I hope a current (I) question isn't out of place on the Volt-Nut list. :-) The fundamental question I have, is how can I measure current in the low micro-amps, 1uA or lower, range and have a low impedance, of say less than 10 Ohms? I'm looking to build up a few test fixtures to profile the current (I) of Boards Under Test, on a production line. I'm looking for problems like flux trapped under a part, and baseline Iq testing. I plan on profiling known good boards, to compare against the new board builds. We are talking about quantities of boards of 50,000 to 100,000 per year, so the test has to be easy and quick, that can be done by minimally trained monkeys (not my choice, I have to work with what I'm given). The products are all battery based, so keeping the current consumption down, especially in Sleep mode, is important. We've found over the years that measuring the system current is a good way to find production problems, such as flux trapped under a IC etc. In an idea world I want to have a current (I) data acquisition system with the following specifications: * Current is measured on the high side. * Works with source voltages as high as 32 volts. * Has continuous current scale of 1nA to 1A, with a low source impedance (less than 10 Ohms, maybe 100 Ohms). * Cost less than $50 per unit to build. (Please don't recommend high end meters the boss will never spend money on, not even from EBay. We need five to ten of these units). * The product under test can not be modified. Some of them were design ten years ago, and aren't going to change. I'll settle for 1uA to 500 mA, in multiple scales, as long as the scale switching is automatic, at 12V. The reason I need a low impedance is that my products are part of a sensor network, which transmits data using RF. The RF section wakes up at random intervals around thirty seconds or so. The time is deliberately random to avoid RF packet collisions. You never know when the current meter is about to have its needle wrapped around the end stop with a nice satisfying "Thunk". If I try to measure low currents with say a 10k or 100k resistor I get my current measurement, but when the transmit comes on the system crashes because it does not have enough current to sustain it. I want to be able to run through a full sleep->transmit->sleep cycle without crashing due to current starvation, and track the current throughout the full cycle. I've found various ideas on Internet such as: Measuring nanoamperes; Measuring low currents can be tricky. Clever analog-design techniques and the right parts and equipment can help. By Paul Rako, Technical Editor -- EDN, 4/26/2007 http://www.edn.com/article/CA6434367.html 10nA to 10mA using a LogAmp: http://circuits.linear.com/Precision/.../Eight_Decade_Current_Sensing_Log_Amplifier TI has their LOG10x series of LogAmps as well. Once we get above 10mA things are fairly simple, lots of ways to do that. The fundamental problem is the micro-amp measurements, while maintaining a low impedance. Having two different parallel measurement systems would be fine, on of mA and one of uA. What I want to know is that any of you have been down this road before, and what suggestions you might have? -- http://www.wearablesmartsensors.com/ http://www.softwaresafety.net/ http://www.designer-iii.com/ http://www.unusualresearch.com/

I'd look at a low offest and bias current opamp with diode FB to make a current input log current meter. The input is a summing junction, so is very low impedance. A pair of good back-to-back parallel diodes would protect the inputs. You might think about a bar graph display instead of a meter. FWIW., -John ============ > I hope a current (I) question isn't out of place on the Volt-Nut list. :-) > > The fundamental question I have, is how can I measure current in the > low micro-amps, 1uA or lower, range and have a low impedance, of say > less than 10 Ohms? > > I'm looking to build up a few test fixtures to profile the current (I) of > Boards Under Test, on a production line. I'm looking for problems > like flux trapped under a part, and baseline Iq testing. I plan on > profiling known good boards, to compare against the new board builds. > We are talking about quantities of boards of 50,000 to 100,000 per > year, so the test has to be easy and quick, that can be done by > minimally trained monkeys (not my choice, I have to work with what I'm > given). > > The products are all battery based, so keeping the current consumption > down, especially in Sleep mode, is important. We've found over the > years that measuring the system current is a good way to find > production problems, such as flux trapped under a IC etc. > > In an idea world I want to have a current (I) data acquisition system > with the following specifications: > > * Current is measured on the high side. > * Works with source voltages as high as 32 volts. > * Has continuous current scale of 1nA to 1A, with a low source > impedance (less than 10 Ohms, maybe 100 Ohms). > > * Cost less than $50 per unit to build. > (Please don't recommend high end meters the boss will never spend > money on, not even from EBay. We need five to ten of these units). > > * The product under test can not be modified. Some of them were > design ten years ago, and aren't going to change. > > I'll settle for 1uA to 500 mA, in multiple scales, as long as the > scale switching is automatic, at 12V. > > The reason I need a low impedance is that my products are part of a > sensor network, which transmits data using RF. The RF section wakes up > at random intervals around thirty seconds or so. The time is > deliberately random to avoid RF packet collisions. You never know when > the current meter is about to have its needle wrapped around the end > stop with a nice satisfying "Thunk". > > If I try to measure low currents with say a 10k or 100k resistor I get > my current measurement, but when the transmit comes on the system > crashes because it does not have enough current to sustain it. I want > to be able to run through a full sleep->transmit->sleep cycle without > crashing due to current starvation, and track the current throughout > the full cycle. > > I've found various ideas on Internet such as: > > Measuring nanoamperes; Measuring low currents can be tricky. Clever > analog-design techniques and the right parts and equipment can help. > By Paul Rako, Technical Editor -- EDN, 4/26/2007 > http://www.edn.com/article/CA6434367.html > > 10nA to 10mA using a LogAmp: > http://circuits.linear.com/Precision/.../Eight_Decade_Current_Sensing_Log_Amplifier > > TI has their LOG10x series of LogAmps as well. > > Once we get above 10mA things are fairly simple, lots of ways to do > that. The fundamental problem is the micro-amp measurements, while > maintaining a low impedance. Having two different parallel measurement > systems would be fine, on of mA and one of uA. > > What I want to know is that any of you have been down this road > before, and what suggestions you might have? > > > -- > http://www.wearablesmartsensors.com/ > http://www.softwaresafety.net/ > http://www.designer-iii.com/ > http://www.unusualresearch.com/ > > _______________________________________________ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there. > >

Use the Patterson transdiode connection with npn and pnp transistors as feedback elements. It works much better/more predictably at low currents than when signal diodes are used. Bruce J. Forster wrote: > I'd look at a low offest and bias current opamp with diode FB to make a > current input log current meter. The input is a summing junction, so is > very low impedance. A pair of good back-to-back parallel diodes would > protect the inputs. You might think about a bar graph display instead of a > meter. > > FWIW., > -John > > ============ > > > >> I hope a current (I) question isn't out of place on the Volt-Nut list. :-) >> >> The fundamental question I have, is how can I measure current in the >> low micro-amps, 1uA or lower, range and have a low impedance, of say >> less than 10 Ohms? >> >> I'm looking to build up a few test fixtures to profile the current (I) of >> Boards Under Test, on a production line. I'm looking for problems >> like flux trapped under a part, and baseline Iq testing. I plan on >> profiling known good boards, to compare against the new board builds. >> We are talking about quantities of boards of 50,000 to 100,000 per >> year, so the test has to be easy and quick, that can be done by >> minimally trained monkeys (not my choice, I have to work with what I'm >> given). >> >> The products are all battery based, so keeping the current consumption >> down, especially in Sleep mode, is important. We've found over the >> years that measuring the system current is a good way to find >> production problems, such as flux trapped under a IC etc. >> >> In an idea world I want to have a current (I) data acquisition system >> with the following specifications: >> >> * Current is measured on the high side. >> * Works with source voltages as high as 32 volts. >> * Has continuous current scale of 1nA to 1A, with a low source >> impedance (less than 10 Ohms, maybe 100 Ohms). >> >> * Cost less than $50 per unit to build. >> (Please don't recommend high end meters the boss will never spend >> money on, not even from EBay. We need five to ten of these units). >> >> * The product under test can not be modified. Some of them were >> design ten years ago, and aren't going to change. >> >> I'll settle for 1uA to 500 mA, in multiple scales, as long as the >> scale switching is automatic, at 12V. >> >> The reason I need a low impedance is that my products are part of a >> sensor network, which transmits data using RF. The RF section wakes up >> at random intervals around thirty seconds or so. The time is >> deliberately random to avoid RF packet collisions. You never know when >> the current meter is about to have its needle wrapped around the end >> stop with a nice satisfying "Thunk". >> >> If I try to measure low currents with say a 10k or 100k resistor I get >> my current measurement, but when the transmit comes on the system >> crashes because it does not have enough current to sustain it. I want >> to be able to run through a full sleep->transmit->sleep cycle without >> crashing due to current starvation, and track the current throughout >> the full cycle. >> >> I've found various ideas on Internet such as: >> >> Measuring nanoamperes; Measuring low currents can be tricky. Clever >> analog-design techniques and the right parts and equipment can help. >> By Paul Rako, Technical Editor -- EDN, 4/26/2007 >> http://www.edn.com/article/CA6434367.html >> >> 10nA to 10mA using a LogAmp: >> http://circuits.linear.com/Precision/.../Eight_Decade_Current_Sensing_Log_Amplifier >> >> TI has their LOG10x series of LogAmps as well. >> >> Once we get above 10mA things are fairly simple, lots of ways to do >> that. The fundamental problem is the micro-amp measurements, while >> maintaining a low impedance. Having two different parallel measurement >> systems would be fine, on of mA and one of uA. >> >> What I want to know is that any of you have been down this road >> before, and what suggestions you might have? >> >> >> -- >> http://www.wearablesmartsensors.com/ >> http://www.softwaresafety.net/ >> http://www.designer-iii.com/ >> http://www.unusualresearch.com/ >> >> _______________________________________________ >> volt-nuts mailing list -- volt-nuts@febo.com >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts >> and follow the instructions there. >> >> >> > > > > _______________________________________________ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there. > >

Bruce Griffiths wrote: > Use the Patterson transdiode connection with npn and pnp transistors as > feedback elements. > It works much better/more predictably at low currents than when signal > diodes are used. > > Bruce > > To handle 1A either a power opamp or an opamp with a high current output buffer will be required. Transistors with a collector current rating of 1A or more will also be necessary. Bruce

On Sat, Sep 26, 2009 at 6:56 PM, Bruce Griffiths <bruce.griffiths@xtra.co.nz> wrote: > Use the Patterson transdiode connection with npn and pnp transistors as > feedback elements. I'm not familiar with Patterson's work, and Google was not much help. Will you point me to a specific paper or schematic please?

> You might think about a bar graph display instead of a > meter. Actual production line tester just lights up Green/Red LEDs, and gives out a Banana, for Pass/Fail. The data is logged so it can be graphed on a PC later.

Bruce Griffiths wrote: > Bruce Griffiths wrote: > >> Use the Patterson transdiode connection with npn and pnp transistors as >> feedback elements. >> It works much better/more predictably at low currents than when signal >> diodes are used. >> >> Bruce >> >> >> > To handle 1A either a power opamp or an opamp with a high current output > buffer will be required. > Transistors with a collector current rating of 1A or more will also be > necessary. > > Bruce > > > The best solution is to use a log amp or equivalent for the 1nA to 10mA range and a conventional current shunt with a DVM or equivalent to measure the 10mA to 1A range where log amps dont work to well unless one takes heroic and expensive measures. The log amp and the conventional shunt are connected in series. The real problem is to ensure that the voltage drop across the log amp input is low even when the current exceeds 10mA. One way of doing this is to use a high current shunt diode across the log amp inputs. When the log amp isn't saturated the voltage across this diode is very low as is the associated diode leakage. When the log amp opamp output current or voltage limits the voltage across the diode increases turning it on. However this introduces a diode drop in series with the supply at high currents. In principle one could sense the voltage across the current shunt and use this to turn on either a high MOSFET shunt switch connected across the diode shunting the log amp inputs. This can in principle reduce the voltage drop to 10mV or less with the right choice of switch. Depending on the desired response time one could even use a relay for this function. Bruce

> To handle 1A either a power opamp or an opamp with a high current output > buffer will be required. Doing two different scales should avoid that. I can use a standard low value current sense resistors and DiffAmp or InAmp for the currents in the mA range, and up. I did just thing that I should add, that the current pulses from the transmitter are narrow in time, a few tens of microseconds. The capacitors on the actual board under test take care of most of that, from being seen by the tester.

Bob Paddock wrote: > On Sat, Sep 26, 2009 at 6:56 PM, Bruce Griffiths > <bruce.griffiths@xtra.co.nz> wrote: > >> Use the Patterson transdiode connection with npn and pnp transistors as >> feedback elements. >> > > I'm not familiar with Patterson's work, and Google was not much help. > > Will you point me to a specific paper or schematic please? > > Its not particularly new, you have probably seen the circuit many times (the name has been around for at least 50 years or so) see: http://www.electronics.dit.ie/staff/ypanarin/Lecture%20Notes/DT021-4/6LogAntiLogAmplifiers.pdf The problem is that transistor bulk resistance and opamp output limitations restrict the maximum operating current to 10mA or less. Bruce

>> I'm not familiar with Patterson's work, and Google was not much help. >> >> Will you point me to a specific paper or schematic please? >> > Its not particularly new, you have probably seen the circuit many times > (the name has been around for at least 50 years or so) see: You're right, I've seen lots of those circuits, just never recall the name Patterson being mentioned related to them. Thank you for the link. > http://www.electronics.dit.ie/staff/ypanarin/Lecture%20Notes/DT021-4/6LogAntiLogAmplifiers.pdf