I'm going to use my Arduino UNO with an external atleast 13-bit adc,Can you suggest some?
Farnell/Newark/Element14 have a very good parametric search for finding things like this. Take a look at the Microchip MCP3301 http://uk.farnell.com/microchip/mcp3301-ci-p/ic-13bit-adc-1ch-dip8-3301/dp/1332099 which looks ideal.