
being close to each other.  Cleaning the flux off from and around those=20
resistors pretty much resolved the issue.

Cheers,
-Neil.


On 9/3/2015 5:06 PM, Bob Blick wrote:
> 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.
>>
>> 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
>> sample has some kind of "leakage" to the value of the next. By increasin=
g
>> 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, wit=
h
>> an external reference of 5V:
>> 0b1xxx01xx
>>
>> Any help would be appreciated.

--=20
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