Electrical – Reading 32 ADC channels

I would argue that the ADC has a 4th input in addition to the three cited by Fred: its clock. At least for some types of ADCs, jitter or phase noise on the clock can impact ADC measurements.

You say you have a 25 MHz oscillator but are running the ADC at 30 MHz, so you have some PLL involved in the generation of its clock. If that is not working well, its irregularity could be a source of conversion noise. Can you try changing the software configuration (even temporarily) to not use the PLL and just run off of a the input clock or divided down from it?

I believe some microcontrollers also have a mechanism for suspending most of the digital circuitry while taking an ADC reading in order to reduce noise. You might look into seeing if something like that is possible.

1. Although a passive solution is possible, often depending on your source impedance and ADC input impedance, it's usually a good idea to use an opamp for the level/gain shift. One option is an inverting circuit such as this:

   

   Simulation:

   

   The benefit is a very low output impedance to drive the ADC (many ADCs require a minimum signal impedance of <10kΩ)
   The input impedance is just R3 (133.2kΩ in this circuit) To prevent loading effects, you want your signal impedance to be less than 10kΩ or so. If it's higher, you can increase R3 and R2 to increase the input impedance (as long as the ratio remains the same, so does the gain)
   Some opamps don't like a gain less than 1, so you need to check the datasheet to make sure it will be stable in this configuration. Also a small capacitor across R2 is usually a good idea to control bandwidth appropriately.

   The readings will be inverted, but it's easy to correct for this in firmware.

   A quad opamp with a decent voltage reference IC and divider to provide the 735mV reference for all non-inverting inputs could be used. The opamp must be able to swing its output down to ground; some op-amps cannot do this, so check the datasheet.

2. For the I2C and MAX127 (which looks fine) option, you could use a level shifter IC to convert between the 3.3V and 5V levels needed. There are quite a few such ICs designed for use with I2C. Have a look on Farnell, or Mouser, Digikey, etc. The PCA9306 is one example.

3. SPI uses "standard" outputs which are driven high/low (as opposed to I2Cs open drain) You can also easily level convert this, since each line is only one direction. A couple of transistors can be used.