Current sensing wires - Page 1
Forum section: electronics / beginners

author: xoom
author_rank: Regular Contributor
timestamp: July 12, 2016, 08:18:20 pm
content: Hi, i have question about current sensing in Lab PSU. Im making Linear PSU and Shunt resistor are on Power Stage Board. Now for measurement (front panel) im thinking to put op-amp (AD8551) and 2x 20bit ADC's (one for Volts other for Amps) for current need opamp to make Gain ~4.5 (Shunt is 0.22Ohm) PSU will be giving 5A max So from Power Stage board there will be 0 - 1.1V max. And cant find answer if its good to do that by wires (from Power board to front panel board) it can induce noise in those wires or not. How much can affect current measurements?Sorry for my badly formulated question, hope someone can understand it

author: Signal32
author_rank: Frequent Contributor
timestamp: July 12, 2016, 08:28:17 pm
content: I think you're asking how to get the voltage on the PSU output terminals back to the ADC for measurement.There will not be much noise induced in the wires as the wires are directly connected to the low-impedance output of the PSU. So yes you can use wires.Here are some extra things to consider:- You won't be able to get 20bit true resolution out of the ADC's. There will be lots of noise. What ADC are you using ?- It's debatable if putting an op-amp in front of the ADC to amplify the voltage by 4.5x will bring any benefit as it will also introduce noise / drift. It would be better to use an ADC with a built-in op-amp ( PGA ).

author: xoom
author_rank: Regular Contributor
timestamp: July 12, 2016, 08:36:24 pm
content: for Voltage measurements there is no problemI'm confused about currentthere is low voltage on shunt resistor .. thats why im afraid. If it goes by wires to front panel then gets amplified.. so if those wires picks a lot of noise and then gets amplified than its not so good

author: Signal32
author_rank: Frequent Contributor
timestamp: July 12, 2016, 08:43:35 pm
content: For current also there won't be much noise in the wires as the wires are shorted directly through the 0.2R shunt. If you get 22mV of noise at the end of the wires then according to ohm's law you would have to have 100mA passing through the shunt resistor to be able to generate that voltage difference. Think of it like this, would you expect to measure any voltage when connecting a voltmeter to both ends of an unconnected 1m copper wire ?Also, what ADC are you using ?PS: You could multiplex the input of the ADC to measure voltage & current on the same ADC.

author: xoom
author_rank: Regular Contributor
timestamp: July 12, 2016, 08:46:27 pm
content: Thank you for clearing that upADC are LTC2420.

author: Signal32
author_rank: Frequent Contributor
timestamp: July 12, 2016, 08:50:37 pm
content: Quote from: xoom on July 12, 2016, 08:46:27 pmQuoteIf you are going with separate ADC's for the voltage/current, then for the current measuring ADC you can use a 1.2V reference, making the input 0-1.2V, so no need to use an opamp front end.

author: xoom
author_rank: Regular Contributor
timestamp: July 12, 2016, 08:56:42 pm
content: hmm.. good ideai have 5V vref chip X60003 (10ppm/C) what if i use resistor divider for making 1.2V from my existing vref?

author: Signal32
author_rank: Frequent Contributor
timestamp: July 12, 2016, 09:03:49 pm
content: Quote from: xoom on July 12, 2016, 08:56:42 pmThe purpose of voltage references is to have them be stable, very stable (10ppm/C). If you're going to be using resistors to divide the voltage, then might as well not use a precision reference to start out with.I would avoid using X60003 with the LTC2420 as you will have to power them between 5.1-5.5V. Any other voltage will not work. I would power them from 5V and use a different reference. How are you planning on generating the 5.1-5.5V voltage ?

author: xoom
author_rank: Regular Contributor
timestamp: July 12, 2016, 09:09:49 pm
content: X60003 working from 5.1V - 9V for it im planing to to feed from 6V ( 7812 with divider) and for ADC's just from 7805. LTC2420 working on 2.7V- 5.5V

author: Signal32
author_rank: Frequent Contributor
timestamp: July 12, 2016, 09:14:58 pm
content: Quote from: xoom on July 12, 2016, 09:09:49 pmYes but the problem is that X60003 may have 5.001V and the 7805 output may be 4.9V(or as low as 4.8V, check your datasheet). Now you're powering the ADC at 4.9V but the reference is 5.0V. But the datasheet for the ADC says that the ref voltage has to be less or equal to power supply voltage = no good.Also if you're using a resistor divider to generate 6V from 12V and then putting a load on those 6V you will find that the 6V will drop. By how much depends on your divider, how much of a load you put on it.

author: xoom
author_rank: Regular Contributor
timestamp: July 12, 2016, 09:21:15 pm
content: 6V im planing to supply to X60003 it only needs 500nA ( but if need to somehow lower vref for current measurement ADC ) then other divider will consume some current. If i understand well X60003 can source 10mA with 10uV drop. If you have any better idea about vref please tell. That would help a lot

author: Signal32
author_rank: Frequent Contributor
timestamp: July 12, 2016, 09:23:27 pm
content: Any 1.2Vref for Current and 4V ref for voltage ADC will do. Then you will be able to use 5V supply for all. Where are you sourcing your parts from ?

author: xoom
author_rank: Regular Contributor
timestamp: July 12, 2016, 09:28:02 pm
content: Got those from sample service long time agohope those are not obsolete.QuoteHmm don't have any of those vrefs.. only 2.5Vref and that X60003 5Vref.

author: Signal32
author_rank: Frequent Contributor
timestamp: July 12, 2016, 09:31:06 pm
content: Quote from: xoom on July 12, 2016, 09:28:02 pmThe 1.2 and 4.0 are better to use, if you don't have any the 2.5 will also work.

author: xoom
author_rank: Regular Contributor
timestamp: July 12, 2016, 09:36:41 pm
content: with 2.5V will get less resolution for current sensing.but if not figure out other way then have to use it