Electronic – LC filter before analog pin of microcontroller

adcfiltermicrocontroller

I am measuring analog DC voltage at the analog pin of dspic33 series microcontroller. The source of the signal is AC. So, I rectified it with diodes and amplify it with op-amps. In order to reduce the ripple I am using RC filter as first phase filtering and LC filter as second phase filtering.

Single RC filter cannot satisfy my need that it outputs a voltage with 100mv of ripple content in a signal which is going to be measured by ADC. Also, I cannot increase the capacitance since it violates my timing constraints of the requirement due to high RC time constant. So, I decided to go for LC filter as second phase filtering to reduce the ripple content of the DC signal.

My doubt is, whether using LC filter near the analog pin of microcontroller is technically ok or not? Is there any interference come with LC filter when the setup undergoes environmental tests and noise tests?

RC filter has 1k ohm resistance and 1uF capacitance. LC filter has 470 mH inductance and 1 uF capacitance.

Help please! enter image description here

Edit : I have added filter portion of the schematic. As per comments I need to clarify my points.

Input to the circuit is AC (0 – 200V)
The output of the circuit is DC (0 – 3.3V)

Timing constraint means, I need to detect the voltage within certain milli seconds to process and do other actions based on this value within a second.

Best Answer

The input impedance of the micro is very high .No wonder there is a pronounced resonant peak as Mattman has shown .Damp the LC filter to get low Q and the nasty peak will go .Your coil should be screened like in a can so it wont pick up noise .Otherwise your proposed filter is valid .

Related Solutions

Signal – Frequency Shift in Microcontroller Analog Signal

In principle, DDS should be easy to achieve in software - fractional accumulator, then lookup sine or cos for each tone you want to generate, then sum. On a PIC, that's going to be a tight loop, though...

I am assuming a 256 element LUT containing a complete cycle of a sine wave.

Each sample period , for each frequency, we add a fraction to the position of its pointer in the LUT, and take the sample at the (integer) part of the position. To illustrate how simple this can be, I played with a spreadsheet:

Fs  =   80000   
Fout=   16400   16700   17000
Fout/Fs 0.205   0.20875 0.2125
*256    52.48   53.44   54.4
integer 52      53      54
rem     0.48    0.44    0.4
*256    122.88  112.64  102.4
rounded 123     113     102
actual  16400.146   16700.439   16999.511

The actual DDS operation translates into pseudo-assembler (not PIC!) as

add acc1L,123
adc acc1H,52
lookup LUT,acc1h
mov sum, lookup result

add acc2L,113
adc acc2H,53
lookup LUT,acc2h
add sum, lookup result

add acc3L,102
adc acc3H,54
lookup LUT,acc3h
add sum, lookup result

out DAC,sum
wait for next sample period

which will expand considerably from what I remember of PIC assembly language, but is starting to look feasible.

Remember that the values in the lookup table must be scaled down so that the addition result in "sum" will not overflow.

Electronic – RC filter infront of Analog port pins in pic microcontroller

A lot of MCUs take a pulse of current to "charge" the internal sample and hold circuitry (part of the integrated ADC). If the series resistance of the signal source is too high, it won't charge the internal cap sufficiently and there will be an error in the conversion.

Applying a capacitor from the pin to ground means that there is a much greater chance of this working because the external cap can supply the current needed. See this SE post (relating to PIC input impedance) and check out the answer.

enter image description here

Of course this makes a low pass filter that is also useful for anti-aliasing.

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