grizedale
Advanced Member level 3
Hello,
Please could you tell me if the following current sense resistor voltage measurement technique, which is the standard procedure at a HUGE international telecoms company, is correct?
It concerns the measurment of current (~16A) consumed by the FPGA at the output of a 500KHz, 0.9V buck converter.
The current is measured by using a current sense resistor of 2 milliohms in series with the buck inductor…….(we cannot place the current sense resistor anywhere else because the PCB trace is too wide to cut away…..so the sense resistor “sees” the buck inductor ripple current)
Anyway, the basic method used is to get a 1 metre coaxial cable assembly and connect it to the current sense resistor, and then put this into a Agilent 34401A Bench multimeter….via a BNC to 4mm-plug adapter.
Agilent 34401A meter:
**broken link removed**
The exact method used was:
Get 1 metre length of 50 Ohm coaxial cable.
Solder a 47R axial resistor to the innner conductor at one end of the cable.
Then solder the free end of this 47R resistor to one end of the current sense resistor.
Then "peel away" some of the outer conductor of the cable, and solder that to the other end of the current sense resistor.
Then put a BNC connector on the other end of the cable.
Then put this into a BNC to 4mm plug adapter.
(-An RCR filter is then added, and this is because page 218 of the 34401A meter datasheet states that it can only measure frequencies up to 300KHz.)
-then finally connect this to the input of a Agilent 34401A bench multimeter.
Anyway, the input impedance of the 34401A multimeter is a lot more than 50 Ohms, (page 219 of manual says its 1Megohm) so why do we bother putting the 47R resistor at the end of the coaxial cable?…it surely won’t provide a matching function?
Also, sometimes it's fiendishly difficult to solder the "peeled away" outer connector of the cable to the current sense resistor, so a short, 1 inch piece of twisted pair wires is then used at the end of this coaxial cable assembly to make it easier to solder-connect it to the current sense resistor.....is this acceptable procedure?,
or would that 1 inch piece of twisted pair wires infect the reading with too much noise?
Also, since we are using a heavy RC filter to filter away the ripple, is it really worth us using a coaxial cable assembly?, after all, all the high frequencies are getting filtered out of the measurement, so why don’t we just twist some wires together and run them from the sense resistor to the 34401A inputs?
Please could you tell me if the following current sense resistor voltage measurement technique, which is the standard procedure at a HUGE international telecoms company, is correct?
It concerns the measurment of current (~16A) consumed by the FPGA at the output of a 500KHz, 0.9V buck converter.
The current is measured by using a current sense resistor of 2 milliohms in series with the buck inductor…….(we cannot place the current sense resistor anywhere else because the PCB trace is too wide to cut away…..so the sense resistor “sees” the buck inductor ripple current)
Anyway, the basic method used is to get a 1 metre coaxial cable assembly and connect it to the current sense resistor, and then put this into a Agilent 34401A Bench multimeter….via a BNC to 4mm-plug adapter.
Agilent 34401A meter:
**broken link removed**
The exact method used was:
Get 1 metre length of 50 Ohm coaxial cable.
Solder a 47R axial resistor to the innner conductor at one end of the cable.
Then solder the free end of this 47R resistor to one end of the current sense resistor.
Then "peel away" some of the outer conductor of the cable, and solder that to the other end of the current sense resistor.
Then put a BNC connector on the other end of the cable.
Then put this into a BNC to 4mm plug adapter.
(-An RCR filter is then added, and this is because page 218 of the 34401A meter datasheet states that it can only measure frequencies up to 300KHz.)
-then finally connect this to the input of a Agilent 34401A bench multimeter.
Anyway, the input impedance of the 34401A multimeter is a lot more than 50 Ohms, (page 219 of manual says its 1Megohm) so why do we bother putting the 47R resistor at the end of the coaxial cable?…it surely won’t provide a matching function?
Also, sometimes it's fiendishly difficult to solder the "peeled away" outer connector of the cable to the current sense resistor, so a short, 1 inch piece of twisted pair wires is then used at the end of this coaxial cable assembly to make it easier to solder-connect it to the current sense resistor.....is this acceptable procedure?,
or would that 1 inch piece of twisted pair wires infect the reading with too much noise?
Also, since we are using a heavy RC filter to filter away the ripple, is it really worth us using a coaxial cable assembly?, after all, all the high frequencies are getting filtered out of the measurement, so why don’t we just twist some wires together and run them from the sense resistor to the 34401A inputs?