ADS1120: Single-ended solution for shifted signal

Hi user650944,

Welcome to the E2E forum!  It is not totally clear what your requirements are and resolution required within the measurement.  When cycling through the mux there can be phase delay and I'm not sure how critical this is to your measurement.  Perhaps a device with simultaneous sampling would be a better choice.  We have specific devices for power monitoring such as the ADS131E04 (current only) or the ADS131E08 (current and voltage monitoring).  There is also the ADS131M04 and ADS131M08 designed specifically metering applications but could also be used to monitor a single point application.

See my comments specific to your questions below.

Best regards,

Bob B

user6500944 said:

Part Number: ADS1120

Hello, Buddies! In first, pardon for my poor English. Secondly, I need help with single-ended signal measurement by using ADS1120, because I have already broke my brain. Introduction:
1) ADC ADS1120 powered by 5 v analog, 3.3 v digital;
2) 5-v current sensor with ratiometric output with 2.5 bias on output with small drift. So, as example, 2.5 v means 0 A, 0.5 v - -50A and 4.5 v - +50A;
3) Desired current frequency is 50...60 Hz, high-order harmonic is not very significant
Questions:
1) What would be better: simply bias AIN(-) by half of supply, and use 5-v supply as reference (further - Vsup). So, output of transducer in 2.5-v would mean code 0, 4.5-v (2/Vsup)*2^15-1 and 0.5-v - (-2/Vsup)*2^15. Also, I can use divider to get 2 V of supply in REFP0 pin to get full-range. [Bob]  You have not stated the resolution required for the sensor you are using.  Noise could be a factor, but by using a voltage divider for the reference, although feasible, will have issues related to initial accuracy and drift.  To reduce these issues you need to use precision resistors that have very low drift. Setting one of the ADC inputs to 2.5V serves 2 purposes.  The first allows use of the PGA enabled.  The second allows for a measurement that uses more of the full-scale range.  Using an external reference at 5V with a gain of 2 is similar to using 2.5V with a gain of 1.  However, there are some common-mode input restrictions for the ADS1120 which limits the actual input range to be measured.
Or, another, I can use 2 opamps to convert single-ended signal to differential. First opamp should be configured as follower, second - as inverting with unity gain. Because all opamp would be in one package, it may be expected that drifts and offset would be rather similar and rejected by differential ADC. So, it is no need to use expensive low-offset opamps. Or no? [Bob] You would only need to do this if the sensor impedance is very large and the ADS1120 requires the PGA to be bypassed.
2) Is it true, that filter on REFPn pins must be similar to ADC input filter for real ratiometry? [Bob] The filters should be similar but do not need to be perfectly matched for the ADS1120.  This would be more important with higher precision ADCs that are 24-bit or higher.  The concept here is to lessen the influence of noise in the measurement. Also, by using divider on AIN(-) pin, is it significant to match resistance of filter on AIN(+) and AIN(-). In other, for example, output resistance of divider is 750 ohm, resistor of AIN(+) is 5.1k, so I need add to AIN(-) pin resistor near 4.3K, not 6.2K? In fact, output resistance of transducer unknown, but minimum recommended load is 4.7K. [Bob] I'm not sure if I follow what you are saying.  The resistor divider (if used) on AIN- would be set to a specific voltage.  AIN+ would be coming directly from the sensor.  The series resistance is specific for the filter and would not be loading the sensor.
3) I like to measure 2 transducers by single ADC: so, I put sampling rate to 2KSPS and simple switch signal by internal multiplexer and then start conversion (in single-shot mode). So I obtain 1KSPS per channel - this is near 20 samples per 50-Hz period. Is it look acceptable to calculate phase shift between 2 currents? (the load is 3-phase motor, but measuring only 2 phase may be enough to see abnormal situation). [Bob] To monitor 3 phases you need to monitor all of them.  It may be possible to detect the 3rd phase loss, but there could be damage done to the motor before it is detected. The best course to follow is to monitor all 3 phases at the same time.
With respect!