Voltage divider(as sensor) interfaced with DSPIC30F4011

[P.Copper]

Hi y'all,

I have build 3 voltage divider circuits for sensing DC voltage, the output of this voltage dividers will then be fed to 3 analog channels of the dspic30f. the problem is once i interface the two circuits(voltage divider and dspic30f) the output voltage of 2 of the voltage dividers drops heavily. at first i thought it was a ground problem but i have a common ground for both circuits.


the adc of the dspic works fine without the voltage divider circuit I've checked it as well.


can anyone who might have came across this situation help. thanks

 

[FvM]

Divider impedance too high?

 

[P.Copper]

can you please explain, the thing i'm using power resistors with rating of 5W. do you think that might be the problem?

 

[FvM]

According to datasheet, ADC source resistance (= voltage divider output resistance) can be up to 5k. Acquisition time must be however increased when using a source resistance in this range.

 

[P.Copper]

if that's the case, that might be why such a huge voltage drop b'cos my voltage divider output resistance was calculated to be 22k.

 

[FvM]

If you can't reduce the voltage divider output resistance, e.g. due to power disspation considerations, you should place a rail-to-rail operational amplifier between voltage divider and ADC input for best performance. Or at least try to increase tacq.

 

[P.Copper]

Thanks for the feedback! let me redo the design and will let you know of the outcome.

 

[P.Copper]

Thanks FvM, the rail-to-rail op-amps solved the problem.