Incorrect values

[ShuffleBit]

Arduino 1.8.13, all libraries and boards up to date.

The hardware, ESP8266 (Wemos D1 Mini R1) running at 80MHz, hardware SPI to MCP3208, 3.415V VDD/VREF. There is a 470uF cap between the VDD/VREF, and ground, plus another 100pF cap in parallel with that. VDD is flat/quiet on an oscilliscope.

Constructor is MCP3208(3415, 0) // VREF, CS_PIN
SPI clock is set to 1.6MHz, MODE0, per examples.

Reading MCP3208::Channel::SINGLE_0, the value 4095 isn't returned by read() until pin 1 is at 4.000 VDC: above the VREF. Shorting pin 1 to pin 16 returns 3910.

I expected a return of 4095 when pin 1 reaches 3.415 VDC, same as the VREF.

The adc_sample.ino example, modified only for ADC_VREF and CS pin, behave the same way my own sketch behaves: 4095 is only returned by read after pin 1 reaches 4.000 V.

Shorting pin 1 to pin 15 (VREF), MCP3208::read() returns only 3915.

It looks like a genuine MCP3208, and it came from Mouser.

4.000V is suspiciously ... round but could just be chance. I haven't tried changing SPI clock frequency.

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All measurements are w/ Fluke 179, Rigol scope, and BK bench supply, though VDD is off the 3v3 pin of the ESP8266 (hence the additional filter caps).