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RC filter before ADC

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frankqt

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I have a circuit where I measure a fairly fast signal (1Mhz in frequency) using ADC. The signal is fed to the ADC using an Opamp, however there is no RC filter following the Opamp to the ADC, I didn't know about the need for such charge capacitor holder. Currently, Opamp output is directly connected to the ADC pin.

Anyway, now my system works, what are the down sides of not having this RC circuit there? If I add this RC circuit, what would be the benefits.
 

Start by thinking about what the capacitor will be doing to different frequencies on your signal line.

Recall that the impedance of a capacitor is Zc = 1/(jωC); as frequency increases (ω = 2*pi*freq), ω gets larger, and Zc gets smaller. A capacitor will act like an "AC short" at higher frequencies. If the capacitor is connected from your signal line to ground, then higher frequencies will get "short circuited" to ground. If you have a short circuit at a particular frequency, then it won't go into the following stage.

Using that basic concept, you can see that a shunt RC is a simple low-pass filter; it will help keep high frequency signals out of your ADC's input. Usually high frequency content will appear as noise and spikes/spurs in your low frequency desired signal, so filtering them off is usually desired.
 

Start by thinking about what the capacitor will be doing to different frequencies on your signal line.

Recall that the impedance of a capacitor is Zc = 1/(jωC); as frequency increases (ω = 2*pi*freq), ω gets larger, and Zc gets smaller. A capacitor will act like an "AC short" at higher frequencies. If the capacitor is connected from your signal line to ground, then higher frequencies will get "short circuited" to ground. If you have a short circuit at a particular frequency, then it won't go into the following stage.

Using that basic concept, you can see that a shunt RC is a simple low-pass filter; it will help keep high frequency signals out of your ADC's input. Usually high frequency content will appear as noise and spikes/spurs in your low frequency desired signal, so filtering them off is usually desired.

Thanks.. I actually have an active filter before the ADC, so I am not too worried about the filter aspect. People talk about smoothing the signal from Opamp to ADC and essentially to avoid Opamp to become unstable. I am more interested in the charge holding feature of the RC capacitor, rather than the filtering capability.
 

The 'charge holding' is what makes it into a filter. Normally, before an ADC you filter out frequencies above the sampling rate to prevent aliasing effects but in your instance you are actually taking several samples within each 1MHz cycle so you probably don't want RC components skewing your waveform or it's amplitude. Your active filter should have a cut off frequency no lower than 2MHz and ideally much higher. Don't confuse external capacitors with the internal one used if you have a sample and hold circuit inside the ADC, they have completely different jobs to do. Also, you probably don't want to capacitively load the op-amp as it may become unstable.


Brian.
 

Why my filter should cut off frequency must be above 2MHz? Currently I set to 1.2MHz.
 

If your filter has flat reponse to 1.2MHz it should be OK. Most filters are characterized by their response being some number of dB down at the cut-off frequency. For example, if your -3dB point is 1.2MHz you will probably still have some drop in level at 1MHz. As you are talking about RC filters, I wouldn't expect a sharp knee in the response so you would need to change the cut-off point further away from the frequency you are measuring. If the frequency will always be 1MHz it may be beneficial to use LC filters but not immediately at the input of the ADC.

Brian.
 

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