200 nA offset error for shunt current measurement, how? - Page 1
Forum section: electronics / projects

author: EarthLord
author_rank: Contributor
timestamp: August 18, 2022, 12:52:36 pm
content: Hello all,https://sendyne.com/Datasheets/Sendyne%20SFP200%20IC%20Datasheet%20V1.0.pdfThis IC claims to be able to measure with an input offset error of 200 nA. Also their system using a 18uOhm resistor can measure current from mA to kA it seems. I am not sure how this is possible, especially since they are using MCP6234 as an amplifier, which has 5 mV Vos.Can anyone help me understand this system of current measurement from mA to kA with a single shunt and single amplifier?Thanks, Prithvi

author: Kleinstein
author_rank: Super Contributor
timestamp: August 18, 2022, 01:04:49 pm
content: The offset ( < 200 nV) is achieved with external chopping before the amplifier. Chances are the version with the MCP6234 would be relatively noisy. I would not consider it a good choice, more like a low cost low power one.How the voltage offset translates into a current depends on the shunt. Similar different shunt values are used to get a large range. So it is not the same circuit measureing mA and kA. One can choose a suitabel shunt (usually 1 at a time) to have range up to the kA or mA (possibly µA) range.

author: Marco
author_rank: Super Contributor
timestamp: August 18, 2022, 03:04:20 pm
content: It has double digit current noise and offset errors with that shunt, so not really a mA range measurement.At a mA the shunt only creates 18 nV ... thermal EMF for a solder joint is 5 uV per degree celsius, so that's a bit optimistic if it's just been heated up by high current.

author: EarthLord
author_rank: Contributor
timestamp: August 19, 2022, 09:09:08 am
content: Hello Kleinstein,It doesn't look like there is a chopper based system from the block diagram and suggested schematic. And they don't have different circuit for the measurement for the different ranges.Hi Marco,Maybe they mean starting from about 100 mA when they mean they can measure in mA too.

author: Marco
author_rank: Super Contributor
timestamp: August 19, 2022, 09:28:48 am
content: It's automatic zeroing, they occasionally zero the input with the analog switches.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 19, 2022, 10:55:19 am
content: They have the external switches (MOSFETs in the more detailed schematics) to do the auto zero / chopping. Using 2 pairs of sense wires to the same shunt is a bit strange, but it can work. So the amplifier does not have to be DC accurate.One circuit would normally not use different shunts, but one could use the chips with different size shunts, e.g. for high current in a truck or low current with an E-bike or model car. It is just that the same chip could be used over the large range. This does not mean that 1 circuit can do it.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 19, 2022, 11:06:46 am
content: The input amps have a gain of 201 diff vs 1. The Vos doent figure much in the scheme of things.Vref finds its way via the shunt I guess.Like this maybe...

author: thm_w
author_rank: Super Contributor
timestamp: August 19, 2022, 10:39:00 pm
content: Quote from: Kleinstein on August 19, 2022, 10:55:19 amYeah I thought at first two channels, use two shunts, but they both go to the same shunt (in this example). Polarity of the two amps is flipped across the shunt.The temp compensation is in software, so nothing to do with that.Their explanation at least seems to not mention accuracy, just reliability:Quote

author: Marco
author_rank: Super Contributor
timestamp: August 20, 2022, 08:25:49 pm
content: They could also use the two channels to get continuous measurement, when one zeroes the other measures.