ADC is only positive in single ended mode!!!

[v8dave]

I have a design with an ADC using the MCP3428 and I have 2 of them. 1 is connected to the common ground of the system and the other is isolated with it's own DC-DC supply. The ADC is specified to work with -2.048V to +2.048 input.

The isolated one has the + and - connected to the external connector and can range from -2V to +2V but the other ADC has the + connected to the input and the - to the same GND as the VSS pin so is set for single ended input. This input is supposed to be -5 to +5 so I have a potential divider to drop the 5V input to within the range of the ADC's +-2.048V

The isolated input can measure the -2V to +2V input and I can read this from the output but the non-isolated ADC only seems to measure 0-5V. If I input -5 I get a reading of approx -1.17V from the ADC. From this it appears that this ADC only works with positive input voltages and not negative if we use the single ended input mode?

This appears to happen because of the diode protection on the input if the -Ve is connected to VSS. If floating and in differential mode it seems to work.

Would someone please confirm what I have found?

[tombi]

Datasheet says (top of page 5)

Absolute Maximum Ratings†
VDD...................................................................................7.0V
All inputs and outputs ............. ..........VSS –0.4V to VDD+0.4V

I think it can read voltages that are negative w.r.t channel X minus but the input must always be > VSS-0.4.

So if you wire Channel1 minus to vref then voltages between 0 and Vref will be negative (i.e. return negative numbers) and Vref-Vdd will be positive. But the voltage musn't go below VSS-0.4. Depending on your gain settings it will likely max out in less than that range however.

Tom

[v8dave]

Thanks Tom

The same datasheet shows above that, that the full scale differential input is +-2.048/PGA

The isolated input doesn't have the the -Ve input connected to the supply ground so this one can actually measure the -2V to +2V input from the sig gen. Thankfully this is the one I needed pos a neg inputs on and can live with the 0-5V input.

It tripped me up as I have used other ADC's where I can do this but like you mention, the -REF is connected to a negative reference. The MCP3428 does not have any ref pins, it's all internal.