A mcu-based milliohm meter

[janengelbrecht]

Really a interesting project. Not until now i had realised the presence of differential ADC in these Atmel chips I will follow this subject with great joy in the future

[dannyf]

AVR's ADC modules are quite unique among 8-bit mcus (and even among 32-bit mcus) in that it not only offers differential capabilities but has a variable gain module there, allowing it to achieve better resolution than spec'd.

The Attiny's ADC module, unfortunately, is a little bit crippled in that its positive input must be at a higher potential than its negative input. Other AVRs, like usb128x or 32u4, don't have that limitation.

Unfortunately, usb128x is quite expensive. 32u4 is used in many of those cheap Pro mini boards. Unfortunately, those designers did not route out the ADC0/ADC1 pins.

The best pick for this would be a Leonardo board - its ADC0/1 pins are full accessible. But it seems an overkill for this purpose.

If you want to build, however, a milliohm / millivolt meter with digital display, a Leonardo + lcd shield would be a perfect choice.

[nuno]

Some (for example ATtiny24, ATtiny261) support the "Bipolar Differential Conversion" mode, in which any of the inputs can be higher than the other and the result will be a signed integer.

[dannyf]

Not sure about 261 but Attiny24/44/84 have the same adc module as the 25/45/85: the positive input needs to be at a higher potential than the negative input.

I will get a pro micro and hand solder out the ADC0/ADC1 pins to experiment.

[nuno]

"As default the ADC converter operates in the unipolar input mode, but the bipolar input mode
can be selected by writting the BIN bit in the ADCSRB to one. In the bipolar input mode two sided
voltage differences are allowed and thus the voltage on the negative input pin can also be
larger than the voltage on the positive input pin."

Datasheet is your friend . In each generation ATMEL introduces some minor improvements.

[dannyf]

That's indeed the case.

Thanks.

[dannyf]

Current consumption: 16.6ma total. The driver circuit consumes about 14.7ma so the chip itself consumes 2ma.

[nctnico]

I have a feeling you can increase the resolution a lot if you change the software to do synchronous integration.

[dannyf]

6) Better topology - a real high precision milliohm meter can be done with an external adc - differential input, differential external reference input, 16 bit or more. Put the dut in serial with the precision resistor. The differential adc across the dut and the differential external reference across the precision resistor. The output is ratiometric to the precision resistor.

An example of it would be here - notice how DUT (R3) is connected with the reference resistor (R2). R2 doesn't need to be a precision resistor. If it is not, you need to calibrate the meter.

You can easily go sub-milliohm with this approach.

[dannyf]

You can reconfigure a typical digital meter, like DT830, in a similar approach to get a very inexpensive milliohm meter out of it, with digital readout too.

[dannyf]

Changed the gain to 1v/100mohm.

The readout is a digital volt meter (1v/100mohm), and a digital current meter (10ua/100mohm)

[dannyf]

I took the attiny85 code and ported it to attiny84 - minor changes.

Here it is, measuring a 0.1ohm resistor on a 1v/0.1ohm scale.

Because of the bipolar output, nulling is very easy here.

[dannyf]

Minor changes -> running on timer1, 10-bit pwm.

[nuno]

Very nice