Electronics > Projects, Designs, and Technical Stuff
ADC input filter
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coppice:

--- Quote from: wraper on February 10, 2019, 01:40:43 pm ---
--- Quote from: coppice on February 10, 2019, 09:36:28 am ---
--- Quote from: wraper on February 10, 2019, 02:04:35 am ---
--- Quote from: coppice on February 10, 2019, 01:35:30 am ---So, I don't know why they recommend a 10 times ratio in the capacitances. A 1000 times ratio is usually a better solution.

--- End quote ---
They recommended at least 10 times. For example, I use 10x ratio in one of my circuits. It's because there may be a HUGE environmental noise starting at few a kHz, yet I need at least 500Hz of bandwidth, and input signal full range is about 10mV. Something like 100pf for bypass is simply not sufficient. And I cannot go too high for cap between inputs as well. So ratio ended up somewhat on low side.

--- End quote ---
Why would that make 100pF insufficient? Its only there to suppress RF, not anything close to the band. The big differential capacitor does that.

--- End quote ---
With 100pF it starts to do anything way above the frequency of CM noise in-amp can cope with, not to say sufficiently suppress it. The typical scenario is that body of a load cell may be electrically connected to body of 1kW PWM driven BLDC motor (from crap driver without output filter or anything). Most likely nothing would be grounded.

--- End quote ---
You haven't answered my question. Clue: most of the switched cap input sigma delta converters have terrific CMRR figures, and all you need is to do is suppress RF.
wraper:

--- Quote from: coppice on February 10, 2019, 01:47:52 pm ---
--- Quote from: wraper on February 10, 2019, 01:40:43 pm ---
--- Quote from: coppice on February 10, 2019, 09:36:28 am ---
--- Quote from: wraper on February 10, 2019, 02:04:35 am ---
--- Quote from: coppice on February 10, 2019, 01:35:30 am ---So, I don't know why they recommend a 10 times ratio in the capacitances. A 1000 times ratio is usually a better solution.

--- End quote ---
They recommended at least 10 times. For example, I use 10x ratio in one of my circuits. It's because there may be a HUGE environmental noise starting at few a kHz, yet I need at least 500Hz of bandwidth, and input signal full range is about 10mV. Something like 100pf for bypass is simply not sufficient. And I cannot go too high for cap between inputs as well. So ratio ended up somewhat on low side.

--- End quote ---
Why would that make 100pF insufficient? Its only there to suppress RF, not anything close to the band. The big differential capacitor does that.

--- End quote ---
With 100pF it starts to do anything way above the frequency of CM noise in-amp can cope with, not to say sufficiently suppress it. The typical scenario is that body of a load cell may be electrically connected to body of 1kW PWM driven BLDC motor (from crap driver without output filter or anything). Most likely nothing would be grounded.

--- End quote ---
You haven't answered my question. Clue: most of the switched cap input sigma delta converters have terrific CMRR figures, and all you need is to do is suppress RF.

--- End quote ---
That was not ADC connected directly. And with ADC it also depends, especially if it has PGA. You may need largish bypass caps or don't need them at all.
coppice:

--- Quote from: wraper on February 10, 2019, 02:07:23 pm ---
--- Quote from: coppice on February 10, 2019, 01:47:52 pm ---
--- Quote from: wraper on February 10, 2019, 01:40:43 pm ---
--- Quote from: coppice on February 10, 2019, 09:36:28 am ---
--- Quote from: wraper on February 10, 2019, 02:04:35 am ---
--- Quote from: coppice on February 10, 2019, 01:35:30 am ---So, I don't know why they recommend a 10 times ratio in the capacitances. A 1000 times ratio is usually a better solution.

--- End quote ---
They recommended at least 10 times. For example, I use 10x ratio in one of my circuits. It's because there may be a HUGE environmental noise starting at few a kHz, yet I need at least 500Hz of bandwidth, and input signal full range is about 10mV. Something like 100pf for bypass is simply not sufficient. And I cannot go too high for cap between inputs as well. So ratio ended up somewhat on low side.

--- End quote ---
Why would that make 100pF insufficient? Its only there to suppress RF, not anything close to the band. The big differential capacitor does that.

--- End quote ---
With 100pF it starts to do anything way above the frequency of CM noise in-amp can cope with, not to say sufficiently suppress it. The typical scenario is that body of a load cell may be electrically connected to body of 1kW PWM driven BLDC motor (from crap driver without output filter or anything). Most likely nothing would be grounded.

--- End quote ---
You haven't answered my question. Clue: most of the switched cap input sigma delta converters have terrific CMRR figures, and all you need is to do is suppress RF.

--- End quote ---
That was not ADC connected directly. And with ADC it also depends, especially if it has PGA. You may need largish bypass caps or don't need them at all.

--- End quote ---
The information in this thread is predicated on the ADC being directly connected. Any other kind of input needs different treatment. The majority of sigma delta converters with switched gains have no real PGA, even though the block diagram frequently shows a PGA to illustrate the functionality. If its a discrete time converter, with a switched capacitor inputs, they usually switch the size of the sampling capacitors to alter the gain.
OM222O:
I have decided to use a 100nF Cdiff and a 10nF X2Y cap for common mode, with resistor value of 1k as it limits the cutoff frequency to about 8KHz which should be sufficient for the sigma delta ADC to take care of the rest. One thing that was lost/ignored in the conversation was: what difference will mismatched resistors make?

If possible I would rather use a resistor array with 1% tolerance (no matching guaranteed) to reduce parts and save space on the PCB.Again, I'm not sure the bias currents are matched that closely anyways?
David Hess:

--- Quote from: coppice on February 10, 2019, 01:35:30 am ---So, I don't know why they recommend a 10 times ratio in the capacitances. A 1000 times ratio is usually a better solution.
--- End quote ---

They are assuming low common mode variation which depends on the application.

My experience is that the two common mode suppression capacitors should be as small as possible and only used to remove radio frequency interference and should *not* be relied on for suppressing AC common mode signals within the bandwidth of the ADC because mismatch will convert common mode AC signals to differential signals corrupting any measurement.  If common mode signals within the bandwidth of the ADC need to be suppressed, then an integrator from the ADC input to a previous stage should be used to remove the common mode signal.
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