ADC EVAL_AD7984_PMDZ voltage source and signal source sharing common ground

Question

EDIT: (problem definition shortened into 2 sentences)

For anyone using EVAL_AD7984_PMDZ and having troubles to apply -2.5, 0, 7.5 [V] and signal to differential probes, because the voltage source and signal source shares ground (then there is 2.5 [V] difference between the two), check the last comment for solution.

ORIGINAL POST:

I got differential ADC EVAL_AD7984_PMDZ (page 17 connection) on ZedBoard working just fine (from -5 to 5 [V]). It requires not only 3.3 [V] from PMOD connection (logic) but also -2.5, 0, 7.5 [V] from external power source (Laboratory DC Power Supply - 2 adjustable outputs, that I have connected in series, thus I have "ground" on the middle pin, while on the other 2 pins there is -2.5 and 7.5 - relative to my "ground").

And here it comes, when I introduce input from another Laboratory DC Power Supply, the readings are fine within whole range.

But when I introduce signal from AFG-2125 Arbitrary Function Generator, there is offset of 2.5 [V] from both of the ADC input pins and it kind of messes up whole measurement that is below 2.5 [V].

May I get a little bit of insight on that ?

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The ground of the ADC probes is the "ground" mentioned above.

Technically I have 3 grounds:

• ZedBoard (and thus PMOD connection)(powered from its own power source)
• External power source for the ADC (with the "ground")
• Signal Generator ground

I am not sure how to resolve that. Would it help if the Signal Generator was battery powered ?

^{simulate this circuit – Schematic created using CircuitLab}

(I am sorry if some electrical engineer finds the above schematic offensive)

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Schematics for EVAL_AD7984_PMDZ

Answer 1

The eval board consists of a unity gain buffer on each input, the output of which goes into the ADC. Barring some details about current and input impedance, it is as though the board inputs are connected to the ADC inputs. For the ADC to work, the input voltages need to be between the ADC's ground pin and the ADC's vref pin (see datasheet table 2 -> ANALOG INPUT -> Voltage Range).

If you are powering it from a DC power supply with the eval board's -2.5V input at mains ground, then the eval board ground and hence the ADC's ground pin are at mains ground +2.5V. The ADC's vref pin is held at eval board ground + 5V by the voltage reference IC.

So when either input drops below mains + 2.5V, the corresponding ADC input drops below ADC ground. That takes the ADC out of spec, so you can't expect it to work properly. Worse, it goes below ADC ground pin -0.3V, which is outside the absolute maximum rating for the chip (datasheet page 6) which could damage the chip. Fortunately, the board includes some 22 ohm resistors which will limit the current and probably protect the ADC from damage.

You seem to be expecting the fact that the amplifier is differential to mean it can take any differential input. That's not the case. Differential means the output of the ADC is the difference between the inputs, but for the ADC to work the inputs need to be within the input limits. Every ADC and differential amplifier has input limits - you wouldn't expect to connect 1000000V to one input and 1000001V to the other and read one volt without fireworks. For this ADC, the limits are ground to Vref (which is 5V on the eval board). Very few differential amps or ADCs accept input voltages outside their power supply voltage range. If you really need that, then you need to look at either an isolating amplifier or a transformer.

Answer 2

Just to finish this thread with short answer. Powering the EVAL_AD7984_PMDZ requires the PMOD connection 3.3[V] and GND (pins 5, 6 and 11, 12 on any 12 pin PMOD header) and -2.5, 0, +7,5 [V] on J3 connector. To prevent the above described problems (shared ground between source and signal generator) provide the voltages (-2.5, 0, +7,5 [V]) from a battery source, OR use Voltage Inverter to obtain -2.5 [V] (thus, the shared grounds are at the same level). The LT1054 Voltage Inverter is easy to use and does the job very well.

http://cds.linear.com/docs/en/datasheet/1054lfh.pdf page 10