Pi interfaced with board with 5V ADC.

[lilzz]

2 ways to handle this

1)change the ADC Vref and VDD to 3.3V instead of 5V. But the channels going into the ch0-ch7 still 5V signals.

or
2) Dout of the ADC with resistor divider to scale to 3.3V compatible to interface with Pi.But Pi is 3.3V and it goes to Din of the ADC. Would the 3.3V signal going into the 5V ADC mess up the logic?

which way is better?

[FLYFISH TECHNOLOGIES]

Hi,

1)change the ADC Vref and VDD to 3.3V instead of 5V. But the channels going into the ch0-ch7 still 5V signals.

Check the ADC's datasheet if you can measure 5V signals when the ADC is supplied with 3.3V (most likely not - you might think here about voltage divider on each ch input).

2) Dout of the ADC with resistor divider to scale to 3.3V compatible to interface with Pi.But Pi is 3.3V and it goes to Din of the ADC. Would the 3.3V signal going into the 5V ADC mess up the logic?

Check the ADC's datasheet.
Most likely, you're out of specifications for high level voltages.

P.S. Datasheets contain valuable information related to applicable component. They're worth reading.


Best wishes, Ivan Zilic.

[mikronauts]

If you are using an SPI based ADC, (2) will work fine, I've been using such a setup for years (not Pi)

3v3 for /CS, CLK, and Din works

Just need to divide Dout to 3v3 (I tend to use 2k4/4k7 resistors)

2) Dout of the ADC with resistor divider to scale to 3.3V compatible to interface with Pi.But Pi is 3.3V and it goes to Din of the ADC. Would the 3.3V signal going into the 5V ADC mess up the logic?

which way is better?

[Richard-TX]

The PCF8591 (8 bit I2C dip 4xADC/DAC) has the following specs:

AIN max voltage is VDD + 0.5 volts
VDD can be either +3.3 or +5.0 volts.
The I2C lines have no internal pullups so the chip can be safely interfaced directly the the Rpi's I2c lines even when powered by +5 volts. While the I2C logic levels are outside of stated spec when VDD == 5 volts, it works.
AGND can be at most VDD - 0.8 volts

[pluggy]

3.3 volts into 5V input will work fine in most cases, a 5 V input will typically have a changeover (hi/lo) at about half way (~2.5v) and since 3.3v is well above 2.5v it will register as a high. Do not put 5v onto a 3.3v input as that will damage something. I use a voltage divider to reduce it, but there are other ways of doing it.

[plugwash]

3.3 volts into 5V input will work fine in most cases

This is mostly true but to clarify

3.3V into a "TTL compatible" 5V input is fine.
3.3V into a regular classic CMOS 5V input is generally slightly out of spec. It will usually work in practice but I wouldn't want to rely on it in anything more than a "quick hack on the bench".
3.3V into a classic CMOS schmitt trigger input generally will not work.

You especially have to watch out with 5V PICs as they have a mixture of TTL compatible inputs and classic CMOS schmitt trigger inputs.