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Adc Vref and Gain consideration(AD7715)

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tlhsglm:
Hi fellows,

I will make a current measurement, application needs a fast sampling rate and high resolution.Thus i bought an AD7715 ANZ5, i will supply this with +5v and 0v (single supply rail), datasheet is here;
https://www.analog.com/media/en/technical-documentation/data-sheets/AD7715.pdf

I'm confused about Vref and Gain. My voltage range is 0V to 1V.Some sources tells Vref's purpose is to let ADC know what voltage ranges will come to ADC and ADC makes 16 bit devision based on that.

1- Is this true;  if tie +Vref to 2.5 volt's(that maximum voltage for my package) and -Vref to ground.Adc will make calculations like this:+2,5volts/65536(16bit).And my max voltage is 1 Volt so i have to put an opamp with gain of 2,5.

2- Is this true; But can i tie +Vref to 1 volts? So there is no neccessary to use an opamp?

newbrain:
I won't comment on the choice of this ADC, not really my turf, but the following comes from reading the DS.

It is possible to use as low as 1V for the reference, but the DS clearly states that noise performance will be affected (page 21, Reference Input)
Still you could set an internal gain of 2, so that the actual range in unipolar mide would be 2.5/2=1.25.

Using an external calibration, it woul be even possible to get the full codes fo the 0-1V range, as the supported span is 0.8*Vref/Gain, but I imagine this might be a little too complicated... probably a Vref of 2(.048)V might be a reasonable compromise, still setting a gain of 2.

You don't specify if the input voltage is referenced to GND, note that for buffered mode the AIN- input cannot be 0V.
Since you mention current sensing, I expect the source impedance to be quite low, so you should be fine with keeping the buffer disabled.

tlhsglm:

--- Quote from: newbrain on December 02, 2019, 09:19:32 pm ---I won't comment on the choice of this ADC, not really my turf, but the following comes from reading the DS.

It is possible to use as low as 1V for the reference, but the DS clearly states that noise performance will be affected (page 21, Reference Input)
Still you could set an internal gain of 2, so that the actual range in unipolar mide would be 2.5/2=1.25.

Using an external calibration, it woul be even possible to get the full codes fo the 0-1V range, as the supported span is 0.8*Vref/Gain, but I imagine this might be a little too complicated... probably a Vref of 2(.048)V might be a reasonable compromise, still setting a gain of 2.

You don't specify if the input voltage is referenced to GND, note that for buffered mode the AIN- input cannot be 0V.
Since you mention current sensing, I expect the source impedance to be quite low, so you should be fine with keeping the buffer disabled.

--- End quote ---

Thank you for detailed answer.I'm confused somewhere again, in page 20 and Bipolar/Unipolar Inputs section, datasheet says;


--- Quote --- As a result, the voltage to which the unipolar and
bipolar signals on the AIN(+) input are referenced is the voltage
on the respective AIN(−) input. For example, if AIN(−) is
2.5 V and the AD7715 is configured for unipolar operation with
a gain of 2 and a VREF of 2.5 V, the input voltage range on the
AIN(+) input is 2.5 V to 3.75 V.
--- End quote ---

I didn't understand this part, Is based on the ground 2.5volts?In my circuit when PWM is maximum AIN+ and AIN- voltage difference is 1 volts.But AIN is 10 volts when looking based on circuit's ground.Do you think is this a problem?My circuit is;

iMo:
AIN+ and AIN- are referenced to the AD's analog ground AGND (0V).
Thus with Vref=2.5V and Gain=2 the max difference (AIN+) - (AN-) will be 2.5V/2 = 1.25V.
And because you set AIN- to +2.5V (against agnd) the ADC range will be from 2.5V till 2.5+1.25=3.75V for unipolar operation.

Mind you can't go with AIN+ and/or AIN- beyond AGND and AVDD. Both inputs must be inside AGND..AVDD, where AVDD is 3..5V.

PS: for bipolar operation, Vref=2.5V, Gain=1 and AIN- set to +2.5V the ADC range will be from +1.25V till 3.75V (ADC will return negative values for voltages < +2.5V because the AIN- is set to +2.5V ).

newbrain:
What imo said.

Some more explanation about unipolar/bipolar vs differential/single ended: this ADC has got differential inputs, so it will measure the difference in voltage between its AIN+ and AIN- terminal.

This difference can either be measured in unipolar mode:
  the code points will span from 0 to maximum (0xFFFF) for positive values of the difference from 0V to Vref/gain V,
or in bipolar mode:
  the total span is the same, but the range is centered on AIN- allowing for positive and negative differences.

The circuit as is will not work, look at page 9 of the DS "Absolute Maximum Rating":

--- Quote ---Analog Input Voltage to AGND            −0.3 V to AVDD+ 0.3V
--- End quote ---
You are largely exceeding this, so no reliable data will be produced (if the chip does not immediately fry...).

So, you need to either move the shunt on the low side, or provide an input stage to offset the range into something usable by the ADC (it might also be a current sense amplifier, such as INA139 or similar).

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