Using ADC to monitor supply voltage down to 1.8 Volts

[AMurp.1]

Hi,

Is it possible to use the ADC to monitor the battery supply voltage down to 1.8V just before the micro stops working. 

Due to the reference (2.048) I can only correctly read down to 2.4V

Thanks am

[TDK · Accepted solution]

Actually, even your readings above 2.0V aren't correct.

VDDA: 3.3V, VREFINT: 1393 --> VDDA = 1.21V * 4095/1393 = 3.56V

VDDA: 3.0V, VREFINT: 1512 --> VDDA = 1.21V * 4095/1512 = 3.28V

VDDA: 2.5V, VREFINT: 1900 --> VDDA = 1.21V * 4095/1900 = 2.61V

Also, your VBAT measurements should always be 4095/4 if VDDA=VBAT=VDD, which they aren't.

Is your sample time sufficient? Make it the longest possible.

[AMurp.1]

Ahh .. Now we are getting there ... give me 5 mins to note the results

[AMurp.1]

Hi TDK,

Changed sample time from ADC_SAMPLETIME_2CYCLES_5 to ADC_SAMPLETIME_640CYCLES_5

With VREFBUF set to external voltage ref and final calculation vdda = (1.21 * VBAT * 3) / VREFINT

Drum roll ......

VDDA: 3.3V, VREFINT: 1443, VBAT: 1309, vdda: 3.290

VDDA: 3.0V, VREFINT: 1592, VBAT: 1309, vdda: 2.986

VDDA: 2.5V, VREFINT: 1924, VBAT: 1310, vdda: 2.472

VDDA: 2.0V, VREFINT: 2424, VBAT: 1313, vdda: 1.964

VDDA: 1.9V, VREFINT: 2559, VBAT: 1314, vdda: 1.861

VDDA: 1.8V, VREFINT: 2704, VBAT: 1314, vdda: 1.763

VDDA: 1.7V, VREFINT: 2867, VBAT: 1314, vdda: 1.664

B.T.W It also seems to work in VREF BUF OFF mode, just the current goes to 110mA !!

This look really good for our application, I will figure out the minimum sample time and then increase it by about 50%

TDK - Excellent job, I was going to give this up much earlier, I really hope I can return the help someday...

If you know how to mark Answered - please do it.

Just gone midnight in the UK, sleep well, stay safe

Alan..

[TDK]

Glad it worked.
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