I need to read a 0-6V buffered analog signal from a sensor with the ATXMega256A3BU ADC. With an external voltage reference I can measure voltage level up to about 3V. I'm going to realize a voltage divider but I need some help in finding the correct equivalent resistance of the voltage divider ( R1||R2 ) that is the voltage divider output inpedence. Atmel says that in the worst case the ADC input resistance Rchannel is 4.5k and the S&H capacitor Csample is 5 pF.
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The ADC characteristics can be found in the datasheet
There is a specific application note from Atmel for Xmega ADC Using the Atmel AVR XMEGA ADC.
If you refer to section 1.8 there is an explanation of how the output impedance of the circuit that feeds the ADC affects the conversion speed:
The lower the sampling frequency the higher output impedance you can use for the divider. If you want to use a universal value that can work for a wide range of sampling frequencies then you can go for about 1k or so.
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\$\begingroup\$ Thanks, I need to observe a slow signal so is not a problem to have a low sample rate. I don't know if there are critical values for the input impedance of the ADC. On the application note i read that an high impedance input can afflict the measure because of the high time of charge. With a low impedance we have a higher power consumption. But what values are considered high or low? \$\endgroup\$– MaicolCommented Feb 6, 2014 at 15:35
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\$\begingroup\$ And.. You suggested me a voltage divider with two 1k resistors or a voltage divider with R1||R2 of about 1k? The source impedance is R1||R2. \$\endgroup\$– MaicolCommented Feb 6, 2014 at 15:38
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\$\begingroup\$ @Maicol The x axis of the graph is the output impedance of the divider and as you can see for about 1k (1E+3) you can use up to 2000ksamples and get correct results (proper charge of the S&H capacitor) but if you don't need that high frequency you can go with much higher impedance. If you set the impedance to 100k then you can go up to 100ksamples. \$\endgroup\$– alexan_eCommented Feb 6, 2014 at 15:51
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\$\begingroup\$ can you confirm me that in a voltage divider the output impedance is R1||R2? Thanks for your patience! \$\endgroup\$– MaicolCommented Feb 6, 2014 at 15:54
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\$\begingroup\$ @Maicol Yes, the impedance is R1||R2 \$\endgroup\$– alexan_eCommented Feb 6, 2014 at 16:44