psoc 5lp provide voltage and adc component question

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user_3716231
Level 4
Level 4
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hello ,

i have two situations and some questions show below.

a)i have a project need to measure AC voltage and current.both of voltage and current sensors Vout have dc offset which depend on how much power source i give them.

i connect CY8CKIT-059 to my computer through usb port and i measure its Vdd,it shows 4.6V.so i provide this Vdd to my sensor then sensor's dc offset goes to 2.3V at AC 0V.

but when i disconnect usb port and connect it again,Vdd will be changed to 4.52V sometimes will be 4.38 or 4.44V.it will cause dc offset change to 2.25V or 2.19V at AC 0V.

1.could someone tell me why Vdd is not stable?

b)i convert adc value to mV,the value should be 2300V at 0V.unfortunately there are some noise to cause value unstable(value would be 2324,2285,2292...).

i have gone through datasheet and don't understand description that i marked.

1.do that mean if i want to filter the noise,i can connect sensor Vout to P0[2] or P0[4]?

2.if i choose Internal Vref,bypassed mode,i can't choose those as analog high-z pins.is that right?

3.what if i set Internal Vref,bypassed mode and higher conversion rate,will the value be more stable than Internal Vref mode?

all i want to do is filter noise and eliminate error as much as i can.but i have no idea about it.

need some suggestions,please.

sean

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Len_CONSULTRON
Level 9
Level 9
Beta tester 500 solutions authored 1000 replies posted

sean,

In general, I never try to use VDD (actually VDDA) as a Vref to ADCs.   This is because if I use the USB power, it can vary too widely.  The differences you are seeing with VDD measurements can be for multiple reasons.  The USB 5V feed can vary from the host (4.75V to 5.5V).  Then there is a 0.1V to 0.3V through the schottky diode (D1) on the KitProg board.  If the load on the CY8CKIT-059 varies, this will cause the schottky diode to drop more voltage and any series resistance in the VDD circuit all the way from the host to see more voltage drop.

I believe the safest practice is to use the bandgap voltage source (1.024V) as a Vref to the ADC.  This voltage source is mostly immune to VDD fluctuations.  Even if VDD drops to 1.8V, the bandgap should be stable.  Additionally, the bandgap source has a tigher tolerance for voltage regulation.  Usually better than most external regulators.

Len

Len
"Engineering is an Art. The Art of Compromise."

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