High Side, High Voltage Shunt Measurement - Isolated Differential ADC? - Page 1
Forum section: electronics / projects

author: Reckless Maker
author_rank: Newbie
timestamp: October 07, 2021, 05:17:05 pm
content: Howdy All,I need to measure the high-side current delivered by an LED driver. The problem is that the output voltage can be as high as 250V. One solution would be to float an ADC and digital isolator just below supply voltage using a zener diode. However I'd like to avoid the extra cost of the isolator and power dissipation of the bias resistor.Another solution might be to simply isolate the ADC ground from the LED driver ground, but I'm having difficulty finding any information on how to do this. There's this diagram from Maxim:and this one (top left) from NI:but that's all I could find. Together, they suggest that this solution is at least possible, but the accompanying pages are short on details. The Maxim one references a non-existent bias resistor, and the NI one biases the inputs to ADC ground rather than Vref/2.So my question is twofold. 1) Is this a viable solution? 2) If so, how would one implement it?

author: jbb
author_rank: Super Contributor
timestamp: October 07, 2021, 11:25:05 pm
content: In principle an isolated ADC is good, but you need to power it. A ballast resistor + Zener shunt could work but might get really hot.An isolated power supply would be better. You can get integrated chips with data isolation and power transformer from TI, Analog Devices and others.The per-chip cost might be a bit high, but how about the TI AMC3306M25 ? It’s an ADC with isolated power built right in. Note the output is a sigma-delta (is is it delta-sigma?) bit steam which will need processing. Lots of microcontrollers offer peripherals to do that now.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 08, 2021, 12:11:47 am
content: If it's always a positive voltage over shared ground, this works:The current sense amp has low quiescent current, and uses a modest minimum voltage drop; it's rated up to 40V, but can be extended arbitrarily by cascoding with a PMOS, or stack thereof.This is better than a diffamp, for a given degree of precision and voltage range -- you don't need tightly matched resistors (to get high CMRR) or high voltage drop.Hall effect sensors are nice, but offer poor dynamic range, and are most effective at higher currents (10A+). They're pretty useless down in the mA, even with many turns on the primary and a magnetic core.You didn't mention what accuracy, bandwidth and voltage or current range you need (other than up to 250V, but not how low), so I kind of have to mention everything here.Also, what kind of LED driver? Does it not have a constant current output, or setpoint? It may be as effective to simply use it as a transconductance amp without feeding back its actual output. (If this is for a discrete switching controller design, consider swapping where the load or sense is, so that peak current mode control can be used (low side switch), or consider level translation on the gate driver instead -- use a bootstrap or transformer coupled gate drive -- rather than trying to shift the analog level.)Tim

author: coromonadalix
author_rank: Super Contributor
timestamp: October 08, 2021, 01:26:23 am
content: You should tell how much current you need to sense, is it AC or DC ??The previous post schematic may help, but the sense resistor value 1 ohm is too big for high currentsAn non invasive current measurement could be done with current sensing coils over the wires or hall effect sensors, a clamp based current meter can do precise measurements too if you calibrate it before use ??

author: Terry Bites
author_rank: Super Contributor
timestamp: October 08, 2021, 02:23:32 pm
content: The AD7403 modulator from Analog Devices is for this very purpose. TI offer some good ones too. eg AMC1306https://www.ti.com/isolation/isolated-adcs/overview.htmlBefore you go down any galvanic isolation rabbit hole, work out what your worst case I to O voltage is. You cant go too high after all. Then go higher! Dont forget tha tthere is always I/O capacitance to consider. How much leakage current can you live with?