PIC18LF27K42 ADC measurement issues

ddyess · Joined: 18 Jun 2019 Posts: 5

I have an issue I've been fighting for a few days. If anyone can help out, would appreciate it.

I have 2 custom boards that have 2 analog inputs (AN1, AN2), using PIC in subject line. Both inputs are buffered by op amps.

On 1 board, if I short both inputs, I read an ADC value of 34. If I apply VDD, I read ADC value of 3874.
On 2nd board, if I short both inputs, I read ADC value of 129. If I apply VDD, I read ADC value of 3969.

No matter how many times I read these inputs, I read same values. If you notice the diff between the boards shorted is 95 and at VDD is 95. They have the same dynamic range of 3840 (0x0F00) ADC counts.

This is how I have the adc setup:

alan · Joined: 12 Nov 2012 Posts: 357 Location: South Africa

Op amp can typically not go rail to rail.

If you work out the input voltage to ADC you get 0.1 and 0.027.
Typical values on the output.

ddyess · Joined: 18 Jun 2019 Posts: 5

Thanks Alan.

I'm using a MCP6441 and I've confirmed that when I short my input, the PIC sees 0.001V and it is very near VDD when I apply VDD to the input.

temtronic · Posted: Tue Jun 18, 2019 10:51 am

while I don't use that PIC...
setup_adc_ports(sAN0|sAN1|sAN2,VSS_VDD);
setup_adc(ADC_LEGACY_MODE|ADC_CLOCK_INTERNAL);

1) is ADC_CLOCK_INTERNAL valid for over 1MHz ? Most( all older..) PICs that's only used during sleep mode.

2)It appears Vref for the ADC is VDD ? If so, you may have a problem between PICs reading the same bit for volts....

ddyess · Joined: 18 Jun 2019 Posts: 5

Temtronic,

The CPU is using the internal oscillator. I've set to different frequencies and also configured the ADC clock to run of FOSC and set for valid ranges per datasheet. I do have a precision reference tied to Vref pin and tried using VSS_VREF. I get the same readings for both the shorted and VDD case.

I find it odd that I can't measure near 0V when I have 0.001V on the input pin of the PIC. I also find it odd that the dynamic range for both cases is always 3840 (0xF00).

ddyess · Joined: 18 Jun 2019 Posts: 5

I am reading expected values now. 0 when input is grounded and about 4095 when VDD applied to input.

I changed...