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We are using the ADS114S06B in our system. We have chosen this chip for it's 16 bit capabilities. This would project out input range (+-32.768V) exactly as 1mV/bit.
To keep our power supplies simple, we took the input voltage, put it through a differential OpAmp with a gain of 0.0625 and a 2.048V offset. Now we have a 0-4.096V
signal we want to measure with the ADS. ADS AVDD is supplied with 5V.
At first we measured it (between AINCOM and AIN0) with 4.096V on the REF and AINCOM to GND. This gave us a correct value, but with 15 bit (this is nowhere to be
found in the datasheet). Reading another question about this issue came up with the idea of connecting AINCOM to half VREF (2.048V). Since we still measure a voltage
of 0-4.096V, we left the VREF at 4.096V. This gives us the correct 0 point, but only half the range. 4.096V in gives 4000h, not 7FFFh and 0V gives BFFFh, not 8000h.
Now we also have half VREF connected to REFp (REFn to GND). With this configuration we get the right measurements and range, but we do not get why this works.
We thought that REFp was for the positive reference voltage and REFn for the negative. But since we only use the positive range, REFn was not necessary. Now it looks
like the ADC creates its own -VREF, inverted from the +VREF.
So if lets say I want to measure a voltage that sweeps between -2.048V and +2.048V, I only need a positive reference voltage of 2.048V on REFp, connect REFn to GND
and measure the input against GND? Given the ADS Analogue supply is +-2.5V.
Hi user3731922,
Welcome to the E2E forum! The full-scale range (FSR) for the ADS114S06B is defined as +/- VREF/GAIN as shown in a couple of places in the datasheet. The first place is equation 4 on page 25. The ADC measures differentially AINP to AINN. If the AINP voltage is greater than AINN the output code is positive. If the AINN voltage is greater than AINP the output code is negative. Note that the input should never exceed the supply rails or the device can be damaged.
In the single-ended input case using unipolar supplies (5V and GND) and where AINN is connected to ground, the AINP voltage will always be greater than AINN and so you will only see the positive codes. That is why you were seeing a limited code range of 15 bits.
As the FSR is directly related to +/-VREF, making a pseudo-differential measurement allows all bits to be evaluated. If your FSR is 4.096V, this is equivalent to +/- 2.048V. So you would set the AINN input to 2.048V which would allow you to measure 2.048V above AINN, and 2.048V below AINN which allows for the full-scale range of 4.096V (+/- 2.048V). This is the case using unipolar supplies with the INA output offset by 2.048V.
If you use bipolar analog supplies (+/- 2.5V) for the INA and ADS114S06B You could measure in a similar way with the 2.048V reference. However, in this case AINN would be connected to ground.
The key point for the FSR is that it is 2*VREF (or +/-VREF) for the binary 2's complement output from the ADC.
Best regards,
Bob B