A question about active filters

Hi all,

I need to use the on chip 10-bit ADC of a microcontroller. In order to get real 10 bit accuracy i need to filter the signal before applying to the ADC input. The question is:

Can an active filter remove/reduce hf noise caused by the circuit, while the supply voltage/ground of the filter circuit is noisy?

The circuit is on a breadboard. I used bypass caps where necessary and paid attention to current return paths but couldn't be able to reduce the noise level of the supply/ground below 1mV. I guess that i need to get rid of the supply/ground noise in order to make the filter do its job. What can i do to reduce the noise and make the filter function properly?


Thanks in advance

Hi,
The active filters placed at the input path reduces mainly the noise in the input signal only. The noise in the power supply or ground lines are reduced using bypass capacitors, a high value electrolytic capacitor (for low frequencies) in parallel with a low value ceramic capacitor ( for high frequencies) placed as close to the supply and ground pins as possible.

One method of reducing the ground noise is to reduce the length of ground tracks and to separate analog grounds from digital ground and finally connect them at one point. When you measure the noise in supply, you should check it across the supply and ground pins of the amplifier as this is what the amplifer is exposed to. Again the amplifier design uses generally differential input gain stages to improve the rejection of power supply noise. By using a good differential input amplifier (generally known as instrumentatin amplifiers), with passive RC filters at the input, it should not be difficult to get 10 bit accurcy.
Regards,
Laktronics

Hello,

apart from what's said regarding optimal operation of on-chip ADC (it's important), it may be difficult "to get real 10 bit accuracy" without filtering the output, at least by averaging several samples. Also DNL and INL specs may limit the actual accuracy to 8.5 or 9 bit. You can see from the datasheet. However, without the measures suggested by laktroniks, you won't get the specified accuracy.

Regards,
Frank