IWe Ivan Govnov
Newbie level 6
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.
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?
2) Is it true, that filter on REFPn pins must be similar to ADC input filter for real ratiometry? 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.
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).
With respect!
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.
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?
2) Is it true, that filter on REFPn pins must be similar to ADC input filter for real ratiometry? 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.
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).
With respect!
