Precision Current loop receiver (24bit) - Page 1
Forum section: electronics / projects

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: June 14, 2016, 08:31:45 am
content: Hi,I wanted to design a 0-25mA current loop receiver with at least 22Bits resolution with 200 Samples per second, can I go in the way of using a precision resistor with a 24bit ADC, or we have a better way of doing it?Do we have a nice app note for doing the job done?

author: ebclr
author_rank: Super Contributor
timestamp: June 14, 2016, 08:49:47 am
content: Ohm law is valid, It's a excellent way to do that, and it's extremely popular KISS ( Keep it simple .......), only objection is to have some kind of protection for some bad user

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: June 14, 2016, 08:53:53 am
content: QuoteThanks, then would you suggest some app note for designing a 24Bit system? we are dealing with 1nA current ranges, every thing is against us,Thanks in advance

author: ebclr
author_rank: Super Contributor
timestamp: June 14, 2016, 09:18:40 am
content: Well 25mA isn't nA , you talking about other "animal"

author: ebclr
author_rank: Super Contributor
timestamp: June 14, 2016, 09:20:33 am
content: http://www.ti.com/lsds/ti/amplifiers-linear/current-sense-amplifiers-overview.page

author: coppice
author_rank: Super Contributor
timestamp: June 14, 2016, 09:23:46 am
content: Quote from: ebclr on June 14, 2016, 09:18:40 am22 bits down on 25mA is just a few nanoamps. I think he's talking about the same animal.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: June 14, 2016, 09:37:46 am
content: QuoteThat's correct, I'm talking about this nA animal

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: June 14, 2016, 09:44:44 am
content: QuoteI'm talking about the nA ranges, our maximum current is 25mA, so measuring it with at least 22bits we should be able to read nA currents,A nice appnote would help me a lot.

author: EEVblog
author_rank: Unknown
timestamp: June 14, 2016, 10:03:58 am
content: It's going to depend upon how much voltage drop you can tolerate across your shunt resistor.Your noise floor will become a problem.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: June 14, 2016, 10:19:30 am
content: QuoteThanks for your response, I should design every thing so selecting the shunt resistor and ADC is by myself, our loop sensor voltage is 15V,I'm stuck with how to calculate noise and selecting the right resistor and ADC, would you show me the path?

author: Kleinstein
author_rank: Super Contributor
timestamp: June 14, 2016, 10:27:01 am
content: High resolution (e.g. 22 Bits) at 200 samples per second will be hard, as this is faster than the usual power line cycle.Unless a very low voltage drop (e.g. < 1 V) is required, the simple shunt and than voltage measurement is OK. The required resolution in the nA range is not that critical - leakage currents from the board, OP input bias-drift and good diodes are well below 1 nA more in the <100 pA range. So this is not yet a problem. There is more trouble from self heating of the shunt and power line related signals coming in.For the shunt you can likely choose a value, so that the voltage drop directly fits the ADC input range. ADCs with 24 Bits are available, however INL and noise levels might be the more interesting parameters. However most signal sources / Sensors also have a limited accuracy.

author: nctnico
author_rank: Super Contributor
timestamp: June 14, 2016, 11:09:57 am
content: I'd like to see diodes with a guaranteed leakage current less than 1nA over the commercial temperature range.

author: Christe4nM
author_rank: Unknown
timestamp: June 14, 2016, 03:29:58 pm
content: Your requirements makes me wonder wether an analog current loop is the right method for your application. I'd be thinking about turning the sensor output into digital before putting it on a wire. Even if you still need to turn a current loop output into digital, you might have the benefit of doing so right at the source and possibly less need for protection as well as better noise immunity.Of course I don't know your application, so it's just a thought.

author: TiN
author_rank: Super Contributor
timestamp: June 14, 2016, 04:47:26 pm
content: Also disregarding front-end capabilities, if we talk about stable 22 bits from ADC then 24-bit is not going to cut it, and will need something like TI ADS1262 or Analog devices AD7177-2.Maybe using dedicated current loop interface solution would be a better option, if project target is to transfer data over the link.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: June 14, 2016, 06:25:11 pm
content: Quotewhich dedicated current loop interface are you talking about?

author: Kleinstein
author_rank: Super Contributor
timestamp: June 14, 2016, 06:56:10 pm
content: Building the sensor side of the current loop might be the greater challenge than the receiver side. So using a digital interface instead of the current loop might be a good idea.There are not many sensors that really need a 22 Bit interface. Also 200 Hz readout is rather unusual, as it is much easier to get a stable reading at 50 Hz.

author: ebclr
author_rank: Super Contributor
timestamp: June 15, 2016, 05:02:47 am
content: 2 important things to consider1 the resistor ( probably will need to be a expensive 2 ppm vishay to reach 22Bit2 the reference voltage off your AD, they have 22 resolution but without a good resistor and Reference voltage your 22 bits will never be acurate

author: David Hess
author_rank: Super Contributor
timestamp: June 15, 2016, 06:01:29 am
content: Quote from: nctnico on June 14, 2016, 11:09:57 amLow leakage diodes are not as available as they used to be but even in the past, it was still common to substitute other devices for them because of their high cost do to both low demand and expensive testing.Low leakage JFETs can achieve this when used as diodes and this is the way to go if you want a device which will meet your specifications without additional testing; a 2N4117A has a maximum specified gate leakage of 2.5 nanoamps at 150C.Many small signal NPNs transistors like the 2N3904 can achieve similarly low leakage with their collector-base and emitter-base junctions but this is not something manufacturers test for or specify beyond some high worse case level like 50 or 100 nanoamps. The collector-base junction is slow but has a high breakdown voltage and the emitter-base junction is fast but has a low breakdown voltage. Leakage current varies between manufacturers, processes, and even lots so you have to do your own grading.

author: ebclr
author_rank: Super Contributor
timestamp: June 15, 2016, 07:49:37 am
content: Take a look on this datasheethttp://www.ti.com/lit/ds/symlink/opa857.pdf