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Current Source and DG408 On-Resistance Problem

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Bob McCloy:
I'm working on a project designing a multi-meter and I am currently building the section to measure resistances. I am using a constant current source design from the Art of Electronics, Third Edition, Page 229. I want to be able to measure resistances from 10Ω to 1MΩ.

I have attached the schematic of my current design. The idea is that the op-amp locks the voltage drop across all the 5 top resistors at 1V and then swapping in one resistor at a time gives a known current through the transistor and down through the load. To measure a resistance I start with the assumption that the load is very large and work down swapping in the next value of resistor if it is within the range of the next current value. Each value of current has a maximum unknown load it can provide a measurement for before the voltage drop across the transistor is too small and the transistor goes into saturation.

The full multi-meter is designed around a micro-controller that will be used to measure the values and display the result. I want to be able to have the micro-controller swap between the resistors automatically so it can auto-range and get a more accurate measurement. I thought about using something like the DG408 Multiplexer. https://www.vishay.com/docs/70062/dg408.pdf and this is where I run into a problem.

My question is, the DG408 has a certain On-Resistance that can be up to 100Ω and also varies with temperature this would seriously effect the current when on the lowest resistance setting of 100Ω and the whole design is based on knowing the current for each of the 5 resistors. Is there any change to my circuit that could negate the effect of the On-Resistance of the DG408. Or any other suggestions on improvements to the design?

Thanks

RES:
https://www.edn.com/design/test-and-measurement/4388713/Measure-resistance-with-a-microcontroller

OM222O:
you are almost correct but a few simple changes can fix your issue.
I have built a very similar circuit with current ranges of 10uA to 1A! works fine to 100mA actually but the 1A range is kind of iffy (I got 700 and a change mA output, not sure what's wrong but trying different parts to see if that will fix the issue).
also replace the pot with a voltage reference. if you desperately want 1V, go for it, but I couldn't find any with decent temperature and long term drift for cheap. If not, use a 1.024V reference, you will need to measure it and do the math in your MCU however.

here is the full thread for my project, I think this is exactly what you want:
https://www.eevblog.com/forum/projects/feedback-on-milliohm-meter-v2-0/msg2201538/#msg2201538

schematic:


Edit: I had better luck using the MCP6002 instead of the TLV9002. you can also include a 100ohm to 1k resistor on the output of the op amp to reduce oscillations, but you must use a darlington pair for higher value resistors.
The parts selection is not final, I made an evaluation board to test different darlington pairs, op amps and voltage references, but the project is paused due to the fact that I have exams at the moment, so if you can wait, I will finish my exams in may. we can work on designing a multi meter together. drop me a message if you are interested.

duak:
You can improve this circuit to make it less sensitive to the analog switch resistances with a Kelvin connection.  This uses two analog switches for each resistor;  one handles the load current and the other handles the sensed voltage. This means that the voltage across the selected load current switch is not fed back to the opamp.  Your circuit can be modified with the following (hopefully my description is clear)

 1. the range selection switches are moved to the right of the resistors
 2. a second set of selection switches connect the inverting input of the opamp to the right sides of the resistors
 3. the switch logic is set up so that the switches for each resistor are enabled at the same time

Very few DMMs use this circuit to generate an accurate constant current for various reasons.  Search for "DMM ohms converter"

Cheers,

OM222O:

--- Quote from: duak on April 03, 2019, 10:00:34 pm ---However, you can improve this circuit to make it less sensitive to the analog switch resistances with a Kelvin connection.  This uses two analog switches for each resistor;  one handles the load current and the other handles the sensed voltage. This means that the voltage across the selected load current switch is not fed back to the opamp.  Your circuit can be modified with the following (hopefully my description is clear)

 1. the range selection switches are moved to the right of the resistors
 2. a second set of selection switches connect the inverting input of the opamp to the right sides of the resistors
 3. the switch logic is set up so that the switches for each resistor are enabled at the same time

Very few DMMs use this circuit to generate an accurate constant current for various reasons.  Search for "DMM ohms converter"

Cheers,

--- End quote ---

using my approach the unused resistors are a part of the negative feedback for the op amp, which has a tiny input bias current. let's say a worst case scenario:1uA input current. that is about 0.1v drop across the resistor chain. I know this is terrible for a precision application, but you also have to consider 1uA is huge! the input current for any half decent amplifier is in the order of nA if not pA! so several orders of magnitude better performance. the error is basically negligible.

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