Re: adc reading errors

DAXU@xxxxxxxxxxx writes:

Hello! John,
Thanks for your reply.

The amp is the instrument amplifier INA128. And I just put the +1.6V
signal to its in+, and the in- connected to the analog V-. The Ref pin
of INA128 is connected to the +2.5V after a Opamp. The Vout of the
INA128 goes to +4.1V, which I measured via a multimeter to confirm it
too. Gain=1. I did put 10Ohm and 1nF low pass filter for the ADC
input signal.

Lots of variations I see, not just 1-3 LSB variations, even the MSB
too. not sure if the uP read the code correctly or because of the
circuit noise etc.

You could be reading the serial interface incorrectly, for example
reading each bit on the wrong edge of the clock so that the data is
changing at that instant. SPI interfaces have a "clock polarity" and a
"clock phase" setting; both these must match the ADC.

Look at the serial output on a scope and see if you still see the MSB
transitions then. I bet you don't. But if you do, remove the amplifier
and try shorting the ADC inputs to ground. Check you have the correct
supplies and reference voltage (you could connect the reference to the
positive supply if unsure).

What do you mean by "The Vref is connected to the Opamp output." ?
--- The Vref of the ADC need a low impedance.

They are plastic breadboards. What should I do? Make a PCB instead of
working on the breadboards?

I would use the "dead bug" prototyping method on plain copper clad PCB
material. (Or make a real PCB). You may also be able to buy an
evaluation board for the chip from the manufacturer, use it as a
starting point.

Would the noise make such big difference on every bits?

No.

--

John Devereux
.

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