spikes in low price adc mcp3918

[hadi.esfandi]

Hello friends
I am working on designing analog front-end of a circuit include high resolution ADC. In order to reduce the cost, we intend to use adc low price (MCP3918 Microchip). for the first step I should decrease the amplitude of the signal to be in range of the adc and I do this work by resistor deviding. Is it appropriate approach? I think it increases the noise level of the signal because of resistor noise(4ktr) and decrease the SNR of the signal.
But the main question is about spikes which are appeared in the signal. If I set OSR of the adc to 32 i see many spikes( about 8 in one minutes) but If I set to 4096 the condition will be better and the number of the spikes will be getting low(about 1 in 10 minutes).however when I see digital signals in matlab the spikes are just one sample and this issue reduce the possibility of analog spikes before adc and I think these spikes are formed in adc. The attached file is the schematic of the adc section of our circuit and spikes which appear in digital signal.
Is there anybody who know about origin of these spikes and how to eliminate them without digital approaches?

 

[KlausST]

Hi,

Reading the datasheet helps.

I use a similar MCP and have to say it works better than the datasheet specificatons.

Your picture is not clear if it shows analog input siganls or digital output signals.

Issues:
* Analog and digital supply need to be separated.
* you need an anti-aliasing filter at the signal inputs. Use a capacitor at the signal inputs. Short wires directely to AGND.
* I don´t see your PCB layout, but it has big influence on performance.
* even the length/layout of the interface lines has influence on the analog performance. Avoid overshot and undershot. (even the timing has influence, although very small)

****
Regarding resistor divider.
Regarding noise there is no better solution than to use a resistor divider. It will add only a small amount of noise.
BUT: a resistor divider makes the node high impedance and therfore more sensitive to external noise. Then an amplifier may improve this situation.

Klaus