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PIC16F1947...Input impedance of ADC is 82k instead of the required 10k

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treez

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Hello,
Page 398 of PIC16F1947 datasheet says that the ADC needs an input impedance of 10k. We have 82k and we need 82k there. We dont have room for a buffer opamp. We take ADC readings every 500usecs. Does this mean our ADC readings will be different to what they would be if we had 10k of input impedance.?

(also, we can’t put 10nF on the ADC pin as the signal would be affected too much. We only have 100pF on the 82k resistor to gnd at the ADC input, which incidentally is the resistor at the output of a current source output light sensor [SFH5711])

PIC16F1947 datasheet
https://ww1.microchip.com/downloads/en/DeviceDoc/40001414E.pdf
 

Hi,

Does this mean our ADC readings will be different to what they would be if we had 10k of input impedance.?
Click to expand...
I didn't read the datasheet. This is your job.
Whether the increased input impedance will affect the ADC value depends on the sampling frequency setup..

500us means 2kHz
82kOhms
...How did you come to 100pF? Calculated? Did you follow nyquist rule, or do you do undersampling technique?

Klaus
 
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82k is O.K. if you can afford 3 bit accuracy loss. Otherwise you should consider to follow the datasheet specifications.
 

The accuracy of the ADC will not suffer, it actually has a very high DC resistance. What matters is how fast the signal on the internal sample & hold circuit can follow changes in input voltage. As the impedance feeding the ADC pin increases, the ability to follow the signal slows down. To achieve the rated speed there is a limit of 10K but if the output of the SFH5711 only changes slowly (which I would guess is the case) you should have no problems.

Brian.
 
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Thanks, the output of the sfh5711 is going up and down as our led lamps turn on and off with the mains half cycle.

So i wonder if the sfh5711, with 82k at its output, (thats 82k to goround) can charge the s&h capacitor quick enough for 500us sampling rate?

We need the sfh5711 output to be changing fast with the actual led light, since we do ambient light sensing at the mains zero crossing, where our leds are temporarily off and wont spoil the ambient light reading.
 

The accuracy of the ADC will not suffer, it actually has a very high DC resistance. What matters is how fast the signal on the internal sample & hold circuit can follow changes in input voltage. As the impedance feeding the ADC pin increases, the ability to follow the signal slows down.
Click to expand...

Setting the focus on "signal follow" probably overlooks the charge injection by the ADC capacitor. Increasing the source resistance without providing a parallel capacitor causes a static ADC error.

- - - Updated - - -

Thanks, the output of the sfh5711 is going up and down as our led lamps turn on and off with the mains half cycle.

So i wonder if the sfh5711, with 82k at its output, (thats 82k to goround) can charge the s&h capacitor quick enough for 500us sampling rate?
Click to expand...
It can't, that's the reason for the discussed specification.

With 10 ms measurement time, why can't you provide a filter capacitor? 10 nF results in 0.82 ms, may be still feasible. Probably you'll go for a moderate 1 to 5 ns filter capacitor, provide as much acquisition time as feasible, accept a certain error amount.

Time for actual tests.
 
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Thanks
The attached shows our situation by way of an ltspice sim.
The biggest threat is adc pin leakage current...and the datasheet makes no mention of what this might be...just 2uA of leakage current could seriously mess our readings up, as the sim shows
 

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  • ADC PIC16F18856.txt
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