ADS1120-Q1: ADS1120-Q1 single-ended input setting

Hi Ian,

All measurements taken with the ADS1120-Q1 are differential, so with a single-ended measurement you will only be using 1/2 of the full-scale range (positive full-scale only) so the resolution will drop to 15-bits.

When making single-ended measurements using a unipolar supply, you must disable the PGA (operate in PGA bypass mode) so that the input will be within the correct common-mode input range for the ADC.

Input connections can be accomplished in one of two ways.  One method is to connect AGND to one of the analog inputs and connect the sensor to one of the three remaining inputs (giving a maximum of three measurements inputs with the MUX connection from the sensor input to the AGND input) or you can use the internal MUX connection to AVSS (which is AGND when using a unipolar supply) which allows for four measurement input channels.  The best way to visualize this is to look at the block diagram in the datasheet to see both the MUX connection possibilities and the PGA bypass switches.

Best regards,

Bob B

Hi Bob,

Thanks for your reply.
In the datasheet p.33, 1 LSB = (2x Vref/Gain)/2^16 = FS/2^15. in this equation, i wonder why Vref is divided by Gain. voltage input at the pin is multiplied by gain so max output(voltage input to the ADC) will be around vref, right? Please correct me if i misunderstood. Thanks.

Ian

Hi Ian,

I apologize for the delayed response.  Input referred or output referred, that is the question.  The converter will output a code based on the voltage reference value and the gain applied.  At a gain of 1 the value of 1 code relates directly to the +/- value of the reference alone.  If gain is applied, the full-scale input range is reduced from the +/- reference voltage to the +/-reference voltage divided by the gain.

For example, if the reference equals 2.048V, then the full-scale range (measurable voltage that can be applied) is +/- 2.048V.  The value of one code is 62.5uV.  If a gain of 128 is used, then the measurable voltage is reduced to +/- 2.048V/128 or +/-16mV.  If you apply 16mV to the inputs, you will see full-scale as the input is equal to the current full-scale measurable range of the converter.  The value of one code changes to 488nV in this case.

As an example, if we apply 15mV to the analog inputs we will see two different values of returned codes relative to the gain.  In the case of a gain of 1 we would see 240 codes.  In the case of gain of 128 we would see returned codes 30737.  So you really need to consider the system as opposed to just the amplifier.  It is true that the PGA amplifier gains up the input voltage.  It is also true that the maximum measurable voltage is relative to the reference voltage.  However, the ADC itself has no idea that you applied gain so this must be accounted for in the calculation.

Best regards,

Bob B