## Differential ADCs and negative signals

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The new AVR DA family has a 12 bit differential ADC. My application involves reading small AC (sine-wave) signals from a CT with a burden resistor across it with one leg connected to ground. With a single-ended ADC, the signal gives a reading in the positive half of the sine wave and reads as 0 during the negative half. Does having a differential ADC mean that it can read the negative part of the signal also, the part which is actually a negative voltage w.r.t ground?

-Sam

Last Edited: Wed. Aug 5, 2020 - 02:40 PM

No.

Instead of reading a single ended ADC input with respect to Ground, it will read the difference between two ADC input pins, but both pins must still be a (+) voltage.

On some of the chips the differential input also goes to an op-amp, so one can have gain inside the uC without adding an external op-amp.

JC

You didn't post a schematic, but perhaps:

Instead of connecting the one end to Ground, you should connect it to a "virtual ground" at Vcc/2.

Now the other end deviates a little bit above Vcc/2 and a little bit below Vcc/2, and you can feed that into the (single end, (+) voltage only) ADC.

The VG is most easily constructed with an op-amp voltage follower.

JC

DocJC wrote:

You didn't post a schematic, but perhaps:

Instead of connecting the one end to Ground, you should connect it to a "virtual ground" at Vcc/2.

Now the other end deviates a little bit above Vcc/2 and a little bit below Vcc/2, and you can feed that into the (single end, (+) voltage only) ADC.

The VG is most easily constructed with an op-amp voltage follower.

JC

This will only work if the source and the microcontroller are galvanic isolated.

You can use two resistors on each of the two differential inputs. Like a voltage divider, but with one side connected to your controllers supply so that the input voltage to the ADC is level shifted and always >0V.

The supply voltage doesn't have to be very stable, it does not affect the ADC result because of the differential inputs.

Unfortunately, the DA's do not have a gain stage. If the input voltages are very low this won't work, in that case you can use an external differential amplifier with an op-amp.

DocJC wrote:
The VG is most easily constructed with an op-amp voltage follower.

The AVR-DA MCUs have a DAC that can be used for this, so no external parts are needed.

eauth wrote:
This will only work if the source and the microcontroller are galvanic isolated.

Indeed. We don't want stuff exploding.

eauth wrote:
Unfortunately, the DA's do not have a gain stage. If the input voltages are very low this won't work, in that case you can use an external differential amplifier with an op-amp.

Well, at least it's possible to use the 1.024V internal reference as full scale, so compared to 5V VDD as reference it's already equivalent to 5x amplification, better than nothing I guess...

The DAC virtual gnd would be 0.512V.

This will only work if the source and the microcontroller are galvanic isolated.

Isn't this the case with a current transformer?

JC

Hopefully yes, unless the MCU is somehow being powered from the transformer, we never know...