Using Current Sense Amplifier for resistance measurement. - Page 1
Forum section: electronics / projects

author: sean87
author_rank: Regular Contributor
timestamp: July 30, 2016, 08:52:46 pm
content: Hi all! I am designing a resistance measurement device based onMAX9611high-side CSA. I will have normal SMD resistors but I want to know their exact resistance, e.g. a 2 ohm resistor might be actually 1.95 ohm!I have no expirience with CSA's at all. but according to the datasheet, I would just put a pad on PCB that I can solder in/out any 0805 resistor as thesense resistorand use a precision current source to force in a known current (e.g. 200mA) and read what the chip says, which obviouslyR = (V voltage drop accross sense resistor) / (the known current from a current source)I have the following questions.1.What will happen if I omit the load resistor and connect the sense resistor after the RS- pin directly to the ground? If the load is essential to be there, what resistor value would be optimal?2.what will be the optimal current source that I would need to get accurate voltages? I am planing to test resistors under 10 ohms.3.How does the current measurement works if I also need to check if my current source works as expected, for this the MAX9611 offers these:Common Mode Voltage Measurement:This is the average of the voltage at RS+ and RS- pins with range from 0V to 57.3V and since it is a 12 bit ADC, then the steps will be roughly 0.013V or 13uV.Sense voltage measurement:(what is this and how can I use its data?)Three programmable gains allow for a wide range of currentsto be read by the ADC. The current-sense amplifiergain can be set to 1x, 4x, or 8x. The full-scale sense voltagesare then 440mV, 110mV, and 55mV, respectively.Here is the part from datasheet regarding this sense voltage measurement:so what is this? does it mean I can measure the current with it? if so, how? from the datasheet I can tell if I select the 1x gain, in order to convert it to voltage I have to multiply the 12bit ADC value by 107.50?? what does it represent? current or voltage? I am a bit confused!for example if my sense resistor is 1 ohm and I force 20mA current, what would be the result?

author: RES
author_rank: Regular Contributor
timestamp: July 30, 2016, 09:14:20 pm
content: If you want to know the exact resistance value you need a accurate constant current source, for example a current mirror. LSB in uV, FS in mV.

author: ajb
author_rank: Super Contributor
timestamp: July 30, 2016, 09:25:35 pm
content: Since you're asking about tying one side of the CSA to ground, the question arises: Why do you want to use a CSA at all? A current sense amplifier is nothing but a differential to single-ended amplifier optimized for a high common-mode voltage range (that is, it tells you what the difference between two inputs is regardless of how close those two inputs are to ground). If you have the option of connecting one end of the device under test to ground, then you have a number of other options--you could do a simple single-ended measurement for modest precision, or still perform a differential (four-wire) measurement for higher precision, but without having to worry about large common mode voltages.QuoteIt should work fine with RS- grounded, as the common mode voltage is specified from 0-60V.QuoteWell, since the full-scale input voltage is specified as 55, 110, or 440 mV depending on the selected gain, you'd want a current source that will develop something like one of those voltages at your highest expected resistance to get the most out of your ADC range.QuoteA current sense amplifier generally works by measuring the voltage drop across a known resistance. From the voltage, Ohm's law will then tell you the current. Out of voltage, current, and resistance, you must know two things to calculate the third. So if you wanted to calibrate your system, you might start by acquiring a reference resistor of precisely known value. Connect that as Rsense, then adjust your current source until you get the expected reading from the CSA.

author: Dbldutch
author_rank: Regular Contributor
timestamp: July 31, 2016, 07:07:12 am
content: Look for the scullcom hobby electronics milliohm meter on youtube.I have built that and is very accurate, even without the amplifier section, using a good 5-6 digit DVM to measure the voltage.Enjoy!

author: dannyf
author_rank: Super Contributor
timestamp: July 31, 2016, 11:30:46 am
content: It can be done: I did an analog solution and a digital solution (attiny85). Search my blog for posts on that.

author: danadak
author_rank: Super Contributor
timestamp: July 31, 2016, 01:28:41 pm
content: This might help -http://r.duckduckgo.com/l/?kh=-1&uddg=http%3A%2F%2Fwww.tek.com%2Fdl%2F1KW-30010-0%252520KeithleyLowLevelHandbook_7Ed.pdfRegards, Dana.

author: dannyf
author_rank: Super Contributor
timestamp: July 31, 2016, 02:08:24 pm
content: Not every opamps can work as current sense amps.However, you can add padding here so that practically any opamp can work here.If the output is analog, a typical dual opamp IC, plus a few resistors can be used here for a very accurate ma or UA meter.